man perlfunc (Commandes) - Perl builtin functions

NAME

perlfunc - Perl builtin functions

DESCRIPTION

The functions in this section can serve as terms in an expression. They fall into two major categories: list operators and named unary operators. These differ in their precedence relationship with a following comma. (See the precedence table in perlop.) List operators take more than one argument, while unary operators can never take more than one argument. Thus, a comma terminates the argument of a unary operator, but merely separates the arguments of a list operator. A unary operator generally provides a scalar context to its argument, while a list operator may provide either scalar or list contexts for its arguments. If it does both, the scalar arguments will be first, and the list argument will follow. (Note that there can ever be only one such list argument.) For instance, splice() has three scalar arguments followed by a list, whereas gethostbyname() has four scalar arguments.

In the syntax descriptions that follow, list operators that expect a list (and provide list context for the elements of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Elements of the LIST should be separated by commas.

Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the parentheses.) If you use the parentheses, the simple (but occasionally surprising) rule is this: It looks like a function, therefore it is a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. And whitespace between the function and left parenthesis doesn't countso you need to be careful sometimes:

    print 1+2+4;        # Prints 7.
    print(1+2) + 4;     # Prints 3.
    print (1+2)+4;      # Also prints 3!
    print +(1+2)+4;     # Prints 7.
    print ((1+2)+4);    # Prints 7.

If you run Perl with the -w switch it can warn you about this. For example, the third line above produces:

    print (...) interpreted as function at - line 1.
    Useless use of integer addition in void context at - line 1.

A few functions take no arguments at all, and therefore work as neither unary nor list operators. These include such functions as CWtime and CWendpwent. For example, CWtime+86_400 always means CWtime() + 86_400.

For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in a scalar context by returning the undefined value, and in a list context by returning the null list.

Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value it would be most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in list context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.

A named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. You can't get a list like CW(1,2,3) into being in scalar context, because the compiler knows the context at compile time. It would generate the scalar comma operator there, not the list construction version of the comma. That means it was never a list to start with.

In general, functions in Perl that serve as wrappers for system calls of the same name (like chown(2), fork(2), closedir(2), etc.) all return true when they succeed and CWundef otherwise, as is usually mentioned in the descriptions below. This is different from the C interfaces, which return CW-1 on failure. Exceptions to this rule are CWwait, CWwaitpid, and CWsyscall. System calls also set the special CW$! variable on failure. Other functions do not, except accidentally.

Perl Functions by Category

Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place.

Functions for SCALARs or strings: CWchomp, CWchop, CWchr, CWcrypt, CWhex, CWindex, CWlc, CWlcfirst, CWlength, CWoct, CWord, CWpack, CWq/STRING/, CWqq/STRING/, CWreverse, CWrindex, CWsprintf, CWsubstr, CWtr///, CWuc, CWucfirst, CWy///
Regular expressions and pattern matching: CWm//, CWpos, CWquotemeta, CWs///, CWsplit, CWstudy, CWqr//
Numeric functions: CWabs, CWatan2, CWcos, CWexp, CWhex, CWint, CWlog, CWoct, CWrand, CWsin, CWsqrt, CWsrand CWpop, CWpush, CWshift, CWsplice, CWunshift
Functions for list data: CWgrep, CWjoin, CWmap, CWqw/STRING/, CWreverse, CWsort, CWunpack CWdelete, CWeach, CWexists, CWkeys, CWvalues
Input and output functions: CWbinmode, CWclose, CWclosedir, CWdbmclose, CWdbmopen, CWdie, CWeof, CWfileno, CWflock, CWformat, CWgetc, CWprint, CWprintf, CWread, CWreaddir, CWrewinddir, CWseek, CWseekdir, CWselect, CWsyscall, CWsysread, CWsysseek, CWsyswrite, CWtell, CWtelldir, CWtruncate, CWwarn, CWwrite
Functions for fixed length data or records: CWpack, CWread, CWsyscall, CWsysread, CWsyswrite, CWunpack, CWvec
Functions for filehandles, files, or directories: CW-CIXCW, CWchdir, CWchmod, CWchown, CWchroot, CWfcntl, CWglob, CWioctl, CWlink, CWlstat, CWmkdir, CWopen, CWopendir, CWreadlink, CWrename, CWrmdir, CWstat, CWsymlink, CWsysopen, CWumask, CWunlink, CWutime
Keywords related to the control flow of your perl program: CWcaller, CWcontinue, CWdie, CWdo, CWdump, CWeval, CWexit, CWgoto, CWlast, CWnext, CWredo, CWreturn, CWsub, CWwantarray
Keywords related to scoping: CWcaller, CWimport, CWlocal, CWmy, CWour, CWpackage, CWuse
Miscellaneous functions: CWdefined, CWdump, CWeval, CWformline, CWlocal, CWmy, CWour, CWreset, CWscalar, CWundef, CWwantarray
Functions for processes and process groups: CWalarm, CWexec, CWfork, CWgetpgrp, CWgetppid, CWgetpriority, CWkill, CWpipe, CWqx/STRING/, CWsetpgrp, CWsetpriority, CWsleep, CWsystem, CWtimes, CWwait, CWwaitpid
Keywords related to perl modules: CWdo, CWimport, CWno, CWpackage, CWrequire, CWuse
Keywords related to classes and object-orientedness: CWbless, CWdbmclose, CWdbmopen, CWpackage, CWref, CWtie, CWtied, CWuntie, CWuse
Low-level socket functions: CWaccept, CWbind, CWconnect, CWgetpeername, CWgetsockname, CWgetsockopt, CWlisten, CWrecv, CWsend, CWsetsockopt, CWshutdown, CWsocket, CWsocketpair
System V interprocess communication functions: CWmsgctl, CWmsgget, CWmsgrcv, CWmsgsnd, CWsemctl, CWsemget, CWsemop, CWshmctl, CWshmget, CWshmread, CWshmwrite
Fetching user and group info: CWendgrent, CWendhostent, CWendnetent, CWendpwent, CWgetgrent, CWgetgrgid, CWgetgrnam, CWgetlogin, CWgetpwent, CWgetpwnam, CWgetpwuid, CWsetgrent, CWsetpwent
Fetching network info: CWendprotoent, CWendservent, CWgethostbyaddr, CWgethostbyname, CWgethostent, CWgetnetbyaddr, CWgetnetbyname, CWgetnetent, CWgetprotobyname, CWgetprotobynumber, CWgetprotoent, CWgetservbyname, CWgetservbyport, CWgetservent, CWsethostent, CWsetnetent, CWsetprotoent, CWsetservent
Time-related functions: CWgmtime, CWlocaltime, CWtime, CWtimes
Functions new in perl5: CWabs, CWbless, CWchomp, CWchr, CWexists, CWformline, CWglob, CWimport, CWlc, CWlcfirst, CWmap, CWmy, CWno, CWour, CWprototype, CWqx, CWqw, CWreadline, CWreadpipe, CWref, CWsub*, CWsysopen, CWtie, CWtied, CWuc, CWucfirst, CWuntie, CWuse * - CWsub was a keyword in perl4, but in perl5 it is an operator, which can be used in expressions.
Functions obsoleted in perl5: CWdbmclose, CWdbmopen

Portability

Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the functionality of some Unix system calls may not be available, or details of the available functionality may differ slightly. The Perl functions affected by this are:

CW-X, CWbinmode, CWchmod, CWchown, CWchroot, CWcrypt, CWdbmclose, CWdbmopen, CWdump, CWendgrent, CWendhostent, CWendnetent, CWendprotoent, CWendpwent, CWendservent, CWexec, CWfcntl, CWflock, CWfork, CWgetgrent, CWgetgrgid, CWgethostbyname, CWgethostent, CWgetlogin, CWgetnetbyaddr, CWgetnetbyname, CWgetnetent, CWgetppid, CWgetprgp, CWgetpriority, CWgetprotobynumber, CWgetprotoent, CWgetpwent, CWgetpwnam, CWgetpwuid, CWgetservbyport, CWgetservent, CWgetsockopt, CWglob, CWioctl, CWkill, CWlink, CWlstat, CWmsgctl, CWmsgget, CWmsgrcv, CWmsgsnd, CWopen, CWpipe, CWreadlink, CWrename, CWselect, CWsemctl, CWsemget, CWsemop, CWsetgrent, CWsethostent, CWsetnetent, CWsetpgrp, CWsetpriority, CWsetprotoent, CWsetpwent, CWsetservent, CWsetsockopt, CWshmctl, CWshmget, CWshmread, CWshmwrite, CWsocket, CWsocketpair, CWstat, CWsymlink, CWsyscall, CWsysopen, CWsystem, CWtimes, CWtruncate, CWumask, CWunlink, CWutime, CWwait, CWwaitpid

For more information about the portability of these functions, see perlport and other available platform-specific documentation.

Alphabetical Listing of Perl Functions

-X FILEHANDLE

-X EXPR

-X

A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename or a filehandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests CW$_, except for CW-t, which tests STDIN. Unless otherwise documented, it returns CW1 for true and CW'' for false, or the undefined value if the file doesn't exist. Despite the funny names, precedence is the same as any other named unary operator, and the argument may be parenthesized like any other unary operator. The operator may be any of:

    -r  File is readable by effective uid/gid.
    -w  File is writable by effective uid/gid.
    -x  File is executable by effective uid/gid.
    -o  File is owned by effective uid.

    -R  File is readable by real uid/gid.
    -W  File is writable by real uid/gid.
    -X  File is executable by real uid/gid.
    -O  File is owned by real uid.

    -e  File exists.
    -z  File has zero size (is empty).
    -s  File has nonzero size (returns size in bytes).

    -f  File is a plain file.
    -d  File is a directory.
    -l  File is a symbolic link.
    -p  File is a named pipe (FIFO), or Filehandle is a pipe.
    -S  File is a socket.
    -b  File is a block special file.
    -c  File is a character special file.
    -t  Filehandle is opened to a tty.

    -u  File has setuid bit set.
    -g  File has setgid bit set.
    -k  File has sticky bit set.

    -T  File is an ASCII text file (heuristic guess).
    -B  File is a "binary" file (opposite of -T).

    -M  Script start time minus file modification time, in days.
    -A  Same for access time.
    -C  Same for inode change time (Unix, may differ for other platforms)

Example:

    while (<>) {
        chomp;
        next unless -f $_;      # ignore specials
        #...
    }

The interpretation of the file permission operators CW-r, CW-R, CW-w, CW-W, CW-x, and CW-X is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file. Such reasons may be for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats. Also note that, for the superuser on the local filesystems, the CW-r, CW-R, CW-w, and CW-W tests always return 1, and CW-x and CW-X return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode of the file, or temporarily set their effective uid to something else. If you are using ACLs, there is a pragma called CWfiletest that may produce more accurate results than the bare stat() mode bits. When under the CWuse filetest 'access' the above-mentioned filetests will test whether the permission can (not) be granted using the access() family of system calls. Also note that the CW-x and CW-X may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Read the documentation for the CWfiletest pragma for more information. Note that CW-s/a/b/ does not do a negated substitution. Saying CW-exp($foo) still works as expected, howeveronly single letters following a minus are interpreted as file tests. The CW-T and CW-B switches work as follows. The first block or so of the file is examined for odd characters such as strange control codes or characters with the high bit set. If too many strange characters (>30%) are found, it's a CW-B file, otherwise it's a CW-T file. Also, any file containing null in the first block is considered a binary file. If CW-T or CW-B is used on a filehandle, the current IO buffer is examined rather than the first block. Both CW-T and CW-B return true on a null file, or a file at EOF when testing a filehandle. Because you have to read a file to do the CW-T test, on most occasions you want to use a CW-f against the file first, as in CWnext unless -f $file && -T $file. If any of the file tests (or either the CWstat or CWlstat operators) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with CW-t, and you need to remember that lstat() and CW-l will leave values in the stat structure for the symbolic link, not the real file.) (Also, if the stat buffer was filled by a CWlstat call, CW-T and CW-B will reset it with the results of CWstat _). Example:

    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);
    print "Readable\n" if -r _;
    print "Writable\n" if -w _;
    print "Executable\n" if -x _;
    print "Setuid\n" if -u _;
    print "Setgid\n" if -g _;
    print "Sticky\n" if -k _;
    print "Text\n" if -T _;
    print "Binary\n" if -B _;

abs VALUE

abs

Returns the absolute value of its argument. If VALUE is omitted, uses CW$_.

accept NEWSOCKET,GENERICSOCKET

Accepts an incoming socket connect, just as the accept(2) system call does. Returns the packed address if it succeeded, false otherwise. See the example in Sockets: Client/Server Communication in perlipc. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in perlvar.

alarm SECONDS

alarm

Arranges to have a SIGALRM delivered to this process after the specified number of wallclock seconds have elapsed. If SECONDS is not specified, the value stored in CW$_ is used. (On some machines, unfortunately, the elapsed time may be up to one second less or more than you specified because of how seconds are counted, and process scheduling may delay the delivery of the signal even further.) Only one timer may be counting at once. Each call disables the previous timer, and an argument of CW0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer. For delays of finer granularity than one second, you may use Perl's four-argument version of select() leaving the first three arguments undefined, or you might be able to use the CWsyscall interface to access setitimer(2) if your system supports it. The Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) may also prove useful. It is usually a mistake to intermix CWalarm and CWsleep calls. (CWsleep may be internally implemented in your system with CWalarm) If you want to use CWalarm to time out a system call you need to use an CWeval/CWdie pair. You can't rely on the alarm causing the system call to fail with CW$! set to CWEINTR because Perl sets up signal handlers to restart system calls on some systems. Using CWeval/CWdie always works, modulo the caveats given in Signals in perlipc.

    eval {
        local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
        alarm $timeout;
        $nread = sysread SOCKET, $buffer, $size;
        alarm 0;
    };
    if ($@) {
        die unless $@ eq "alarm\n";   # propagate unexpected errors
        # timed out
    }
    else {
        # didn't
    }

For more information see perlipc.

atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI. For the tangent operation, you may use the CWMath::Trig::tan function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }

bind SOCKET,NAME

Binds a network address to a socket, just as the bind system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in Sockets: Client/Server Communication in perlipc.

binmode FILEHANDLE, LAYER

binmode FILEHANDLE

Arranges for FILEHANDLE to be read or written in binary or text mode on systems where the run-time libraries distinguish between binary and text files. If FILEHANDLE is an expression, the value is taken as the name of the filehandle. Returns true on success, otherwise it returns CWundef and sets CW$! (errno). On some systems (in general, DOS and Windows-based systems) binmode() is necessary when you're not working with a text file. For the sake of portability it is a good idea to always use it when appropriate, and to never use it when it isn't appropriate. Also, people can set their I/O to be by default UTF-8 encoded Unicode, not bytes. In other words: regardless of platform, use binmode() on binary data, like for example images. If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the file handle. When LAYER is present using binmode on text file makes sense. If LAYER is omitted or specified as CW:raw the filehandle is made suitable for passing binary data. This includes turning off possible CRLF translation and marking it as bytes (as opposed to Unicode characters). Note that, despite what may be implied in Programming Perl (the Camel) or elsewhere, CW:raw is not the simply inverse of CW:crlf other layers which would affect binary nature of the stream are also disabled. See PerlIO, perlrun and the discussion about the PERLIO environment variable. The CW:bytes, CW:crlf, and CW:utf8, and any other directives of the form CW:..., are called I/O layers. The CWopen pragma can be used to establish default I/O layers. See open. The LAYER parameter of the binmode() function is described as DISCIPLINE in Programming Perl, 3rd Edition. However, since the publishing of this book, by many known as Camel III, the consensus of the naming of this functionality has moved from discipline to layer. All documentation of this version of Perl therefore refers to layers rather than to disciplines. Now back to the regularly scheduled documentation... To mark FILEHANDLE as UTF-8, use CW:utf8. In general, binmode() should be called after open() but before any I/O is done on the filehandle. Calling binmode() will normally flush any pending buffered output data (and perhaps pending input data) on the handle. An exception to this is the CW:encoding layer that changes the default character encoding of the handle, see open. The CW:encoding layer sometimes needs to be called in mid-stream, and it doesn't flush the stream. The CW:encoding also implicitly pushes on top of itself the CW:utf8 layer because internally Perl will operate on UTF-8 encoded Unicode characters. The operating system, device drivers, C libraries, and Perl run-time system all work together to let the programmer treat a single character (CW\n) as the line terminator, irrespective of the external representation. On many operating systems, the native text file representation matches the internal representation, but on some platforms the external representation of CW\n is made up of more than one character. Mac OS, all variants of Unix, and Stream_LF files on VMS use a single character to end each line in the external representation of text (even though that single character is CARRIAGE RETURN on Mac OS and LINE FEED on Unix and most VMS files). In other systems like OS/2, DOS and the various flavors of MS-Windows your program sees a CW\n as a simple CW\cJ, but what's stored in text files are the two characters CW\cM\cJ. That means that, if you don't use binmode() on these systems, CW\cM\cJ sequences on disk will be converted to CW\n on input, and any CW\n in your program will be converted back to CW\cM\cJ on output. This is what you want for text files, but it can be disastrous for binary files. Another consequence of using binmode() (on some systems) is that special end-of-file markers will be seen as part of the data stream. For systems from the Microsoft family this means that if your binary data contains CW\cZ, the I/O subsystem will regard it as the end of the file, unless you use binmode(). binmode() is not only important for readline() and print() operations, but also when using read(), seek(), sysread(), syswrite() and tell() (see perlport for more details). See the CW$/ and CW$\ variables in perlvar for how to manually set your input and output line-termination sequences.

bless REF,CLASSNAME

bless REF

This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package. If CLASSNAME is omitted, the current package is used. Because a CWbless is often the last thing in a constructor, it returns the reference for convenience. Always use the two-argument version if the function doing the blessing might be inherited by a derived class. See perltoot and perlobj for more about the blessing (and blessings) of objects. Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase names, so to prevent confusion, you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value. See Perl Modules in perlmod.

caller EXPR

caller

Returns the context of the current subroutine call. In scalar context, returns the caller's package name if there is a caller, that is, if we're in a subroutine or CWeval or CWrequire, and the undefined value otherwise. In list context, returns

    ($package, $filename, $line) = caller;

With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.

    ($package, $filename, $line, $subroutine, $hasargs,
    $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);

Here CW$subroutine may be CW(eval) if the frame is not a subroutine call, but an CWeval. In such a case additional elements CW$evaltext and CW$is_require are set: CW$is_require is true if the frame is created by a CWrequire or CWuse statement, CW$evaltext contains the text of the CWeval EXPR statement. In particular, for an CWeval BLOCK statement, CW$filename is CW(eval), but CW$evaltext is undefined. (Note also that each CWuse statement creates a CWrequire frame inside an CWeval EXPR frame.) CW$subroutine may also be CW(unknown) if this particular subroutine happens to have been deleted from the symbol table. CW$hasargs is true if a new instance of CW@_ was set up for the frame. CW$hints and CW$bitmask contain pragmatic hints that the caller was compiled with. The CW$hints and CW$bitmask values are subject to change between versions of Perl, and are not meant for external use. Furthermore, when called from within the DB package, caller returns more detailed information: it sets the list variable CW@DB::args to be the arguments with which the subroutine was invoked. Be aware that the optimizer might have optimized call frames away before CWcaller had a chance to get the information. That means that CWcaller(N) might not return information about the call frame you expect it do, for CWN > 1. In particular, CW@DB::args might have information from the previous time CWcaller was called.

chdir EXPR

Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the directory specified by CW$ENV{HOME}, if set; if not, changes to the directory specified by CW$ENV{LOGDIR}. (Under VMS, the variable CW$ENV{SYS$LOGIN} is also checked, and used if it is set.) If neither is set, CWchdir does nothing. It returns true upon success, false otherwise. See the example under CWdie.

chmod LIST

Changes the permissions of a list of files. The first element of the list must be the numerical mode, which should probably be an octal number, and which definitely should not be a string of octal digits: CW0644 is okay, CW'0644' is not. Returns the number of files successfully changed. See also oct, if all you have is a string.

    $cnt = chmod 0755, 'foo', 'bar';
    chmod 0755, @executables;
    $mode = '0644'; chmod $mode, 'foo';      # !!! sets mode to
                                             # --w----r-T
    $mode = '0644'; chmod oct($mode), 'foo'; # this is better
    $mode = 0644;   chmod $mode, 'foo';      # this is best

You can also import the symbolic CWS_I* constants from the Fcntl module:

    use Fcntl ':mode';

    chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
    # This is identical to the chmod 0755 of the above example.

chomp VARIABLE

chomp( LIST )

chomp

This safer version of chop removes any trailing string that corresponds to the current value of CW$/ (also known as CW$INPUT_RECORD_SEPARATOR in the CWEnglish module). It returns the total number of characters removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried that the final record may be missing its newline. When in paragraph mode (CW$/ = ""), it removes all trailing newlines from the string. When in slurp mode (CW$/ = undef) or fixed-length record mode (CW$/ is a reference to an integer or the like, see perlvar) chomp() won't remove anything. If VARIABLE is omitted, it chomps CW$_. Example:

    while (<>) {
        chomp;  # avoid \n on last field
        @array = split(/:/);
        # ...
    }

If VARIABLE is a hash, it chomps the hash's values, but not its keys. You can actually chomp anything that's an lvalue, including an assignment:

    chomp($cwd = `pwd`);
    chomp($answer = <STDIN>);

If you chomp a list, each element is chomped, and the total number of characters removed is returned. If the CWencoding pragma is in scope then the lengths returned are calculated from the length of CW$/ in Unicode characters, which is not always the same as the length of CW$/ in the native encoding. Note that parentheses are necessary when you're chomping anything that is not a simple variable. This is because CWchomp $cwd = `pwd`; is interpreted as CW(chomp $cwd) = `pwd`;, rather than as CWchomp( $cwd = `pwd` ) which you might expect. Similarly, CWchomp $a, $b is interpreted as CWchomp($a), $b rather than as CWchomp($a, $b).

chop VARIABLE

chop( LIST )

chop

Chops off the last character of a string and returns the character chopped. It is much more efficient than CWs/.$//s because it neither scans nor copies the string. If VARIABLE is omitted, chops CW$_. If VARIABLE is a hash, it chops the hash's values, but not its keys. You can actually chop anything that's an lvalue, including an assignment. If you chop a list, each element is chopped. Only the value of the last CWchop is returned. Note that CWchop returns the last character. To return all but the last character, use CWsubstr($string, 0, -1). See also chomp.

chown LIST

Changes the owner (and group) of a list of files. The first two elements of the list must be the numeric uid and gid, in that order. A value of -1 in either position is interpreted by most systems to leave that value unchanged. Returns the number of files successfully changed.

    $cnt = chown $uid, $gid, 'foo', 'bar';
    chown $uid, $gid, @filenames;

Here's an example that looks up nonnumeric uids in the passwd file:

    print "User: ";
    chomp($user = <STDIN>);
    print "Files: ";
    chomp($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
        or die "$user not in passwd file";

    @ary = glob($pattern);      # expand filenames
    chown $uid, $gid, @ary;

On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption. On POSIX systems, you can detect this condition this way:

    use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
    $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);

chr NUMBER

chr

Returns the character represented by that NUMBER in the character set. For example, CWchr(6) is CW"A" in either ASCII or Unicode, and chr(0) is a Unicode smiley face. Note that characters from 128 to 255 (inclusive) are by default not encoded in UTF-8 Unicode for backward compatibility reasons (but see encoding). If NUMBER is omitted, uses CW$_. For the reverse, use ord. Note that under the CWbytes pragma the NUMBER is masked to the low eight bits. See perlunicode and encoding for more about Unicode.

chroot FILENAME

chroot

This function works like the system call by the same name: it makes the named directory the new root directory for all further pathnames that begin with a CW/ by your process and all its children. (It doesn't change your current working directory, which is unaffected.) For security reasons, this call is restricted to the superuser. If FILENAME is omitted, does a CWchroot to CW$_.

close FILEHANDLE

close

Closes the file or pipe associated with the file handle, returning true only if IO buffers are successfully flushed and closes the system file descriptor. Closes the currently selected filehandle if the argument is omitted. You don't have to close FILEHANDLE if you are immediately going to do another CWopen on it, because CWopen will close it for you. (See CWopen.) However, an explicit CWclose on an input file resets the line counter (CW$.), while the implicit close done by CWopen does not. If the file handle came from a piped open, CWclose will additionally return false if one of the other system calls involved fails, or if the program exits with non-zero status. (If the only problem was that the program exited non-zero, CW$! will be set to CW0.) Closing a pipe also waits for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards, and implicitly puts the exit status value of that command into CW$?. Prematurely closing the read end of a pipe (i.e. before the process writing to it at the other end has closed it) will result in a SIGPIPE being delivered to the writer. If the other end can't handle that, be sure to read all the data before closing the pipe. Example:

    open(OUTPUT, '|sort >foo')  # pipe to sort
        or die "Can't start sort: $!";
    #...                        # print stuff to output
    close OUTPUT                # wait for sort to finish
        or warn $! ? "Error closing sort pipe: $!"
                   : "Exit status $? from sort";
    open(INPUT, 'foo')          # get sort's results
        or die "Can't open 'foo' for input: $!";

FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the real filehandle name.

closedir DIRHANDLE

Closes a directory opened by CWopendir and returns the success of that system call.

connect SOCKET,NAME

Attempts to connect to a remote socket, just as the connect system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in Sockets: Client/Server Communication in perlipc.

continue BLOCK

Actually a flow control statement rather than a function. If there is a CWcontinue BLOCK attached to a BLOCK (typically in a CWwhile or CWforeach), it is always executed just before the conditional is about to be evaluated again, just like the third part of a CWfor loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the CWnext statement (which is similar to the C CWcontinue statement). CWlast, CWnext, or CWredo may appear within a CWcontinue block. CWlast and CWredo will behave as if they had been executed within the main block. So will CWnext, but since it will execute a CWcontinue block, it may be more entertaining.

    while (EXPR) {
        ### redo always comes here
        do_something;
    } continue {
        ### next always comes here
        do_something_else;
        # then back the top to re-check EXPR
    }
    ### last always comes here

Omitting the CWcontinue section is semantically equivalent to using an empty one, logically enough. In that case, CWnext goes directly back to check the condition at the top of the loop.

cos EXPR

cos

Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes cosine of CW$_. For the inverse cosine operation, you may use the CWMath::Trig::acos() function, or use this relation:

    sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

crypt PLAINTEXT,SALT

Encrypts a string exactly like the crypt(3) function in the C library (assuming that you actually have a version there that has not been extirpated as a potential munition). This can prove useful for checking the password file for lousy passwords, amongst other things. Only the guys wearing white hats should do this. Note that crypt is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) corresponding decrypt function (in other words, the crypt() is a one-way hash function). As a result, this function isn't all that useful for cryptography. (For that, see your nearby CPAN mirror.) When verifying an existing encrypted string you should use the encrypted text as the salt (like CWcrypt($plain, $crypted) eq $crypted). This allows your code to work with the standard crypt and with more exotic implementations. In other words, do not assume anything about the returned string itself, or how many bytes in the encrypted string matter. Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11 bytes from the set CW[./0-9A-Za-z], and only the first eight bytes of the encrypted string mattered, but alternative hashing schemes (like MD5), higher level security schemes (like C2), and implementations on non-UNIX platforms may produce different strings. When choosing a new salt create a random two character string whose characters come from the set CW[./0-9A-Za-z] (like CWjoin '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]). This set of characters is just a recommendation; the characters allowed in the salt depend solely on your system's crypt library, and Perl can't restrict what salts CWcrypt() accepts. Here's an example that makes sure that whoever runs this program knows their own password:

    $pwd = (getpwuid($<))[1];

    system "stty -echo";
    print "Password: ";
    chomp($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $pwd) ne $pwd) {
        die "Sorry...\n";
    } else {
        print "ok\n";
    }

Of course, typing in your own password to whoever asks you for it is unwise. The crypt function is unsuitable for encrypting large quantities of data, not least of all because you can't get the information back. Look at the by-module/Crypt and by-module/PGP directories on your favorite CPAN mirror for a slew of potentially useful modules. If using crypt() on a Unicode string (which potentially has characters with codepoints above 255), Perl tries to make sense of the situation by trying to downgrade (a copy of the string) the string back to an eight-bit byte string before calling crypt() (on that copy). If that works, good. If not, crypt() dies with CWWide character in crypt.

dbmclose HASH

[This function has been largely superseded by the CWuntie function.] Breaks the binding between a DBM file and a hash.

dbmopen HASH,DBNAME,MASK

[This function has been largely superseded by the CWtie function.] This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal CWopen, the first argument is not a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If the database does not exist, it is created with protection specified by MASK (as modified by the CWumask). If your system supports only the older DBM functions, you may perform only one CWdbmopen in your program. In older versions of Perl, if your system had neither DBM nor ndbm, calling CWdbmopen produced a fatal error; it now falls back to sdbm(3). If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an CWeval, which will trap the error. Note that functions such as CWkeys and CWvalues may return huge lists when used on large DBM files. You may prefer to use the CWeach function to iterate over large DBM files. Example:

    # print out history file offsets
    dbmopen(%HIST,'/usr/lib/news/history',0666);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    dbmclose(%HIST);

See also AnyDBM_File for a more general description of the pros and cons of the various dbm approaches, as well as DB_File for a particularly rich implementation. You can control which DBM library you use by loading that library before you call dbmopen():

    use DB_File;
    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
        or die "Can't open netscape history file: $!";

defined EXPR

defined

Returns a Boolean value telling whether EXPR has a value other than the undefined value CWundef. If EXPR is not present, CW$_ will be checked. Many operations return CWundef to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish CWundef from other values. (A simple Boolean test will not distinguish among CWundef, zero, the empty string, and CW"0", which are all equally false.) Note that since CWundef is a valid scalar, its presence doesn't necessarily indicate an exceptional condition: CWpop returns CWundef when its argument is an empty array, or when the element to return happens to be CWundef. You may also use CWdefined(&func) to check whether subroutine CW&func has ever been defined. The return value is unaffected by any forward declarations of CW&func. Note that a subroutine which is not defined may still be callable: its package may have an CWAUTOLOAD method that makes it spring into existence the first time that it is called see perlsub. Use of CWdefined on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate has ever been allocated. This behavior may disappear in future versions of Perl. You should instead use a simple test for size:

    if (@an_array) { print "has array elements\n" }
    if (%a_hash)   { print "has hash members\n"   }

When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use exists for the latter purpose. Examples:

    print if defined $switch{'D'};
    print "$val\n" while defined($val = pop(@ary));
    die "Can't readlink $sym: $!"
        unless defined($value = readlink $sym);
    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
    $debugging = 0 unless defined $debugging;

Note: Many folks tend to overuse CWdefined, and then are surprised to discover that the number CW0 and CW"" (the zero-length string) are, in fact, defined values. For example, if you say

    "ab" =~ /a(.*)b/;

The pattern match succeeds, and CW$1 is defined, despite the fact that it matched nothing. But it didn't really match nothingrather, it matched something that happened to be zero characters long. This is all very above-board and honest. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should use CWdefined only when you're questioning the integrity of what you're trying to do. At other times, a simple comparison to CW0 or CW"" is what you want. See also undef, exists, ref.

delete EXPR

Given an expression that specifies a hash element, array element, hash slice, or array slice, deletes the specified element(s) from the hash or array. In the case of an array, if the array elements happen to be at the end, the size of the array will shrink to the highest element that tests true for exists() (or 0 if no such element exists). Returns a list with the same number of elements as the number of elements for which deletion was attempted. Each element of that list consists of either the value of the element deleted, or the undefined value. In scalar context, this means that you get the value of the last element deleted (or the undefined value if that element did not exist).

    %hash = (foo => 11, bar => 22, baz => 33);
    $scalar = delete $hash{foo};             # $scalar is 11
    $scalar = delete @hash{qw(foo bar)};     # $scalar is 22
    @array  = delete @hash{qw(foo bar baz)}; # @array  is (undef,undef,33)

Deleting from CW%ENV modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. Deleting from a CWtied hash or array may not necessarily return anything. Deleting an array element effectively returns that position of the array to its initial, uninitialized state. Subsequently testing for the same element with exists() will return false. Note that deleting array elements in the middle of an array will not shift the index of the ones after them downuse splice() for that. See exists. The following (inefficiently) deletes all the values of CW%HASH and CW@ARRAY:

    foreach $key (keys %HASH) {
        delete $HASH{$key};
    }

    foreach $index (0 .. $#ARRAY) {
        delete $ARRAY[$index];
    }

And so do these:

    delete @HASH{keys %HASH};

    delete @ARRAY[0 .. $#ARRAY];

But both of these are slower than just assigning the empty list or undefining CW%HASH or CW@ARRAY:

    %HASH = ();         # completely empty %HASH
    undef %HASH;        # forget %HASH ever existed

    @ARRAY = ();        # completely empty @ARRAY
    undef @ARRAY;       # forget @ARRAY ever existed

Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash element, array element, hash slice, or array slice lookup:

    delete $ref->[$x][$y]{$key};
    delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

    delete $ref->[$x][$y][$index];
    delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];

die LIST

Outside an CWeval, prints the value of LIST to CWSTDERR and exits with the current value of CW$! (errno). If CW$! is CW0, exits with the value of CW($? >> 8) (backtick `command` status). If CW($? >> 8) is CW0, exits with CW255. Inside an CWeval(), the error message is stuffed into CW$@ and the CWeval is terminated with the undefined value. This makes CWdie the way to raise an exception. Equivalent examples:

    die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
    chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"

If the last element of LIST does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. Note that the input line number (also known as chunk) is subject to whatever notion of line happens to be currently in effect, and is also available as the special variable CW$.. See $/ in perlvar and $. in perlvar. Hint: sometimes appending CW", stopped" to your message will cause it to make better sense when the string CW"at foo line 123" is appended. Suppose you are running script canasta.

    die "/etc/games is no good";
    die "/etc/games is no good, stopped";

produce, respectively

    /etc/games is no good at canasta line 123.
    /etc/games is no good, stopped at canasta line 123.

See also exit(), warn(), and the Carp module. If LIST is empty and CW$@ already contains a value (typically from a previous eval) that value is reused after appending CW"\t...propagated". This is useful for propagating exceptions:

    eval { ... };
    die unless $@ =~ /Expected exception/;

If LIST is empty and CW$@ contains an object reference that has a CWPROPAGATE method, that method will be called with additional file and line number parameters. The return value replaces the value in CW$@. ie. as if CW$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; were called. If CW$@ is empty then the string CW"Died" is used. die() can also be called with a reference argument. If this happens to be trapped within an eval(), $@ contains the reference. This behavior permits a more elaborate exception handling implementation using objects that maintain arbitrary state about the nature of the exception. Such a scheme is sometimes preferable to matching particular string values of $@ using regular expressions. Here's an example:

    eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
    if ($@) {
        if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
            # handle Some::Module::Exception
        }
        else {
            # handle all other possible exceptions
        }
    }

Because perl will stringify uncaught exception messages before displaying them, you may want to overload stringification operations on such custom exception objects. See overload for details about that. You can arrange for a callback to be run just before the CWdie does its deed, by setting the CW$SIG{__DIE__} hook. The associated handler will be called with the error text and can change the error message, if it sees fit, by calling CWdie again. See $SIG{expr} in perlvar for details on setting CW%SIG entries, and eval BLOCK for some examples. Although this feature was meant to be run only right before your program was to exit, this is not currently the casethe CW$SIG{__DIE__} hook is currently called even inside eval()ed blocks/strings! If one wants the hook to do nothing in such situations, put

        die @_ if $^S;

as the first line of the handler (see $^S in perlvar). Because this promotes strange action at a distance, this counterintuitive behavior may be fixed in a future release.

do BLOCK

Not really a function. Returns the value of the last command in the sequence of commands indicated by BLOCK. When modified by a loop modifier, executes the BLOCK once before testing the loop condition. (On other statements the loop modifiers test the conditional first.) CWdo BLOCK does not count as a loop, so the loop control statements CWnext, CWlast, or CWredo cannot be used to leave or restart the block. See perlsyn for alternative strategies.

do SUBROUTINE(LIST)

A deprecated form of subroutine call. See perlsub.

do EXPR

Uses the value of EXPR as a filename and executes the contents of the file as a Perl script.

    do 'stat.pl';

is just like

    eval `cat stat.pl`;

except that it's more efficient and concise, keeps track of the current filename for error messages, searches the CW@INC directories, and updates CW%INC if the file is found. See Predefined Names in perlvar for these variables. It also differs in that code evaluated with CWdo FILENAME cannot see lexicals in the enclosing scope; CWeval STRING does. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop. If CWdo cannot read the file, it returns undef and sets CW$! to the error. If CWdo can read the file but cannot compile it, it returns undef and sets an error message in CW$@. If the file is successfully compiled, CWdo returns the value of the last expression evaluated. Note that inclusion of library modules is better done with the CWuse and CWrequire operators, which also do automatic error checking and raise an exception if there's a problem. You might like to use CWdo to read in a program configuration file. Manual error checking can be done this way:

    # read in config files: system first, then user
    for $file ("/share/prog/defaults.rc",
               "$ENV{HOME}/.someprogrc")
   {
        unless ($return = do $file) {
            warn "couldn't parse $file: $@" if $@;
            warn "couldn't do $file: $!"    unless defined $return;
            warn "couldn't run $file"       unless $return;
        }
    }

dump LABEL

dump

This function causes an immediate core dump. See also the -u command-line switch in perlrun, which does the same thing. Primarily this is so that you can use the undump program (not supplied) to turn your core dump into an executable binary after having initialized all your variables at the beginning of the program. When the new binary is executed it will begin by executing a CWgoto LABEL (with all the restrictions that CWgoto suffers). Think of it as a goto with an intervening core dump and reincarnation. If CWLABEL is omitted, restarts the program from the top. WARNING: Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting confusion on the part of Perl. This function is now largely obsolete, partly because it's very hard to convert a core file into an executable, and because the real compiler backends for generating portable bytecode and compilable C code have superseded it. That's why you should now invoke it as CWCORE::dump(), if you don't want to be warned against a possible typo. If you're looking to use dump to speed up your program, consider generating bytecode or native C code as described in perlcc. If you're just trying to accelerate a CGI script, consider using the CWmod_perl extension to Apache, or the CPAN module, CGI::Fast. You might also consider autoloading or selfloading, which at least make your program appear to run faster.

each HASH

When called in list context, returns a 2-element list consisting of the key and value for the next element of a hash, so that you can iterate over it. When called in scalar context, returns only the key for the next element in the hash. Entries are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be in the same order as either the CWkeys or CWvalues function would produce on the same (unmodified) hash. Since Perl 5.8.1 the ordering is different even between different runs of Perl for security reasons (see Algorithmic Complexity Attacks in perlsec). When the hash is entirely read, a null array is returned in list context (which when assigned produces a false (CW0) value), and CWundef in scalar context. The next call to CWeach after that will start iterating again. There is a single iterator for each hash, shared by all CWeach, CWkeys, and CWvalues function calls in the program; it can be reset by reading all the elements from the hash, or by evaluating CWkeys HASH or CWvalues HASH. If you add or delete elements of a hash while you're iterating over it, you may get entries skipped or duplicated, so don't. Exception: It is always safe to delete the item most recently returned by CWeach(), which means that the following code will work:

        while (($key, $value) = each %hash) {
          print $key, "\n";
          delete $hash{$key};   # This is safe
        }

The following prints out your environment like the printenv(1) program, only in a different order:

    while (($key,$value) = each %ENV) {
        print "$key=$value\n";
    }

See also exp for the inverse operation.

lstat EXPR

lstat

Does the same thing as the CWstat function (including setting the special CW_ filehandle) but stats a symbolic link instead of the file the symbolic link points to. If symbolic links are unimplemented on your system, a normal CWstat is done. For much more detailed information, please see the documentation for stat. If EXPR is omitted, stats CW$_.

m//

The match operator. See perlop.

map BLOCK LIST

map EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting CW$_ to each element) and returns the list value composed of the results of each such evaluation. In scalar context, returns the total number of elements so generated. Evaluates BLOCK or EXPR in list context, so each element of LIST may produce zero, one, or more elements in the returned value.

    @chars = map(chr, @nums);

translates a list of numbers to the corresponding characters. And

    %hash = map { getkey($_) => $_ } @array;

is just a funny way to write

    %hash = ();
    foreach $_ (@array) {
        $hash{getkey($_)} = $_;
    }

Note that CW$_ is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Using a regular CWforeach loop for this purpose would be clearer in most cases. See also grep for an array composed of those items of the original list for which the BLOCK or EXPR evaluates to true. CW{ starts both hash references and blocks, so CWmap { ... could be either the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look ahead for the closing CW} it has to take a guess at which its dealing with based what it finds just after the CW{. Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the CW} and encounters the missing (or unexpected) comma. The syntax error will be reported close to the CW} but you'll need to change something near the CW{ such as using a unary CW+ to give perl some help:

    %hash = map {  "\L$_", 1  } @array  # perl guesses EXPR.  wrong
    %hash = map { +"\L$_", 1  } @array  # perl guesses BLOCK. right
    %hash = map { ("\L$_", 1) } @array  # this also works
    %hash = map {  lc($_), 1  } @array  # as does this.
    %hash = map +( lc($_), 1 ), @array  # this is EXPR and works!

    %hash = map  ( lc($_), 1 ), @array  # evaluates to (1, @array)

or to force an anon hash constructor use CW+{

   @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end

and you get list of anonymous hashes each with only 1 entry.

mkdir FILENAME,MASK

mkdir FILENAME

Creates the directory specified by FILENAME, with permissions specified by MASK (as modified by CWumask). If it succeeds it returns true, otherwise it returns false and sets CW$! (errno). If omitted, MASK defaults to 0777. In general, it is better to create directories with permissive MASK, and let the user modify that with their CWumask, than it is to supply a restrictive MASK and give the user no way to be more permissive. The exceptions to this rule are when the file or directory should be kept private (mail files, for instance). The perlfunc(1) entry on CWumask discusses the choice of MASK in more detail. Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing slashes. Some operating and filesystems do not get this right, so Perl automatically removes all trailing slashes to keep everyone happy.

msgctl ID,CMD,ARG

Calls the System V IPC function msgctl(2). You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions. If CMD is CWIPC_STAT, then ARG must be a variable which will hold the returned CWmsqid_ds structure. Returns like CWioctl: the undefined value for error, CW"0 but true" for zero, or the actual return value otherwise. See also SysV IPC in perlipc, CWIPC::SysV, and CWIPC::Semaphore documentation.

msgget KEY,FLAGS

Calls the System V IPC function msgget(2). Returns the message queue id, or the undefined value if there is an error. See also SysV IPC in perlipc and CWIPC::SysV and CWIPC::Msg documentation.

msgrcv ID,VAR,SIZE,TYPE,FLAGS

Calls the System V IPC function msgrcv to receive a message from message queue ID into variable VAR with a maximum message size of SIZE. Note that when a message is received, the message type as a native long integer will be the first thing in VAR, followed by the actual message. This packing may be opened with CWunpack("l! a*"). Taints the variable. Returns true if successful, or false if there is an error. See also SysV IPC in perlipc, CWIPC::SysV, and CWIPC::SysV::Msg documentation.

msgsnd ID,MSG,FLAGS

Calls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG must begin with the native long integer message type, and be followed by the length of the actual message, and finally the message itself. This kind of packing can be achieved with CWpack("l! a*", $type, $message). Returns true if successful, or false if there is an error. See also CWIPC::SysV and CWIPC::SysV::Msg documentation.

my EXPR

my TYPE EXPR

my EXPR : ATTRS

my TYPE EXPR : ATTRS

A CWmy declares the listed variables to be local (lexically) to the enclosing block, file, or CWeval. If more than one value is listed, the list must be placed in parentheses. The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of CWfields pragma, and attributes are handled using the CWattributes pragma, or starting from Perl 5.8.0 also via the CWAttribute::Handlers module. See Private Variables via my() in perlsub for details, and fields, attributes, and Attribute::Handlers.

next LABEL

next

The CWnext command is like the CWcontinue statement in C; it starts the next iteration of the loop:

    LINE: while (<STDIN>) {
        next LINE if /^#/;      # discard comments
        #...
    }

Note that if there were a CWcontinue block on the above, it would get executed even on discarded lines. If the LABEL is omitted, the command refers to the innermost enclosing loop. CWnext cannot be used to exit a block which returns a value such as CWeval {}, CWsub {} or CWdo {}, and should not be used to exit a grep() or map() operation. Note that a block by itself is semantically identical to a loop that executes once. Thus CWnext will exit such a block early. See also continue for an illustration of how CWlast, CWnext, and CWredo work.

no Module VERSION LIST

no Module VERSION

no Module LIST

no Module

See the CWuse function, which CWno is the opposite of.

oct EXPR

oct

Interprets EXPR as an octal string and returns the corresponding value. (If EXPR happens to start off with CW0x, interprets it as a hex string. If EXPR starts off with CW0b, it is interpreted as a binary string. Leading whitespace is ignored in all three cases.) The following will handle decimal, binary, octal, and hex in the standard Perl or C notation:

    $val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses CW$_. To go the other way (produce a number in octal), use sprintf() or printf():

    $perms = (stat("filename"))[2] & 07777;
    $oct_perms = sprintf "%lo", $perms;

The oct() function is commonly used when a string such as CW644 needs to be converted into a file mode, for example. (Although perl will automatically convert strings into numbers as needed, this automatic conversion assumes base 10.)

open FILEHANDLE,EXPR

open FILEHANDLE,MODE,EXPR

open FILEHANDLE,MODE,EXPR,LIST

open FILEHANDLE,MODE,REFERENCE

open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE. (The following is a comprehensive reference to open(): for a gentler introduction you may consider perlopentut.) If FILEHANDLE is an undefined scalar variable (or array or hash element) the variable is assigned a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. (This is considered a symbolic reference, so CWuse strict 'refs' should not be in effect.) If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. (Note that lexical variablesthose declared with CWmy--will not work for this purpose; so if you're using CWmy, specify EXPR in your call to open.) If three or more arguments are specified then the mode of opening and the file name are separate. If MODE is CW'<' or nothing, the file is opened for input. If MODE is CW'>', the file is truncated and opened for output, being created if necessary. If MODE is CW'>>', the file is opened for appending, again being created if necessary. You can put a CW'+' in front of the CW'>' or CW'<' to indicate that you want both read and write access to the file; thus CW'+<' is almost always preferred for read/write updatesthe CW'+>' mode would clobber the file first. You can't usually use either read-write mode for updating textfiles, since they have variable length records. See the -i switch in perlrun for a better approach. The file is created with permissions of CW0666 modified by the process' CWumask value. These various prefixes correspond to the fopen(3) modes of CW'r', CW'r+', CW'w', CW'w+', CW'a', and CW'a+'. In the 2-arguments (and 1-argument) form of the call the mode and filename should be concatenated (in this order), possibly separated by spaces. It is possible to omit the mode in these forms if the mode is CW'<'. If the filename begins with CW'|', the filename is interpreted as a command to which output is to be piped, and if the filename ends with a CW'|', the filename is interpreted as a command which pipes output to us. See Using open() for IPC in perlipc for more examples of this. (You are not allowed to CWopen to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and Bidirectional Communication with Another Process in perlipc for alternatives.) For three or more arguments if MODE is CW'|-', the filename is interpreted as a command to which output is to be piped, and if MODE is CW'-|', the filename is interpreted as a command which pipes output to us. In the 2-arguments (and 1-argument) form one should replace dash (CW'-') with the command. See Using open() for IPC in perlipc for more examples of this. (You are not allowed to CWopen to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and Bidirectional Communication in perlipc for alternatives.) In the three-or-more argument form of pipe opens, if LIST is specified (extra arguments after the command name) then LIST becomes arguments to the command invoked if the platform supports it. The meaning of CWopen with more than three arguments for non-pipe modes is not yet specified. Experimental layers may give extra LIST arguments meaning. In the 2-arguments (and 1-argument) form opening CW'-' opens STDIN and opening CW'>-' opens STDOUT. You may use the three-argument form of open to specify IO layers (sometimes also referred to as disciplines) to be applied to the handle that affect how the input and output are processed (see open and PerlIO for more details). For example

  open(FH, "<:utf8", "file")

will open the UTF-8 encoded file containing Unicode characters, see perluniintro. (Note that if layers are specified in the three-arg form then default layers set by the CWopen pragma are ignored.) Open returns nonzero upon success, the undefined value otherwise. If the CWopen involved a pipe, the return value happens to be the pid of the subprocess. If you're running Perl on a system that distinguishes between text files and binary files, then you should check out binmode for tips for dealing with this. The key distinction between systems that need CWbinmode and those that don't is their text file formats. Systems like Unix, Mac OS, and Plan 9, which delimit lines with a single character, and which encode that character in C as CW"\n", do not need CWbinmode. The rest need it. When opening a file, it's usually a bad idea to continue normal execution if the request failed, so CWopen is frequently used in connection with CWdie. Even if CWdie won't do what you want (say, in a CGI script, where you want to make a nicely formatted error message (but there are modules that can help with that problem)) you should always check the return value from opening a file. The infrequent exception is when working with an unopened filehandle is actually what you want to do. As a special case the 3 arg form with a read/write mode and the third argument being CWundef:

    open(TMP, "+>", undef) or die ...

opens a filehandle to an anonymous temporary file. Also using +< works for symmetry, but you really should consider writing something to the temporary file first. You will need to seek() to do the reading. Since v5.8.0, perl has built using PerlIO by default. Unless you've changed this (ie Configure -Uuseperlio), you can open file handles to in memory files held in Perl scalars via:

    open($fh, '>', \$variable) || ..

Though if you try to re-open CWSTDOUT or CWSTDERR as an in memory file, you have to close it first:

    close STDOUT;
    open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";

Examples:

    $ARTICLE = 100;
    open ARTICLE or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...

    open(LOG, '>>/usr/spool/news/twitlog');     # (log is reserved)
    # if the open fails, output is discarded

    open(DBASE, '+<', 'dbase.mine')             # open for update
        or die "Can't open 'dbase.mine' for update: $!";

    open(DBASE, '+<dbase.mine')                 # ditto
        or die "Can't open 'dbase.mine' for update: $!";

    open(ARTICLE, '-|', "caesar <$article")     # decrypt article
        or die "Can't start caesar: $!";

    open(ARTICLE, "caesar <$article |")         # ditto
        or die "Can't start caesar: $!";

    open(EXTRACT, "|sort >Tmp$$")               # $$ is our process id
        or die "Can't start sort: $!";

    # in memory files
    open(MEMORY,'>', \$var)
        or die "Can't open memory file: $!";
    print MEMORY "foo!\n";                      # output will end up in $var

    # process argument list of files along with any includes

    foreach $file (@ARGV) {
        process($file, 'fh00');
    }

    sub process {
        my($filename, $input) = @_;
        $input++;               # this is a string increment
        unless (open($input, $filename)) {
            print STDERR "Can't open $filename: $!\n";
            return;
        }

        local $_;
        while (<$input>) {              # note use of indirection
            if (/^#include "(.*)"/) {
                process($1, $input);
                next;
            }
            #...                # whatever
        }
    }

See perliol for detailed info on PerlIO. You may also, in the Bourne shell tradition, specify an EXPR beginning with CW'>&', in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) to be duped (as dup(2)) and opened. You may use CW& after CW>, CW>>, CW<, CW+>, CW+>>, and CW+<. The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of IO buffers.) If you use the 3 arg form then you can pass either a number, the name of a filehandle or the normal reference to a glob. Here is a script that saves, redirects, and restores CWSTDOUT and CWSTDERR using various methods:

    #!/usr/bin/perl
    open my $oldout, ">&STDOUT"     or die "Can't dup STDOUT: $!";
    open OLDERR,     ">&", \*STDERR or die "Can't dup STDERR: $!";

    open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
    open STDERR, ">&STDOUT"     or die "Can't dup STDOUT: $!";

    select STDERR; $| = 1;      # make unbuffered
    select STDOUT; $| = 1;      # make unbuffered

    print STDOUT "stdout 1\n";  # this works for
    print STDERR "stderr 1\n";  # subprocesses too

    open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
    open STDERR, ">&OLDERR"    or die "Can't dup OLDERR: $!";

    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";

If you specify CW'<&=X', where CWX is a file descriptor number or a filehandle, then Perl will do an equivalent of C's CWfdopen of that file descriptor (and not call dup(2)); this is more parsimonious of file descriptors. For example:

    # open for input, reusing the fileno of $fd
    open(FILEHANDLE, "<&=$fd")

    open(FILEHANDLE, "<&=", $fd)

    # open for append, using the fileno of OLDFH
    open(FH, ">>&=", OLDFH)

    open(FH, ">>&=OLDFH")

Being parsimonious on filehandles is also useful (besides being parsimonious) for example when something is dependent on file descriptors, like for example locking using flock(). If you do just CWopen(A, '>>&B'), the filehandle A will not have the same file descriptor as B, and therefore flock(A) will not flock(B), and vice versa. But with CWopen(A, '>>&=B') the filehandles will share the same file descriptor. Note that if you are using Perls older than 5.8.0, Perl will be using the standard C libraries' fdopen() to implement the = functionality. On many UNIX systems fdopen() fails when file descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is most often the default. You can see whether Perl has been compiled with PerlIO or not by running CWperl -V and looking for CWuseperlio= line. If CWuseperlio is CWdefine, you have PerlIO, otherwise you don't. If you open a pipe on the command CW'-', i.e., either CW'|-' or CW'-|' with 2-arguments (or 1-argument) form of open(), then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and CW0 within the child process. (Use CWdefined($pid) to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't openedi/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following triples are more or less equivalent:

    open(FOO, "|tr '[a-z]' '[A-Z]'");
    open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
    open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
    open(FOO, '|-', "tr", '[a-z]', '[A-Z]');

    open(FOO, "cat -n '$file'|");
    open(FOO, '-|', "cat -n '$file'");
    open(FOO, '-|') || exec 'cat', '-n', $file;
    open(FOO, '-|', "cat", '-n', $file);

The last example in each block shows the pipe as list form, which is not yet supported on all platforms. A good rule of thumb is that if your platform has true CWfork() (in other words, if your platform is UNIX) you can use the list form. See Safe Pipe Opens in perlipc for more examples of this. Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set CW$| ($AUTOFLUSH in English) or call the CWautoflush() method of CWIO::Handle on any open handles. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of $^F. See $^F in perlvar. Closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in CW$?. The filename passed to 2-argument (or 1-argument) form of open() will have leading and trailing whitespace deleted, and the normal redirection characters honored. This property, known as magic open, can often be used to good effect. A user could specify a filename of rsh cat file |, or you could change certain filenames as needed:

    $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
    open(FH, $filename) or die "Can't open $filename: $!";

Use 3-argument form to open a file with arbitrary weird characters in it,

    open(FOO, '<', $file);

otherwise it's necessary to protect any leading and trailing whitespace:

    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0");

(this may not work on some bizarre filesystems). One should conscientiously choose between the magic and 3-arguments form of open():

    open IN, $ARGV[0];

will allow the user to specify an argument of the form CW"rsh cat file |", but will not work on a filename which happens to have a trailing space, while

    open IN, '<', $ARGV[0];

will have exactly the opposite restrictions. If you want a real C CWopen (see open(2) on your system), then you should use the CWsysopen function, which involves no such magic (but may use subtly different filemodes than Perl open(), which is mapped to C fopen()). This is another way to protect your filenames from interpretation. For example:

    use IO::Handle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
        or die "sysopen $path: $!";
    $oldfh = select(HANDLE); $| = 1; select($oldfh);
    print HANDLE "stuff $$\n";
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;

Using the constructor from the CWIO::Handle package (or one of its subclasses, such as CWIO::File or CWIO::Socket), you can generate anonymous filehandles that have the scope of whatever variables hold references to them, and automatically close whenever and however you leave that scope:

    use IO::File;
    #...
    sub read_myfile_munged {
        my $ALL = shift;
        my $handle = new IO::File;
        open($handle, "myfile") or die "myfile: $!";
        $first = <$handle>
            or return ();     # Automatically closed here.
        mung $first or die "mung failed";       # Or here.
        return $first, <$handle> if $ALL;       # Or here.
        $first;                                 # Or here.
    }

See seek for some details about mixing reading and writing.

opendir DIRHANDLE,EXPR

Opens a directory named EXPR for processing by CWreaddir, CWtelldir, CWseekdir, CWrewinddir, and CWclosedir. Returns true if successful. DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined scalar variable (or array or hash element), the variable is assigned a reference to a new anonymous dirhandle. DIRHANDLEs have their own namespace separate from FILEHANDLEs.

ord EXPR

ord

Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC, or Unicode) value of the first character of EXPR. If EXPR is omitted, uses CW$_. For the reverse, see chr. See perlunicode and encoding for more about Unicode.

our EXPR

our EXPR TYPE

our EXPR : ATTRS

our TYPE EXPR : ATTRS

An CWour declares the listed variables to be valid globals within the enclosing block, file, or CWeval. That is, it has the same scoping rules as a my declaration, but does not create a local variable. If more than one value is listed, the list must be placed in parentheses. The CWour declaration has no semantic effect unless use strict vars is in effect, in which case it lets you use the declared global variable without qualifying it with a package name. (But only within the lexical scope of the CWour declaration. In this it differs from use vars, which is package scoped.) An CWour declaration declares a global variable that will be visible across its entire lexical scope, even across package boundaries. The package in which the variable is entered is determined at the point of the declaration, not at the point of use. This means the following behavior holds:

    package Foo;
    our $bar;           # declares $Foo::bar for rest of lexical scope
    $bar = 20;

    package Bar;
    print $bar;         # prints 20

Multiple CWour declarations in the same lexical scope are allowed if they are in different packages. If they happened to be in the same package, Perl will emit warnings if you have asked for them.

    use warnings;
    package Foo;
    our $bar;           # declares $Foo::bar for rest of lexical scope
    $bar = 20;

    package Bar;
    our $bar = 30;      # declares $Bar::bar for rest of lexical scope
    print $bar;         # prints 30

    our $bar;           # emits warning

An CWour declaration may also have a list of attributes associated with it. The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of CWfields pragma, and attributes are handled using the CWattributes pragma, or starting from Perl 5.8.0 also via the CWAttribute::Handlers module. See Private Variables via my() in perlsub for details, and fields, attributes, and Attribute::Handlers. The only currently recognized CWour() attribute is CWunique which indicates that a single copy of the global is to be used by all interpreters should the program happen to be running in a multi-interpreter environment. (The default behaviour would be for each interpreter to have its own copy of the global.) Examples:

    our @EXPORT : unique = qw(foo);
    our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
    our $VERSION : unique = "1.00";

Note that this attribute also has the effect of making the global readonly when the first new interpreter is cloned (for example, when the first new thread is created). Multi-interpreter environments can come to being either through the fork() emulation on Windows platforms, or by embedding perl in a multi-threaded application. The CWunique attribute does nothing in all other environments. Warning: the current implementation of this attribute operates on the typeglob associated with the variable; this means that CWour $x : unique also has the effect of CWour @x : unique; our %x : unique. This may be subject to change.

pack TEMPLATE,LIST

Takes a LIST of values and converts it into a string using the rules given by the TEMPLATE. The resulting string is the concatenation of the converted values. Typically, each converted value looks like its machine-level representation. For example, on 32-bit machines a converted integer may be represented by a sequence of 4 bytes. The TEMPLATE is a sequence of characters that give the order and type of values, as follows:

    a   A string with arbitrary binary data, will be null padded.
    A   A text (ASCII) string, will be space padded.
    Z   A null terminated (ASCIZ) string, will be null padded.

    b   A bit string (ascending bit order inside each byte, like vec()).
    B   A bit string (descending bit order inside each byte).
    h   A hex string (low nybble first).
    H   A hex string (high nybble first).

    c   A signed char value.
    C   An unsigned char value.  Only does bytes.  See U for Unicode.

    s   A signed short value.
    S   An unsigned short value.
          (This 'short' is _exactly_ 16 bits, which may differ from
           what a local C compiler calls 'short'.  If you want
           native-length shorts, use the '!' suffix.)

    i   A signed integer value.
    I   An unsigned integer value.
          (This 'integer' is _at_least_ 32 bits wide.  Its exact
           size depends on what a local C compiler calls 'int',
           and may even be larger than the 'long' described in
           the next item.)

    l   A signed long value.
    L   An unsigned long value.
          (This 'long' is _exactly_ 32 bits, which may differ from
           what a local C compiler calls 'long'.  If you want
           native-length longs, use the '!' suffix.)

    n   An unsigned short in "network" (big-endian) order.
    N   An unsigned long in "network" (big-endian) order.
    v   An unsigned short in "VAX" (little-endian) order.
    V   An unsigned long in "VAX" (little-endian) order.
          (These 'shorts' and 'longs' are _exactly_ 16 bits and
           _exactly_ 32 bits, respectively.)

    q   A signed quad (64-bit) value.
    Q   An unsigned quad value.
          (Quads are available only if your system supports 64-bit
           integer values _and_ if Perl has been compiled to support those.
           Causes a fatal error otherwise.)

    j   A signed integer value (a Perl internal integer, IV).
    J   An unsigned integer value (a Perl internal unsigned integer, UV).

    f   A single-precision float in the native format.
    d   A double-precision float in the native format.

    F   A floating point value in the native native format
           (a Perl internal floating point value, NV).
    D   A long double-precision float in the native format.
          (Long doubles are available only if your system supports long
           double values _and_ if Perl has been compiled to support those.
           Causes a fatal error otherwise.)

    p   A pointer to a null-terminated string.
    P   A pointer to a structure (fixed-length string).

    u   A uuencoded string.
    U   A Unicode character number.  Encodes to UTF-8 internally
        (or UTF-EBCDIC in EBCDIC platforms).

    w   A BER compressed integer (not an ASN.1 BER, see perlpacktut for
        details).  Its bytes represent an unsigned integer in base 128,
        most significant digit first, with as few digits as possible.  Bit
        eight (the high bit) is set on each byte except the last.

    x   A null byte.
    X   Back up a byte.
    @   Null fill to absolute position, counted from the start of
        the innermost ()-group.
    (   Start of a ()-group.

The following rules apply:

*

Each letter may optionally be followed by a number giving a repeat count. With all types except CWa, CWA, CWZ, CWb, CWB, CWh, CWH, CW@, CWx, CWX and CWP the pack function will gobble up that many values from the LIST. A CW* for the repeat count means to use however many items are left, except for CW@, CWx, CWX, where it is equivalent to CW0, and CWu, where it is equivalent to 1 (or 45, what is the same). A numeric repeat count may optionally be enclosed in brackets, as in CWpack 'C[80]', @arr. One can replace the numeric repeat count by a template enclosed in brackets; then the packed length of this template in bytes is used as a count. For example, CWx[L] skips a long (it skips the number of bytes in a long); the template CW$t X[$t] $t unpack()s twice what CW$t unpacks. If the template in brackets contains alignment commands (such as CWx![d]), its packed length is calculated as if the start of the template has the maximal possible alignment. When used with CWZ, CW* results in the addition of a trailing null byte (so the packed result will be one longer than the byte CWlength of the item). The repeat count for CWu is interpreted as the maximal number of bytes to encode per line of output, with 0 and 1 replaced by 45.

*

The CWa, CWA, and CWZ types gobble just one value, but pack it as a string of length count, padding with nulls or spaces as necessary. When unpacking, CWA strips trailing spaces and nulls, CWZ strips everything after the first null, and CWa returns data verbatim. When packing, CWa, and CWZ are equivalent. If the value-to-pack is too long, it is truncated. If too long and an explicit count is provided, CWZ packs only CW$count-1 bytes, followed by a null byte. Thus CWZ always packs a trailing null byte under all circumstances.

*

Likewise, the CWb and CWB fields pack a string that many bits long. Each byte of the input field of pack() generates 1 bit of the result. Each result bit is based on the least-significant bit of the corresponding input byte, i.e., on CWord($byte)%2. In particular, bytes CW"0" and CW"1" generate bits 0 and 1, as do bytes CW"\0" and CW"\1". Starting from the beginning of the input string of pack(), each 8-tuple of bytes is converted to 1 byte of output. With format CWb the first byte of the 8-tuple determines the least-significant bit of a byte, and with format CWB it determines the most-significant bit of a byte. If the length of the input string is not exactly divisible by 8, the remainder is packed as if the input string were padded by null bytes at the end. Similarly, during unpack()ing the extra bits are ignored. If the input string of pack() is longer than needed, extra bytes are ignored. A CW* for the repeat count of pack() means to use all the bytes of the input field. On unpack()ing the bits are converted to a string of CW"0"s and CW"1"s.

*

The CWh and CWH fields pack a string that many nybbles (4-bit groups, representable as hexadecimal digits, 0-9a-f) long. Each byte of the input field of pack() generates 4 bits of the result. For non-alphabetical bytes the result is based on the 4 least-significant bits of the input byte, i.e., on CWord($byte)%16. In particular, bytes CW"0" and CW"1" generate nybbles 0 and 1, as do bytes CW"\0" and CW"\1". For bytes CW"a".."f" and CW"A".."F" the result is compatible with the usual hexadecimal digits, so that CW"a" and CW"A" both generate the nybble CW0xa==10. The result for bytes CW"g".."z" and CW"G".."Z" is not well-defined. Starting from the beginning of the input string of pack(), each pair of bytes is converted to 1 byte of output. With format CWh the first byte of the pair determines the least-significant nybble of the output byte, and with format CWH it determines the most-significant nybble. If the length of the input string is not even, it behaves as if padded by a null byte at the end. Similarly, during unpack()ing the extra nybbles are ignored. If the input string of pack() is longer than needed, extra bytes are ignored. A CW* for the repeat count of pack() means to use all the bytes of the input field. On unpack()ing the bits are converted to a string of hexadecimal digits.

*

The CWp type packs a pointer to a null-terminated string. You are responsible for ensuring the string is not a temporary value (which can potentially get deallocated before you get around to using the packed result). The CWP type packs a pointer to a structure of the size indicated by the length. A NULL pointer is created if the corresponding value for CWp or CWP is CWundef, similarly for unpack().

*

The CW/ template character allows packing and unpacking of strings where the packed structure contains a byte count followed by the string itself. You write length-itemCW/string-item. The length-item can be any CWpack template letter, and describes how the length value is packed. The ones likely to be of most use are integer-packing ones like CWn (for Java strings), CWw (for ASN.1 or SNMP) and CWN (for Sun XDR). For CWpack, the string-item must, at present, be CW"A*", CW"a*" or CW"Z*". For CWunpack the length of the string is obtained from the length-item, but if you put in the '*' it will be ignored. For all other codes, CWunpack applies the length value to the next item, which must not have a repeat count.

    unpack 'C/a', "\04Gurusamy";        gives 'Guru'
    unpack 'a3/A* A*', '007 Bond  J ';  gives (' Bond','J')
    pack 'n/a* w/a*','hello,','world';  gives "\000\006hello,\005world"

The length-item is not returned explicitly from CWunpack. Adding a count to the length-item letter is unlikely to do anything useful, unless that letter is CWA, CWa or CWZ. Packing with a length-item of CWa or CWZ may introduce CW"\000" characters, which Perl does not regard as legal in numeric strings.

*

The integer types CWs, CWS, CWl, and CWL may be immediately followed by a CW! suffix to signify native shorts or longsas you can see from above for example a bare CWl does mean exactly 32 bits, the native CWlong (as seen by the local C compiler) may be larger. This is an issue mainly in 64-bit platforms. You can see whether using CW! makes any difference by

        print length(pack("s")), " ", length(pack("s!")), "\n";
        print length(pack("l")), " ", length(pack("l!")), "\n";

CWi! and CWI! also work but only because of completeness; they are identical to CWi and CWI. The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform where Perl was built are also available via Config:

       use Config;
       print $Config{shortsize},    "\n";
       print $Config{intsize},      "\n";
       print $Config{longsize},     "\n";
       print $Config{longlongsize}, "\n";

(The CW$Config{longlongsize} will be undefined if your system does not support long longs.)

*

The integer formats CWs, CWS, CWi, CWI, CWl, CWL, CWj, and CWJ are inherently non-portable between processors and operating systems because they obey the native byteorder and endianness. For example a 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively (arranged in and handled by the CPU registers) into bytes as

        0x12 0x34 0x56 0x78     # big-endian
        0x78 0x56 0x34 0x12     # little-endian

Basically, the Intel and VAX CPUs are little-endian, while everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode. The names `big-endian' and `little-endian' are comic references to the classic Gulliver's Travels (via the paper On Holy Wars and a Plea for Peace by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and the egg-eating habits of the Lilliputians. Some systems may have even weirder byte orders such as

        0x56 0x78 0x12 0x34
        0x34 0x12 0x78 0x56

You can see your system's preference with

        print join(" ", map { sprintf "%#02x", $_ }
                            unpack("C*",pack("L",0x12345678))), "\n";

The byteorder on the platform where Perl was built is also available via Config:

        use Config;
        print $Config{byteorder}, "\n";

Byteorders CW'1234' and CW'12345678' are little-endian, CW'4321' and CW'87654321' are big-endian. If you want portable packed integers use the formats CWn, CWN, CWv, and CWV, their byte endianness and size are known. See also perlport.

*

Real numbers (floats and doubles) are in the native machine format only; due to the multiplicity of floating formats around, and the lack of a standard network representation, no facility for interchange has been made. This means that packed floating point data written on one machine may not be readable on another - even if both use IEEE floating point arithmetic (as the endian-ness of the memory representation is not part of the IEEE spec). See also perlport. Note that Perl uses doubles internally for all numeric calculation, and converting from double into float and thence back to double again will lose precision (i.e., CWunpack("f", pack("f", $foo)) will not in general equal CW$foo).

*

If the pattern begins with a CWU, the resulting string will be treated as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string with an initial CWU0, and the bytes that follow will be interpreted as Unicode characters. If you don't want this to happen, you can begin your pattern with CWC0 (or anything else) to force Perl not to UTF-8 encode your string, and then follow this with a CWU* somewhere in your pattern.

*

You must yourself do any alignment or padding by inserting for example enough CW'x'es while packing. There is no way to pack() and unpack() could know where the bytes are going to or coming from. Therefore CWpack (and CWunpack) handle their output and input as flat sequences of bytes.

*

A ()-group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat count, both as postfix, and for unpack() also via the CW/ template character. Within each repetition of a group, positioning with CW@ starts again at 0. Therefore, the result of

    pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )

is the string \0a\0\0bc.

*

CWx and CWX accept CW! modifier. In this case they act as alignment commands: they jump forward/back to the closest position aligned at a multiple of CWcount bytes. For example, to pack() or unpack() C's CWstruct {char c; double d; char cc[2]} one may need to use the template CWC x![d] d C[2]; this assumes that doubles must be aligned on the double's size. For alignment commands CWcount of 0 is equivalent to CWcount of 1; both result in no-ops.

*

A comment in a TEMPLATE starts with CW# and goes to the end of line. White space may be used to separate pack codes from each other, but a CW! modifier and a repeat count must follow immediately.

*

If TEMPLATE requires more arguments to pack() than actually given, pack() assumes additional CW"" arguments. If TEMPLATE requires less arguments to pack() than actually given, extra arguments are ignored. Examples:

    $foo = pack("CCCC",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("C4",65,66,67,68);
    # same thing
    $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
    # same thing with Unicode circled letters

    $foo = pack("ccxxcc",65,66,67,68);
    # foo eq "AB\0\0CD"

    # note: the above examples featuring "C" and "c" are true
    # only on ASCII and ASCII-derived systems such as ISO Latin 1
    # and UTF-8.  In EBCDIC the first example would be
    # $foo = pack("CCCC",193,194,195,196);

    $foo = pack("s2",1,2);
    # "\1\0\2\0" on little-endian
    # "\0\1\0\2" on big-endian

    $foo = pack("a4","abcd","x","y","z");
    # "abcd"

    $foo = pack("aaaa","abcd","x","y","z");
    # "axyz"

    $foo = pack("a14","abcdefg");
    # "abcdefg\0\0\0\0\0\0\0"

    $foo = pack("i9pl", gmtime);
    # a real struct tm (on my system anyway)

    $utmp_template = "Z8 Z8 Z16 L";
    $utmp = pack($utmp_template, @utmp1);
    # a struct utmp (BSDish)

    @utmp2 = unpack($utmp_template, $utmp);
    # "@utmp1" eq "@utmp2"

    sub bintodec {
        unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
    }

    $foo = pack('sx2l', 12, 34);
    # short 12, two zero bytes padding, long 34
    $bar = pack('s@4l', 12, 34);
    # short 12, zero fill to position 4, long 34
    # $foo eq $bar

The same template may generally also be used in unpack().

package NAMESPACE

package

Declares the compilation unit as being in the given namespace. The scope of the package declaration is from the declaration itself through the end of the enclosing block, file, or eval (the same as the CWmy operator). All further unqualified dynamic identifiers will be in this namespace. A package statement affects only dynamic variablesincluding those you've used CWlocal onbut not lexical variables, which are created with CWmy. Typically it would be the first declaration in a file to be included by the CWrequire or CWuse operator. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the identifier with the package name and a double colon: CW$Package::Variable. If the package name is null, the CWmain package as assumed. That is, CW$::sail is equivalent to CW$main::sail (as well as to CW$main'sail, still seen in older code). If NAMESPACE is omitted, then there is no current package, and all identifiers must be fully qualified or lexicals. However, you are strongly advised not to make use of this feature. Its use can cause unexpected behaviour, even crashing some versions of Perl. It is deprecated, and will be removed from a future release. See Packages in perlmod for more information about packages, modules, and classes. See perlsub for other scoping issues.

pipe READHANDLE,WRITEHANDLE

Opens a pair of connected pipes like the corresponding system call. Note that if you set up a loop of piped processes, deadlock can occur unless you are very careful. In addition, note that Perl's pipes use IO buffering, so you may need to set CW$| to flush your WRITEHANDLE after each command, depending on the application. See IPC::Open2, IPC::Open3, and Bidirectional Communication in perlipc for examples of such things. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors as determined by the value of $^F. See $^F in perlvar.

pop ARRAY

pop

Pops and returns the last value of the array, shortening the array by one element. Has an effect similar to

    $ARRAY[$#ARRAY--]

If there are no elements in the array, returns the undefined value (although this may happen at other times as well). If ARRAY is omitted, pops the CW@ARGV array in the main program, and the CW@_ array in subroutines, just like CWshift.

pos SCALAR

pos

Returns the offset of where the last CWm//g search left off for the variable in question (CW$_ is used when the variable is not specified). Note that 0 is a valid match offset, while CWundef indicates that the search position is reset (usually due to match failure, but can also be because no match has yet been performed on the scalar). CWpos directly accesses the location used by the regexp engine to store the offset, so assigning to CWpos will change that offset, and so will also influence the CW\G zero-width assertion in regular expressions. Because a failed CWm//gc match doesn't reset the offset, the return from CWpos won't change either in this case. See perlre and perlop.

print FILEHANDLE LIST

print LIST

print

Prints a string or a list of strings. Returns true if successful. FILEHANDLE may be a scalar variable name, in which case the variable contains the name of or a reference to the filehandle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a CW+ or put parentheses around the arguments.) If FILEHANDLE is omitted, prints by default to standard output (or to the last selected output channelsee select). If LIST is also omitted, prints CW$_ to the currently selected output channel. To set the default output channel to something other than STDOUT use the select operation. The current value of CW$, (if any) is printed between each LIST item. The current value of CW$\ (if any) is printed after the entire LIST has been printed. Because print takes a LIST, anything in the LIST is evaluated in list context, and any subroutine that you call will have one or more of its expressions evaluated in list context. Also be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the printinterpose a CW+ or put parentheses around all the arguments. Note that if you're storing FILEHANDLES in an array or other expression, you will have to use a block returning its value instead:

    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";

printf FILEHANDLE FORMAT, LIST

printf FORMAT, LIST

Equivalent to CWprint FILEHANDLE sprintf(FORMAT, LIST), except that CW$\ (the output record separator) is not appended. The first argument of the list will be interpreted as the CWprintf format. See CWsprintf for an explanation of the format argument. If CWuse locale is in effect, the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC locale. See perllocale. Don't fall into the trap of using a CWprintf when a simple CWprint would do. The CWprint is more efficient and less error prone.

prototype FUNCTION

Returns the prototype of a function as a string (or CWundef if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve. If FUNCTION is a string starting with CWCORE::, the rest is taken as a name for Perl builtin. If the builtin is not overridable (such as CWqw//) or its arguments cannot be expressed by a prototype (such as CWsystem) returns CWundef because the builtin does not really behave like a Perl function. Otherwise, the string describing the equivalent prototype is returned.

push ARRAY,LIST

Treats ARRAY as a stack, and pushes the values of LIST onto the end of ARRAY. The length of ARRAY increases by the length of LIST. Has the same effect as

    for $value (LIST) {
        $ARRAY[++$#ARRAY] = $value;
    }

but is more efficient. Returns the new number of elements in the array.

q/STRING/

qq/STRING/

qr/STRING/

qx/STRING/

qw/STRING/

Generalized quotes. See Regexp Quote-Like Operators in perlop.

quotemeta EXPR

quotemeta

Returns the value of EXPR with all non-word characters backslashed. (That is, all characters not matching CW/[A-Za-z_0-9]/ will be preceded by a backslash in the returned string, regardless of any locale settings.) This is the internal function implementing the CW\Q escape in double-quoted strings. If EXPR is omitted, uses CW$_.

rand EXPR

rand

Returns a random fractional number greater than or equal to CW0 and less than the value of EXPR. (EXPR should be positive.) If EXPR is omitted, the value CW1 is used. Currently EXPR with the value CW0 is also special-cased as CW1 - this has not been documented before perl 5.8.0 and is subject to change in future versions of perl. Automatically calls CWsrand unless CWsrand has already been called. See also CWsrand. Apply CWint() to the value returned by CWrand() if you want random integers instead of random fractional numbers. For example,

    int(rand(1))

returns a random integer between CW0 and CW9, inclusive. (Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was probably compiled with the wrong number of RANDBITS.)

read FILEHANDLE,SCALAR,LENGTH,OFFSET

read FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE. Returns the number of characters actually read, CW0 at end of file, or undef if there was an error (in the latter case CW$! is also set). SCALAR will be grown or shrunk so that the last character actually read is the last character of the scalar after the read. An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with CW"\0" bytes before the result of the read is appended. The call is actually implemented in terms of either Perl's or system's fread() call. To get a true read(2) system call, see CWsysread. Note the characters: depending on the status of the filehandle, either (8-bit) bytes or characters are read. By default all filehandles operate on bytes, but for example if the filehandle has been opened with the CW:utf8 I/O layer (see open, and the CWopen pragma, open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the CW:encoding pragma: in that case pretty much any characters can be read.

readdir DIRHANDLE

Returns the next directory entry for a directory opened by CWopendir. If used in list context, returns all the rest of the entries in the directory. If there are no more entries, returns an undefined value in scalar context or a null list in list context. If you're planning to filetest the return values out of a CWreaddir, you'd better prepend the directory in question. Otherwise, because we didn't CWchdir there, it would have been testing the wrong file.

    opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
    @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
    closedir DIR;

readline EXPR

Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, each call reads and returns the next line, until end-of-file is reached, whereupon the subsequent call returns undef. In list context, reads until end-of-file is reached and returns a list of lines. Note that the notion of line used here is however you may have defined it with CW$/ or CW$INPUT_RECORD_SEPARATOR). See $/ in perlvar. When CW$/ is set to CWundef, when readline() is in scalar context (i.e. file slurp mode), and when an empty file is read, it returns CW'' the first time, followed by CWundef subsequently. This is the internal function implementing the CW<EXPR> operator, but you can use it directly. The CW<EXPR> operator is discussed in more detail in I/O Operators in perlop.

    $line = <STDIN>;
    $line = readline(*STDIN);           # same thing

If readline encounters an operating system error, CW$! will be set with the corresponding error message. It can be helpful to check CW$! when you are reading from filehandles you don't trust, such as a tty or a socket. The following example uses the operator form of CWreadline, and takes the necessary steps to ensure that CWreadline was successful.

    for (;;) {
        undef $!;
        unless (defined( $line = <> )) {
            die $! if $!;
            last; # reached EOF
        }
        # ...
    }

readlink EXPR

readlink

Returns the value of a symbolic link, if symbolic links are implemented. If not, gives a fatal error. If there is some system error, returns the undefined value and sets CW$! (errno). If EXPR is omitted, uses CW$_.

readpipe EXPR

EXPR is executed as a system command. The collected standard output of the command is returned. In scalar context, it comes back as a single (potentially multi-line) string. In list context, returns a list of lines (however you've defined lines with CW$/ or CW$INPUT_RECORD_SEPARATOR). This is the internal function implementing the CWqx/EXPR/ operator, but you can use it directly. The CWqx/EXPR/ operator is discussed in more detail in I/O Operators in perlop.

recv SOCKET,SCALAR,LENGTH,FLAGS

Receives a message on a socket. Attempts to receive LENGTH characters of data into variable SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk to the length actually read. Takes the same flags as the system call of the same name. Returns the address of the sender if SOCKET's protocol supports this; returns an empty string otherwise. If there's an error, returns the undefined value. This call is actually implemented in terms of recvfrom(2) system call. See UDP: Message Passing in perlipc for examples. Note the characters: depending on the status of the socket, either (8-bit) bytes or characters are received. By default all sockets operate on bytes, but for example if the socket has been changed using binmode() to operate with the CW:utf8 I/O layer (see the CWopen pragma, open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the CW:encoding pragma: in that case pretty much any characters can be read.

redo LABEL

redo

The CWredo command restarts the loop block without evaluating the conditional again. The CWcontinue block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclosing loop. This command is normally used by programs that want to lie to themselves about what was just input:

    # a simpleminded Pascal comment stripper
    # (warning: assumes no { or } in strings)
    LINE: while (<STDIN>) {
        while (s|({.*}.*){.*}|$1 |) {}
        s|{.*}| |;
        if (s|{.*| |) {
            $front = $_;
            while (<STDIN>) {
                if (/}/) {      # end of comment?
                    s|^|$front\{|;
                    redo LINE;
                }
            }
        }
        print;
    }

CWredo cannot be used to retry a block which returns a value such as CWeval {}, CWsub {} or CWdo {}, and should not be used to exit a grep() or map() operation. Note that a block by itself is semantically identical to a loop that executes once. Thus CWredo inside such a block will effectively turn it into a looping construct. See also continue for an illustration of how CWlast, CWnext, and CWredo work.

ref EXPR

ref

Returns a non-empty string if EXPR is a reference, the empty string otherwise. If EXPR is not specified, CW$_ will be used. The value returned depends on the type of thing the reference is a reference to. Builtin types include:

    SCALAR
    ARRAY
    HASH
    CODE
    REF
    GLOB
    LVALUE

If the referenced object has been blessed into a package, then that package name is returned instead. You can think of CWref as a CWtypeof operator.

    if (ref($r) eq "HASH") {
        print "r is a reference to a hash.\n";
    }
    unless (ref($r)) {
        print "r is not a reference at all.\n";
    }
    if (UNIVERSAL::isa($r, "HASH")) {  # for subclassing
        print "r is a reference to something that isa hash.\n";
    }

See also perlref.

rename OLDNAME,NEWNAME

Changes the name of a file; an existing file NEWNAME will be clobbered. Returns true for success, false otherwise. Behavior of this function varies wildly depending on your system implementation. For example, it will usually not work across file system boundaries, even though the system mv command sometimes compensates for this. Other restrictions include whether it works on directories, open files, or pre-existing files. Check perlport and either the rename(2) manpage or equivalent system documentation for details.

require VERSION

require EXPR

require

Demands a version of Perl specified by VERSION, or demands some semantics specified by EXPR or by CW$_ if EXPR is not supplied. VERSION may be either a numeric argument such as 5.006, which will be compared to CW$], or a literal of the form v5.6.1, which will be compared to CW$^V (aka CW$PERL_VERSION). A fatal error is produced at run time if VERSION is greater than the version of the current Perl interpreter. Compare with use, which can do a similar check at compile time. Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads to misleading error messages under earlier versions of Perl which do not support this syntax. The equivalent numeric version should be used instead.

    require v5.6.1;     # run time version check
    require 5.6.1;      # ditto
    require 5.006_001;  # ditto; preferred for backwards compatibility

Otherwise, demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of CWeval. Has semantics similar to the following subroutine:

    sub require {
       my ($filename) = @_;
       if (exists $INC{$filename}) {
           return 1 if $INC{$filename};
           die "Compilation failed in require";
       }
       my ($realfilename,$result);
       ITER: {
           foreach $prefix (@INC) {
               $realfilename = "$prefix/$filename";
               if (-f $realfilename) {
                   $INC{$filename} = $realfilename;
                   $result = do $realfilename;
                   last ITER;
               }
           }
           die "Can't find $filename in \@INC";
       }
       if ($@) {
           $INC{$filename} = undef;
           die $@;
       } elsif (!$result) {
           delete $INC{$filename};
           die "$filename did not return true value";
       } else {
           return $result;
       }
    }

Note that the file will not be included twice under the same specified name. The file must return true as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with CW1; unless you're sure it'll return true otherwise. But it's better just to put the CW1;, in case you add more statements. If EXPR is a bareword, the require assumes a ".pm extension and replaces :: with /" in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace. In other words, if you try this:

        require Foo::Bar;    # a splendid bareword

The require function will actually look for the "Foo/Bar.pm" file in the directories specified in the CW@INC array. But if you try this:

        $class = 'Foo::Bar';
        require $class;      # $class is not a bareword
    #or
        require "Foo::Bar";  # not a bareword because of the ""

The require function will look for the "Foo::Bar file in the CW@INC array and will complain about not finding Foo::Bar" there. In this case you can do:

        eval "require $class";

Now that you understand how CWrequire looks for files in the case of a bareword argument, there is a little extra functionality going on behind the scenes. Before CWrequire looks for a ".pm extension, it will first look for a filename with a .pmc" extension. A file with this extension is assumed to be Perl bytecode generated by B::Bytecode. If this file is found, and its modification time is newer than a coinciding ".pm non-compiled file, it will be loaded in place of that non-compiled file ending in a .pm" extension. You can also insert hooks into the import facility, by putting directly Perl code into the CW@INC array. There are three forms of hooks: subroutine references, array references and blessed objects. Subroutine references are the simplest case. When the inclusion system walks through CW@INC and encounters a subroutine, this subroutine gets called with two parameters, the first being a reference to itself, and the second the name of the file to be included (e.g. "Foo/Bar.pm"). The subroutine should return CWundef or a filehandle, from which the file to include will be read. If CWundef is returned, CWrequire will look at the remaining elements of CW@INC. If the hook is an array reference, its first element must be a subroutine reference. This subroutine is called as above, but the first parameter is the array reference. This enables to pass indirectly some arguments to the subroutine. In other words, you can write:

    push @INC, \&my_sub;
    sub my_sub {
        my ($coderef, $filename) = @_;  # $coderef is \&my_sub
        ...
    }

or:

    push @INC, [ \&my_sub, $x, $y, ... ];
    sub my_sub {
        my ($arrayref, $filename) = @_;
        # Retrieve $x, $y, ...
        my @parameters = @$arrayref[1..$#$arrayref];
        ...
    }

If the hook is an object, it must provide an INC method, that will be called as above, the first parameter being the object itself. (Note that you must fully qualify the sub's name, as it is always forced into package CWmain.) Here is a typical code layout:

    # In Foo.pm
    package Foo;
    sub new { ... }
    sub Foo::INC {
        my ($self, $filename) = @_;
        ...
    }

    # In the main program
    push @INC, new Foo(...);

Note that these hooks are also permitted to set the CW%INC entry corresponding to the files they have loaded. See %INC in perlvar. For a yet-more-powerful import facility, see use and perlmod.

reset EXPR

reset

Generally used in a CWcontinue block at the end of a loop to clear variables and reset CW?? searches so that they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables and arrays beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches (CW?pattern?) are reset to match again. Resets only variables or searches in the current package. Always returns 1. Examples:

    reset 'X';          # reset all X variables
    reset 'a-z';        # reset lower case variables
    reset;              # just reset ?one-time? searches

Resetting CW"A-Z" is not recommended because you'll wipe out your CW@ARGV and CW@INC arrays and your CW%ENV hash. Resets only package variableslexical variables are unaffected, but they clean themselves up on scope exit anyway, so you'll probably want to use them instead. See my.

return EXPR

return

Returns from a subroutine, CWeval, or CWdo FILE with the value given in EXPR. Evaluation of EXPR may be in list, scalar, or void context, depending on how the return value will be used, and the context may vary from one execution to the next (see CWwantarray). If no EXPR is given, returns an empty list in list context, the undefined value in scalar context, and (of course) nothing at all in a void context. (Note that in the absence of an explicit CWreturn, a subroutine, eval, or do FILE will automatically return the value of the last expression evaluated.)

reverse LIST

In list context, returns a list value consisting of the elements of LIST in the opposite order. In scalar context, concatenates the elements of LIST and returns a string value with all characters in the opposite order.

    print reverse <>;           # line tac, last line first

    undef $/;                   # for efficiency of <>
    print scalar reverse <>;    # character tac, last line tsrif

Used without arguments in scalar context, reverse() reverses CW$_. This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash, such as from a DBM file.

    %by_name = reverse %by_address;     # Invert the hash

rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the CWreaddir routine on DIRHANDLE.

rindex STR,SUBSTR,POSITION

rindex STR,SUBSTR

Works just like index() except that it returns the position of the LAST occurrence of SUBSTR in STR. If POSITION is specified, returns the last occurrence at or before that position.

rmdir FILENAME

rmdir

Deletes the directory specified by FILENAME if that directory is empty. If it succeeds it returns true, otherwise it returns false and sets CW$! (errno). If FILENAME is omitted, uses CW$_.

s///

The substitution operator. See perlop.

scalar EXPR

Forces EXPR to be interpreted in scalar context and returns the value of EXPR.

    @counts = ( scalar @a, scalar @b, scalar @c );

There is no equivalent operator to force an expression to be interpolated in list context because in practice, this is never needed. If you really wanted to do so, however, you could use the construction CW@{[ (some expression) ]}, but usually a simple CW(some expression) suffices. Because CWscalar is unary operator, if you accidentally use for EXPR a parenthesized list, this behaves as a scalar comma expression, evaluating all but the last element in void context and returning the final element evaluated in scalar context. This is seldom what you want. The following single statement:

        print uc(scalar(&foo,$bar)),$baz;

is the moral equivalent of these two:

        &foo;
        print(uc($bar),$baz);

See perlop for more details on unary operators and the comma operator.

seek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's position, just like the CWfseek call of CWstdio. FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are CW0 to set the new position in bytes to POSITION, CW1 to set it to the current position plus POSITION, and CW2 to set it to EOF plus POSITION (typically negative). For WHENCE you may use the constants CWSEEK_SET, CWSEEK_CUR, and CWSEEK_END (start of the file, current position, end of the file) from the Fcntl module. Returns CW1 upon success, CW0 otherwise. Note the in bytes: even if the filehandle has been set to operate on characters (for example by using the CW:utf8 open layer), tell() will return byte offsets, not character offsets (because implementing that would render seek() and tell() rather slow). If you want to position file for CWsysread or CWsyswrite, don't use CWseek--buffering makes its effect on the file's system position unpredictable and non-portable. Use CWsysseek instead. Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of CW1 (CWSEEK_CUR) is useful for not moving the file position:

    seek(TEST,0,1);

This is also useful for applications emulating CWtail -f. Once you hit EOF on your read, and then sleep for a while, you might have to stick in a seek() to reset things. The CWseek doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next CW<FILE> makes Perl try again to read something. We hope. If that doesn't work (some IO implementations are particularly cantankerous), then you may need something more like this:

    for (;;) {
        for ($curpos = tell(FILE); $_ = <FILE>;
             $curpos = tell(FILE)) {
            # search for some stuff and put it into files
        }
        sleep($for_a_while);
        seek(FILE, $curpos, 0);
    }

seekdir DIRHANDLE,POS

Sets the current position for the CWreaddir routine on DIRHANDLE. POS must be a value returned by CWtelldir. Has the same caveats about possible directory compaction as the corresponding system library routine.

select FILEHANDLE

select

Returns the currently selected filehandle. Sets the current default filehandle for output, if FILEHANDLE is supplied. This has two effects: first, a CWwrite or a CWprint without a filehandle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel. For example, if you have to set the top of form format for more than one output channel, you might do the following:

    select(REPORT1);
    $^ = 'report1_top';
    select(REPORT2);
    $^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:

    $oldfh = select(STDERR); $| = 1; select($oldfh);

Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as:

    use IO::Handle;
    STDERR->autoflush(1);

select RBITS,WBITS,EBITS,TIMEOUT

This calls the select(2) system call with the bit masks specified, which can be constructed using CWfileno and CWvec, along these lines:

    $rin = $win = $ein = '';
    vec($rin,fileno(STDIN),1) = 1;
    vec($win,fileno(STDOUT),1) = 1;
    $ein = $rin | $win;

If you want to select on many filehandles you might wish to write a subroutine:

    sub fhbits {
        my(@fhlist) = split(' ',$_[0]);
        my($bits);
        for (@fhlist) {
            vec($bits,fileno($_),1) = 1;
        }
        $bits;
    }
    $rin = fhbits('STDIN TTY SOCK');

The usual idiom is:

    ($nfound,$timeleft) =
      select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

or to block until something becomes ready just do this

    $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

Most systems do not bother to return anything useful in CW$timeleft, so calling select() in scalar context just returns CW$nfound. Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning the CW$timeleft. If not, they always return CW$timeleft equal to the supplied CW$timeout. You can effect a sleep of 250 milliseconds this way:

    select(undef, undef, undef, 0.25);

Note that whether CWselect gets restarted after signals (say, SIGALRM) is implementation-dependent. See also perlport for notes on the portability of CWselect. On error, CWselect behaves like the select(2) system call : it returns -1 and sets CW$!. Note: on some Unixes, the select(2) system call may report a socket file descriptor as ready for reading, when actually no data is available, thus a subsequent read blocks. It can be avoided using always the O_NONBLOCK flag on the socket. See select(2) and fcntl(2) for further details. WARNING: One should not attempt to mix buffered I/O (like CWread or <FH>) with CWselect, except as permitted by POSIX, and even then only on POSIX systems. You have to use CWsysread instead.

semctl ID,SEMNUM,CMD,ARG

Calls the System V IPC function CWsemctl. You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable which will hold the returned semid_ds structure or semaphore value array. Returns like CWioctl: the undefined value for error, "CW0 but true" for zero, or the actual return value otherwise. The ARG must consist of a vector of native short integers, which may be created with CWpack("s!",(0)x$nsem). See also SysV IPC in perlipc, CWIPC::SysV, CWIPC::Semaphore documentation.

semget KEY,NSEMS,FLAGS

Calls the System V IPC function semget. Returns the semaphore id, or the undefined value if there is an error. See also SysV IPC in perlipc, CWIPC::SysV, CWIPC::SysV::Semaphore documentation.

semop KEY,OPSTRING

Calls the System V IPC function semop to perform semaphore operations such as signalling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with CWpack("s!3", $semnum, $semop, $semflag). The number of semaphore operations is implied by the length of OPSTRING. Returns true if successful, or false if there is an error. As an example, the following code waits on semaphore CW$semnum of semaphore id CW$semid:

    $semop = pack("s!3", $semnum, -1, 0);
    die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace CW-1 with CW1. See also SysV IPC in perlipc, CWIPC::SysV, and CWIPC::SysV::Semaphore documentation.

send SOCKET,MSG,FLAGS,TO

send SOCKET,MSG,FLAGS

Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET filehandle. Takes the same flags as the system call of the same name. On unconnected sockets you must specify a destination to send TO, in which case it does a C CWsendto. Returns the number of characters sent, or the undefined value if there is an error. The C system call sendmsg(2) is currently unimplemented. See UDP: Message Passing in perlipc for examples. Note the characters: depending on the status of the socket, either (8-bit) bytes or characters are sent. By default all sockets operate on bytes, but for example if the socket has been changed using binmode() to operate with the CW:utf8 I/O layer (see open, or the CWopen pragma, open), the I/O will operate on UTF-8 encoded Unicode characters, not bytes. Similarly for the CW:encoding pragma: in that case pretty much any characters can be sent.

setpgrp PID,PGRP

Sets the current process group for the specified PID, CW0 for the current process. Will produce a fatal error if used on a machine that doesn't implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, it defaults to CW0,0. Note that the BSD 4.2 version of CWsetpgrp does not accept any arguments, so only CWsetpgrp(0,0) is portable. See also CWPOSIX::setsid().

setpriority WHICH,WHO,PRIORITY

Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Will produce a fatal error if used on a machine that doesn't implement setpriority(2).

setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Sets the socket option requested. Returns undefined if there is an error. OPTVAL may be specified as CWundef if you don't want to pass an argument.

shift ARRAY

shift

Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down. If there are no elements in the array, returns the undefined value. If ARRAY is omitted, shifts the CW@_ array within the lexical scope of subroutines and formats, and the CW@ARGV array at file scopes or within the lexical scopes established by the CWeval '', CWBEGIN {}, CWINIT {}, CWCHECK {}, and CWEND {} constructs. See also CWunshift, CWpush, and CWpop. CWshift and CWunshift do the same thing to the left end of an array that CWpop and CWpush do to the right end.

shmctl ID,CMD,ARG

Calls the System V IPC function shmctl. You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions. If CMD is CWIPC_STAT, then ARG must be a variable which will hold the returned CWshmid_ds structure. Returns like ioctl: the undefined value for error, "CW0 but true" for zero, or the actual return value otherwise. See also SysV IPC in perlipc and CWIPC::SysV documentation.

shmget KEY,SIZE,FLAGS

Calls the System V IPC function shmget. Returns the shared memory segment id, or the undefined value if there is an error. See also SysV IPC in perlipc and CWIPC::SysV documentation.

shmread ID,VAR,POS,SIZE

shmwrite ID,STRING,POS,SIZE

Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable that will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out SIZE bytes. Return true if successful, or false if there is an error. shmread() taints the variable. See also SysV IPC in perlipc, CWIPC::SysV documentation, and the CWIPC::Shareable module from CPAN.

shutdown SOCKET,HOW

Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the system call of the same name.

    shutdown(SOCKET, 0);    # I/we have stopped reading data
    shutdown(SOCKET, 1);    # I/we have stopped writing data
    shutdown(SOCKET, 2);    # I/we have stopped using this socket

This is useful with sockets when you want to tell the other side you're done writing but not done reading, or vice versa. It's also a more insistent form of close because it also disables the file descriptor in any forked copies in other processes.

sin EXPR

sin

Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of CW$_. For the inverse sine operation, you may use the CWMath::Trig::asin function, or use this relation:

    sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

sleep EXPR

sleep

Causes the script to sleep for EXPR seconds, or forever if no EXPR. May be interrupted if the process receives a signal such as CWSIGALRM. Returns the number of seconds actually slept. You probably cannot mix CWalarm and CWsleep calls, because CWsleep is often implemented using CWalarm. On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount. They may appear to sleep longer than that, however, because your process might not be scheduled right away in a busy multitasking system. For delays of finer granularity than one second, you may use Perl's CWsyscall interface to access setitimer(2) if your system supports it, or else see select above. The Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) may also help. See also the POSIX module's CWpause function.

socket SOCKET,DOMAIN,TYPE,PROTOCOL

Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the system call of the same name. You should CWuse Socket first to get the proper definitions imported. See the examples in Sockets: Client/Server Communication in perlipc. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in perlvar.

socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as for the system call of the same name. If unimplemented, yields a fatal error. Returns true if successful. On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors, as determined by the value of $^F. See $^F in perlvar. Some systems defined CWpipe in terms of CWsocketpair, in which a call to CWpipe(Rdr, Wtr) is essentially:

    use Socket;
    socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
    shutdown(Rdr, 1);        # no more writing for reader
    shutdown(Wtr, 0);        # no more reading for writer

See perlipc for an example of socketpair use. Perl 5.8 and later will emulate socketpair using IP sockets to localhost if your system implements sockets but not socketpair.

sort SUBNAME LIST

sort BLOCK LIST

sort LIST

In list context, this sorts the LIST and returns the sorted list value. In scalar context, the behaviour of CWsort() is undefined. If SUBNAME or BLOCK is omitted, CWsorts in standard string comparison order. If SUBNAME is specified, it gives the name of a subroutine that returns an integer less than, equal to, or greater than CW0, depending on how the elements of the list are to be ordered. (The CW<=> and CWcmp operators are extremely useful in such routines.) SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine. If the subroutine's prototype is CW($$), the elements to be compared are passed by reference in CW@_, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables CW$a and CW$b (see example below). Note that in the latter case, it is usually counter-productive to declare CW$a and CW$b as lexicals. In either case, the subroutine may not be recursive. The values to be compared are always passed by reference, so don't modify them. You also cannot exit out of the sort block or subroutine using any of the loop control operators described in perlsyn or with CWgoto. When CWuse locale is in effect, CWsort LIST sorts LIST according to the current collation locale. See perllocale. Perl 5.6 and earlier used a quicksort algorithm to implement sort. That algorithm was not stable, and could go quadratic. (A stable sort preserves the input order of elements that compare equal. Although quicksort's run time is O(N) when averaged over all arrays of length N, the time can be O(N**2), quadratic behavior, for some inputs.) In 5.7, the quicksort implementation was replaced with a stable mergesort algorithm whose worst case behavior is O(N). But benchmarks indicated that for some inputs, on some platforms, the original quicksort was faster. 5.8 has a sort pragma for limited control of the sort. Its rather blunt control of the underlying algorithm may not persist into future perls, but the ability to characterize the input or output in implementation independent ways quite probably will. See sort. Examples:

    # sort lexically
    @articles = sort @files;

    # same thing, but with explicit sort routine
    @articles = sort {$a cmp $b} @files;

    # now case-insensitively
    @articles = sort {uc($a) cmp uc($b)} @files;

    # same thing in reversed order
    @articles = sort {$b cmp $a} @files;

    # sort numerically ascending
    @articles = sort {$a <=> $b} @files;

    # sort numerically descending
    @articles = sort {$b <=> $a} @files;

    # this sorts the %age hash by value instead of key
    # using an in-line function
    @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

    # sort using explicit subroutine name
    sub byage {
        $age{$a} <=> $age{$b};  # presuming numeric
    }
    @sortedclass = sort byage @class;

    sub backwards { $b cmp $a }
    @harry  = qw(dog cat x Cain Abel);
    @george = qw(gone chased yz Punished Axed);
    print sort @harry;
            # prints AbelCaincatdogx
    print sort backwards @harry;
            # prints xdogcatCainAbel
    print sort @george, 'to', @harry;
            # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

    # inefficiently sort by descending numeric compare using
    # the first integer after the first = sign, or the
    # whole record case-insensitively otherwise

    @new = sort {
        ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
                            ||
                    uc($a)  cmp  uc($b)
    } @old;

    # same thing, but much more efficiently;
    # we'll build auxiliary indices instead
    # for speed
    @nums = @caps = ();
    for (@old) {
        push @nums, /=(\d+)/;
        push @caps, uc($_);
    }

    @new = @old[ sort {
                        $nums[$b] <=> $nums[$a]
                                 ||
                        $caps[$a] cmp $caps[$b]
                       } 0..$#old
               ];

    # same thing, but without any temps
    @new = map { $_->[0] }
           sort { $b->[1] <=> $a->[1]
                           ||
                  $a->[2] cmp $b->[2]
           } map { [$_, /=(\d+)/, uc($_)] } @old;

    # using a prototype allows you to use any comparison subroutine
    # as a sort subroutine (including other package's subroutines)
    package other;
    sub backwards ($$) { $_[1] cmp $_[0]; }     # $a and $b are not set here

    package main;
    @new = sort other::backwards @old;

    # guarantee stability, regardless of algorithm
    use sort 'stable';
    @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

    # force use of mergesort (not portable outside Perl 5.8)
    use sort '_mergesort';  # note discouraging _
    @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

If you're using strict, you must not declare CW$a and CW$b as lexicals. They are package globals. That means if you're in the CWmain package and type

    @articles = sort {$b <=> $a} @files;

then CW$a and CW$b are CW$main::a and CW$main::b (or CW$::a and CW$::b), but if you're in the CWFooPack package, it's the same as typing

    @articles = sort {$FooPack::b <=> $FooPack::a} @files;

The comparison function is required to behave. If it returns inconsistent results (sometimes saying CW$x[1] is less than CW$x[2] and sometimes saying the opposite, for example) the results are not well-defined. Because CW<=> returns CWundef when either operand is CWNaN (not-a-number), and because CWsort will trigger a fatal error unless the result of a comparison is defined, when sorting with a comparison function like CW$a <=> $b, be careful about lists that might contain a CWNaN. The following example takes advantage of the fact that CWNaN != NaN to eliminate any CWNaNs from the input.

    @result = sort { $a <=> $b } grep { $_ == $_ } @input;

splice ARRAY,OFFSET,LENGTH,LIST

splice ARRAY,OFFSET,LENGTH

splice ARRAY,OFFSET

splice ARRAY

Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. In list context, returns the elements removed from the array. In scalar context, returns the last element removed, or CWundef if no elements are removed. The array grows or shrinks as necessary. If OFFSET is negative then it starts that far from the end of the array. If LENGTH is omitted, removes everything from OFFSET onward. If LENGTH is negative, removes the elements from OFFSET onward except for -LENGTH elements at the end of the array. If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is past the end of the array, perl issues a warning, and splices at the end of the array. The following equivalences hold (assuming CW$[ == 0 and $#a >= $i )

    push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
    pop(@a)             splice(@a,-1)
    shift(@a)           splice(@a,0,1)
    unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
    $a[$i] = $y         splice(@a,$i,1,$y)

Example, assuming array lengths are passed before arrays:

    sub aeq {   # compare two list values
        my(@a) = splice(@_,0,shift);
        my(@b) = splice(@_,0,shift);
        return 0 unless @a == @b;       # same len?
        while (@a) {
            return 0 if pop(@a) ne pop(@b);
        }
        return 1;
    }
    if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }

split /PATTERN/,EXPR,LIMIT

split /PATTERN/,EXPR

split /PATTERN/

split

Splits the string EXPR into a list of strings and returns that list. By default, empty leading fields are preserved, and empty trailing ones are deleted. (If all fields are empty, they are considered to be trailing.) In scalar context, returns the number of fields found and splits into the CW@_ array. Use of split in scalar context is deprecated, however, because it clobbers your subroutine arguments. If EXPR is omitted, splits the CW$_ string. If PATTERN is also omitted, splits on whitespace (after skipping any leading whitespace). Anything matching PATTERN is taken to be a delimiter separating the fields. (Note that the delimiter may be longer than one character.) If LIMIT is specified and positive, it represents the maximum number of fields the EXPR will be split into, though the actual number of fields returned depends on the number of times PATTERN matches within EXPR. If LIMIT is unspecified or zero, trailing null fields are stripped (which potential users of CWpop would do well to remember). If LIMIT is negative, it is treated as if an arbitrarily large LIMIT had been specified. Note that splitting an EXPR that evaluates to the empty string always returns the empty list, regardless of the LIMIT specified. A pattern matching the null string (not to be confused with a null pattern CW//, which is just one member of the set of patterns matching a null string) will split the value of EXPR into separate characters at each point it matches that way. For example:

    print join(':', split(/ */, 'hi there'));

produces the output 'h:i:t:h:e:r:e'. As a special case for CWsplit, using the empty pattern CW// specifically matches only the null string, and is not be confused with the regular use of CW// to mean the last successful pattern match. So, for CWsplit, the following:

    print join(':', split(//, 'hi there'));

produces the output 'h:i: :t:h:e:r:e'. Empty leading (or trailing) fields are produced when there are positive width matches at the beginning (or end) of the string; a zero-width match at the beginning (or end) of the string does not produce an empty field. For example:

   print join(':', split(/(?=\w)/, 'hi there!'));

produces the output 'h:i :t:h:e:r:e!'. The LIMIT parameter can be used to split a line partially

    ($login, $passwd, $remainder) = split(/:/, $_, 3);

When assigning to a list, if LIMIT is omitted, or zero, Perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need. If the PATTERN contains parentheses, additional list elements are created from each matching substring in the delimiter.

    split(/([,-])/, "1-10,20", 3);

produces the list value

    (1, '-', 10, ',', 20)

If you had the entire header of a normal Unix email message in CW$header, you could split it up into fields and their values this way:

    $header =~ s/\n\s+/ /g;  # fix continuation lines
    %hdrs   =  (UNIX_FROM => split /^(\S*?):\s*/m, $header);

The pattern CW/PATTERN/ may be replaced with an expression to specify patterns that vary at runtime. (To do runtime compilation only once, use CW/$variable/o.) As a special case, specifying a PATTERN of space (CW' ') will split on white space just as CWsplit with no arguments does. Thus, CWsplit(' ') can be used to emulate awk's default behavior, whereas CWsplit(/ /) will give you as many null initial fields as there are leading spaces. A CWsplit on CW/\s+/ is like a CWsplit(' ') except that any leading whitespace produces a null first field. A CWsplit with no arguments really does a CWsplit(' ', $_) internally. A PATTERN of CW/^/ is treated as if it were CW/^/m, since it isn't much use otherwise. Example:

    open(PASSWD, '/etc/passwd');
    while (<PASSWD>) {
        chomp;
        ($login, $passwd, $uid, $gid,
         $gcos, $home, $shell) = split(/:/);
        #...
    }

As with regular pattern matching, any capturing parentheses that are not matched in a CWsplit() will be set to CWundef when returned:

    @fields = split /(A)|B/, "1A2B3";
    # @fields is (1, 'A', 2, undef, 3)

sprintf FORMAT, LIST

Returns a string formatted by the usual CWprintf conventions of the C library function CWsprintf. See below for more details and see sprintf(3) or printf(3) on your system for an explanation of the general principles. For example:

        # Format number with up to 8 leading zeroes
        $result = sprintf("%08d", $number);

        # Round number to 3 digits after decimal point
        $rounded = sprintf("%.3f", $number);

Perl does its own CWsprintf formattingit emulates the C function CWsprintf, but it doesn't use it (except for floating-point numbers, and even then only the standard modifiers are allowed). As a result, any non-standard extensions in your local CWsprintf are not available from Perl. Unlike CWprintf, CWsprintf does not do what you probably mean when you pass it an array as your first argument. The array is given scalar context, and instead of using the 0th element of the array as the format, Perl will use the count of elements in the array as the format, which is almost never useful. Perl's CWsprintf permits the following universally-known conversions:

   %%   a percent sign
   %c   a character with the given number
   %s   a string
   %d   a signed integer, in decimal
   %u   an unsigned integer, in decimal
   %o   an unsigned integer, in octal
   %x   an unsigned integer, in hexadecimal
   %e   a floating-point number, in scientific notation
   %f   a floating-point number, in fixed decimal notation
   %g   a floating-point number, in %e or %f notation

In addition, Perl permits the following widely-supported conversions:

   %X   like %x, but using upper-case letters
   %E   like %e, but using an upper-case "E"
   %G   like %g, but with an upper-case "E" (if applicable)
   %b   an unsigned integer, in binary
   %p   a pointer (outputs the Perl value's address in hexadecimal)
   %n   special: *stores* the number of characters output so far
        into the next variable in the parameter list

Finally, for backward (and we do mean backward) compatibility, Perl permits these unnecessary but widely-supported conversions:

   %i   a synonym for %d
   %D   a synonym for %ld
   %U   a synonym for %lu
   %O   a synonym for %lo
   %F   a synonym for %f

Note that the number of exponent digits in the scientific notation produced by CW%e, CW%E, CW%g and CW%G for numbers with the modulus of the exponent less than 100 is system-dependent: it may be three or less (zero-padded as necessary). In other words, 1.23 times ten to the 99th may be either 1.23e99 or 1.23e099. Between the CW% and the format letter, you may specify a number of additional attributes controlling the interpretation of the format. In order, these are:

format parameter index

An explicit format parameter index, such as CW2$. By default sprintf will format the next unused argument in the list, but this allows you to take the arguments out of order. Eg:

  printf '%2$d %1$d', 12, 34;      # prints "34 12"
  printf '%3$d %d %1$d', 1, 2, 3;  # prints "3 1 1"

flags

one or more of: space prefix positive number with a space + prefix positive number with a plus sign - left-justify within the field 0 use zeros, not spaces, to right-justify # prefix non-zero octal with 0, non-zero hex with 0x, non-zero binary with 0b For example:

  printf '<% d>', 12;   # prints "< 12>"
  printf '<%+d>', 12;   # prints "<+12>"
  printf '<%6s>', 12;   # prints "<    12>"
  printf '<%-6s>', 12;  # prints "<12    >"
  printf '<%06s>', 12;  # prints "<000012>"
  printf '<%#x>', 12;   # prints "<0xc>"

vector flag

The vector flag CWv, optionally specifying the join string to use. This flag tells perl to interpret the supplied string as a vector of integers, one for each character in the string, separated by a given string (a dot CW. by default). This can be useful for displaying ordinal values of characters in arbitrary strings:

  printf "version is v%vd\n", $^V;     # Perl's version

Put an asterisk CW* before the CWv to override the string to use to separate the numbers:

  printf "address is %*vX\n", ":", $addr;   # IPv6 address
  printf "bits are %0*v8b\n", " ", $bits;   # random bitstring

You can also explicitly specify the argument number to use for the join string using eg CW*2$v:

  printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":";   # 3 IPv6 addresses

(minimum) width

Arguments are usually formatted to be only as wide as required to display the given value. You can override the width by putting a number here, or get the width from the next argument (with CW*) or from a specified argument (with eg CW*2$):

  printf '<%s>', "a";       # prints "<a>"
  printf '<%6s>', "a";      # prints "<     a>"
  printf '<%*s>', 6, "a";   # prints "<     a>"
  printf '<%*2$s>', "a", 6; # prints "<     a>"
  printf '<%2s>', "long";   # prints "<long>" (does not truncate)

If a field width obtained through CW* is negative, it has the same effect as the CW- flag: left-justification.

precision, or maximum width

You can specify a precision (for numeric conversions) or a maximum width (for string conversions) by specifying a CW. followed by a number. For floating point formats, with the exception of 'g' and 'G', this specifies the number of decimal places to show (the default being 6), eg:

  # these examples are subject to system-specific variation
  printf '<%f>', 1;    # prints "<1.000000>"
  printf '<%.1f>', 1;  # prints "<1.0>"
  printf '<%.0f>', 1;  # prints "<1>"
  printf '<%e>', 10;   # prints "<1.000000e+01>"
  printf '<%.1e>', 10; # prints "<1.0e+01>"

For 'g' and 'G', this specifies the maximum number of digits to show, including prior to the decimal point as well as after it, eg:

  # these examples are subject to system-specific variation
  printf '<%g>', 1;        # prints "<1>"
  printf '<%.10g>', 1;     # prints "<1>"
  printf '<%g>', 100;      # prints "<100>"
  printf '<%.1g>', 100;    # prints "<1e+02>"
  printf '<%.2g>', 100.01; # prints "<1e+02>"
  printf '<%.5g>', 100.01; # prints "<100.01>"
  printf '<%.4g>', 100.01; # prints "<100>"

For integer conversions, specifying a precision implies that the output of the number itself should be zero-padded to this width:

  printf '<%.6x>', 1;      # prints "<000001>"
  printf '<%#.6x>', 1;     # prints "<0x000001>"
  printf '<%-10.6x>', 1;   # prints "<000001    >"

For string conversions, specifying a precision truncates the string to fit in the specified width:

  printf '<%.5s>', "truncated";   # prints "<trunc>"
  printf '<%10.5s>', "truncated"; # prints "<     trunc>"

You can also get the precision from the next argument using CW.*:

  printf '<%.6x>', 1;       # prints "<000001>"
  printf '<%.*x>', 6, 1;    # prints "<000001>"

You cannot currently get the precision from a specified number, but it is intended that this will be possible in the future using eg CW.*2$:

  printf '<%.*2$x>', 1, 6;   # INVALID, but in future will print "<000001>"

size

For numeric conversions, you can specify the size to interpret the number as using CWl, CWh, CWV, CWq, CWL, or CWll. For integer conversions (CWd u o x X b i D U O), numbers are usually assumed to be whatever the default integer size is on your platform (usually 32 or 64 bits), but you can override this to use instead one of the standard C types, as supported by the compiler used to build Perl:

   l           interpret integer as C type "long" or "unsigned long"
   h           interpret integer as C type "short" or "unsigned short"
   q, L or ll  interpret integer as C type "long long", "unsigned long long".
               or "quads" (typically 64-bit integers)

The last will produce errors if Perl does not understand quads in your installation. (This requires that either the platform natively supports quads or Perl was specifically compiled to support quads.) You can find out whether your Perl supports quads via Config:

        use Config;
        ($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
                print "quads\n";

For floating point conversions (CWe f g E F G), numbers are usually assumed to be the default floating point size on your platform (double or long double), but you can force 'long double' with CWq, CWL, or CWll if your platform supports them. You can find out whether your Perl supports long doubles via Config:

        use Config;
        $Config{d_longdbl} eq 'define' && print "long doubles\n";

You can find out whether Perl considers 'long double' to be the default floating point size to use on your platform via Config:

        use Config;
        ($Config{uselongdouble} eq 'define') &&
                print "long doubles by default\n";

It can also be the case that long doubles and doubles are the same thing:

        use Config;
        ($Config{doublesize} == $Config{longdblsize}) &&
                print "doubles are long doubles\n";

The size specifier CWV has no effect for Perl code, but it is supported for compatibility with XS code; it means 'use the standard size for a Perl integer (or floating-point number)', which is already the default for Perl code.

order of arguments

Normally, sprintf takes the next unused argument as the value to format for each format specification. If the format specification uses CW* to require additional arguments, these are consumed from the argument list in the order in which they appear in the format specification before the value to format. Where an argument is specified using an explicit index, this does not affect the normal order for the arguments (even when the explicitly specified index would have been the next argument in any case). So:

  printf '<%*.*s>', $a, $b, $c;

would use CW$a for the width, CW$b for the precision and CW$c as the value to format, while:

  print '<%*1$.*s>', $a, $b;

would use CW$a for the width and the precision, and CW$b as the value to format. Here are some more examples - beware that when using an explicit index, the CW$ may need to be escaped:

  printf "%2\$d %d\n",    12, 34;               # will print "34 12\n"
  printf "%2\$d %d %d\n", 12, 34;               # will print "34 12 34\n"
  printf "%3\$d %d %d\n", 12, 34, 56;           # will print "56 12 34\n"
  printf "%2\$*3\$d %d\n", 12, 34, 3;           # will print " 34 12\n"

If CWuse locale is in effect, the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC locale. See perllocale.

sqrt EXPR

sqrt

Return the square root of EXPR. If EXPR is omitted, returns square root of CW$_. Only works on non-negative operands, unless you've loaded the standard Math::Complex module.

    use Math::Complex;
    print sqrt(-2);    # prints 1.4142135623731i

srand EXPR

srand

Sets the random number seed for the CWrand operator. The point of the function is to seed the CWrand function so that CWrand can produce a different sequence each time you run your program. If srand() is not called explicitly, it is called implicitly at the first use of the CWrand operator. However, this was not the case in versions of Perl before 5.004, so if your script will run under older Perl versions, it should call CWsrand. Most programs won't even call srand() at all, except those that need a cryptographically-strong starting point rather than the generally acceptable default, which is based on time of day, process ID, and memory allocation, or the /dev/urandom device, if available. You can call srand($seed) with the same CW$seed to reproduce the same sequence from rand(), but this is usually reserved for generating predictable results for testing or debugging. Otherwise, don't call srand() more than once in your program. Do not call srand() (i.e. without an argument) more than once in a script. The internal state of the random number generator should contain more entropy than can be provided by any seed, so calling srand() again actually loses randomness. Most implementations of CWsrand take an integer and will silently truncate decimal numbers. This means CWsrand(4) will usually produce the same results as CWsrand(42.1). To be safe, always pass CWsrand an integer. In versions of Perl prior to 5.004 the default seed was just the current CWtime. This isn't a particularly good seed, so many old programs supply their own seed value (often CWtime ^ $$ or CWtime ^ ($$ + ($$ << 15))), but that isn't necessary any more. Note that you need something much more random than the default seed for cryptographic purposes. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:

    srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);

If you're particularly concerned with this, see the CWMath::TrulyRandom module in CPAN. Frequently called programs (like CGI scripts) that simply use

    time ^ $$

for a seed can fall prey to the mathematical property that

    a^b == (a+1)^(b+1)

one-third of the time. So don't do that.

stat FILEHANDLE

stat EXPR

stat

Returns a 13-element list giving the status info for a file, either the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it stats CW$_. Returns a null list if the stat fails. Typically used as follows:

    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);

Not all fields are supported on all filesystem types. Here are the meanings of the fields:

  0 dev      device number of filesystem
  1 ino      inode number
  2 mode     file mode  (type and permissions)
  3 nlink    number of (hard) links to the file
  4 uid      numeric user ID of file's owner
  5 gid      numeric group ID of file's owner
  6 rdev     the device identifier (special files only)
  7 size     total size of file, in bytes
  8 atime    last access time in seconds since the epoch
  9 mtime    last modify time in seconds since the epoch
 10 ctime    inode change time in seconds since the epoch (*)
 11 blksize  preferred block size for file system I/O
 12 blocks   actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.) (*) Not all fields are supported on all filesystem types. Notably, the ctime field is non-portable. In particular, you cannot expect it to be a creation time, see Files and Filesystems in perlport for details. If CWstat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last CWstat, CWlstat, or filetest are returned. Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
        print "$file is executable NFS file\n";
    }

(This works on machines only for which the device number is negative under NFS.) Because the mode contains both the file type and its permissions, you should mask off the file type portion and (s)printf using a CW"%o" if you want to see the real permissions.

    $mode = (stat($filename))[2];
    printf "Permissions are %04o\n", $mode & 07777;

In scalar context, CWstat returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle CW_. The File::stat module provides a convenient, by-name access mechanism:

    use File::stat;
    $sb = stat($filename);
    printf "File is %s, size is %s, perm %04o, mtime %s\n",
        $filename, $sb->size, $sb->mode & 07777,
        scalar localtime $sb->mtime;

You can import symbolic mode constants (CWS_IF*) and functions (CWS_IS*) from the Fcntl module:

    use Fcntl ':mode';

    $mode = (stat($filename))[2];

    $user_rwx      = ($mode & S_IRWXU) >> 6;
    $group_read    = ($mode & S_IRGRP) >> 3;
    $other_execute =  $mode & S_IXOTH;

    printf "Permissions are %04o\n", S_IMODE($mode), "\n";

    $is_setuid     =  $mode & S_ISUID;
    $is_setgid     =  S_ISDIR($mode);

You could write the last two using the CW-u and CW-d operators. The commonly available CWS_IF* constants are

    # Permissions: read, write, execute, for user, group, others.

    S_IRWXU S_IRUSR S_IWUSR S_IXUSR
    S_IRWXG S_IRGRP S_IWGRP S_IXGRP
    S_IRWXO S_IROTH S_IWOTH S_IXOTH

    # Setuid/Setgid/Stickiness/SaveText.
    # Note that the exact meaning of these is system dependent.

    S_ISUID S_ISGID S_ISVTX S_ISTXT

    # File types.  Not necessarily all are available on your system.

    S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT

    # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.

    S_IREAD S_IWRITE S_IEXEC

and the CWS_IF* functions are

    S_IMODE($mode)      the part of $mode containing the permission bits
                        and the setuid/setgid/sticky bits

    S_IFMT($mode)       the part of $mode containing the file type
                        which can be bit-anded with e.g. S_IFREG
                        or with the following functions

    # The operators -f, -d, -l, -b, -c, -p, and -S.

    S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
    S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)

    # No direct -X operator counterpart, but for the first one
    # the -g operator is often equivalent.  The ENFMT stands for
    # record flocking enforcement, a platform-dependent feature.

    S_ISENFMT($mode) S_ISWHT($mode)

See your native chmod(2) and stat(2) documentation for more details about the CWS_* constants. To get status info for a symbolic link instead of the target file behind the link, use the CWlstat function.

study SCALAR

study

Takes extra time to study SCALAR (CW$_ if unspecified) in anticipation of doing many pattern matches on the string before it is next modified. This may or may not save time, depending on the nature and number of patterns you are searching on, and on the distribution of character frequencies in the string to be searchedyou probably want to compare run times with and without it to see which runs faster. Those loops which scan for many short constant strings (including the constant parts of more complex patterns) will benefit most. You may have only one CWstudy active at a timeif you study a different scalar the first is unstudied. (The way CWstudy works is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the CW'k' characters are. From each search string, the rarest character is selected, based on some static frequency tables constructed from some C programs and English text. Only those places that contain this rarest character are examined.) For example, here is a loop that inserts index producing entries before any line containing a certain pattern:

    while (<>) {
        study;
        print ".IX foo\n"       if /\bfoo\b/;
        print ".IX bar\n"       if /\bbar\b/;
        print ".IX blurfl\n"    if /\bblurfl\b/;
        # ...
        print;
    }

In searching for CW/\bfoo\b/, only those locations in CW$_ that contain CWf will be looked at, because CWf is rarer than CWo. In general, this is a big win except in pathological cases. The only question is whether it saves you more time than it took to build the linked list in the first place. Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and CWeval that to avoid recompiling all your patterns all the time. Together with undefining CW$/ to input entire files as one record, this can be very fast, often faster than specialized programs like fgrep(1). The following scans a list of files (CW@files) for a list of words (CW@words), and prints out the names of those files that contain a match:

    $search = 'while (<>) { study;';
    foreach $word (@words) {
        $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
    }
    $search .= "}";
    @ARGV = @files;
    undef $/;
    eval $search;               # this screams
    $/ = "\n";          # put back to normal input delimiter
    foreach $file (sort keys(%seen)) {
        print $file, "\n";
    }

sub NAME BLOCK

sub NAME (PROTO) BLOCK

sub NAME : ATTRS BLOCK

sub NAME (PROTO) : ATTRS BLOCK

This is subroutine definition, not a real function per se. Without a BLOCK it's just a forward declaration. Without a NAME, it's an anonymous function declaration, and does actually return a value: the CODE ref of the closure you just created. See perlsub and perlref for details about subroutines and references, and attributes and Attribute::Handlers for more information about attributes.

substr EXPR,OFFSET,LENGTH,REPLACEMENT

substr EXPR,OFFSET,LENGTH

substr EXPR,OFFSET

Extracts a substring out of EXPR and returns it. First character is at offset CW0, or whatever you've set CW$[ to (but don't do that). If OFFSET is negative (or more precisely, less than CW$[), starts that far from the end of the string. If LENGTH is omitted, returns everything to the end of the string. If LENGTH is negative, leaves that many characters off the end of the string. You can use the substr() function as an lvalue, in which case EXPR must itself be an lvalue. If you assign something shorter than LENGTH, the string will shrink, and if you assign something longer than LENGTH, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using CWsprintf. If OFFSET and LENGTH specify a substring that is partly outside the string, only the part within the string is returned. If the substring is beyond either end of the string, substr() returns the undefined value and produces a warning. When used as an lvalue, specifying a substring that is entirely outside the string is a fatal error. Here's an example showing the behavior for boundary cases:

    my $name = 'fred';
    substr($name, 4) = 'dy';            # $name is now 'freddy'
    my $null = substr $name, 6, 2;      # returns '' (no warning)
    my $oops = substr $name, 7;         # returns undef, with warning
    substr($name, 7) = 'gap';           # fatal error

An alternative to using substr() as an lvalue is to specify the replacement string as the 4th argument. This allows you to replace parts of the EXPR and return what was there before in one operation, just as you can with splice(). If the lvalue returned by substr is used after the EXPR is changed in any way, the behaviour may not be as expected and is subject to change. This caveat includes code such as CWprint(substr($foo,$a,$b)=$bar) or CW(substr($foo,$a,$b)=$bar)=$fud (where CW$foo is changed via the substring assignment, and then the substr is used again), or where a substr() is aliased via a CWforeach loop or passed as a parameter or a reference to it is taken and then the alias, parameter, or deref'd reference either is used after the original EXPR has been changed or is assigned to and then used a second time.

symlink OLDFILE,NEWFILE

Creates a new filename symbolically linked to the old filename. Returns CW1 for success, CW0 otherwise. On systems that don't support symbolic links, produces a fatal error at run time. To check for that, use eval:

    $symlink_exists = eval { symlink("",""); 1 };

syscall NUMBER, LIST

Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, produces a fatal error. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. You can't use a string literal (or other read-only string) as an argument to CWsyscall because Perl has to assume that any string pointer might be written through. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add CW0 to them to force them to look like numbers. This emulates the CWsyswrite function (or vice versa):

    require 'syscall.ph';               # may need to run h2ph
    $s = "hi there\n";
    syscall(&SYS_write, fileno(STDOUT), $s, length $s);

Note that Perl supports passing of up to only 14 arguments to your system call, which in practice should usually suffice. Syscall returns whatever value returned by the system call it calls. If the system call fails, CWsyscall returns CW-1 and sets CW$! (errno). Note that some system calls can legitimately return CW-1. The proper way to handle such calls is to assign CW$!=0; before the call and check the value of CW$! if syscall returns CW-1. There's a problem with CWsyscall(&SYS_pipe): it returns the file number of the read end of the pipe it creates. There is no way to retrieve the file number of the other end. You can avoid this problem by using CWpipe instead.

sysopen FILEHANDLE,FILENAME,MODE

sysopen FILEHANDLE,FILENAME,MODE,PERMS

Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. This function calls the underlying operating system's CWopen function with the parameters FILENAME, MODE, PERMS. The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module CWFcntl. See the documentation of your operating system's CWopen to see which values and flag bits are available. You may combine several flags using the CW|-operator. Some of the most common values are CWO_RDONLY for opening the file in read-only mode, CWO_WRONLY for opening the file in write-only mode, and CWO_RDWR for opening the file in read-write mode. For historical reasons, some values work on almost every system supported by perl: zero means read-only, one means write-only, and two means read/write. We know that these values do not work under OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to use them in new code. If the file named by FILENAME does not exist and the CWopen call creates it (typically because MODE includes the CWO_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument to CWsysopen, Perl uses the octal value CW0666. These permission values need to be in octal, and are modified by your process's current CWumask. In many systems the CWO_EXCL flag is available for opening files in exclusive mode. This is not locking: exclusiveness means here that if the file already exists, sysopen() fails. CWO_EXCL may not work on network filesystems, and has no effect unless the CWO_CREAT flag is set as well. Setting CWO_CREAT|O_EXCL prevents the file from being opened if it is a symbolic link. It does not protect against symbolic links in the file's path. Sometimes you may want to truncate an already-existing file. This can be done using the CWO_TRUNC flag. The behavior of CWO_TRUNC with CWO_RDONLY is undefined. You should seldom if ever use CW0644 as argument to CWsysopen, because that takes away the user's option to have a more permissive umask. Better to omit it. See the perlfunc(1) entry on CWumask for more on this. Note that CWsysopen depends on the fdopen() C library function. On many UNIX systems, fdopen() is known to fail when file descriptors exceed a certain value, typically 255. If you need more file descriptors than that, consider rebuilding Perl to use the CWsfio library, or perhaps using the POSIX::open() function. See perlopentut for a kinder, gentler explanation of opening files.

sysread FILEHANDLE,SCALAR,LENGTH,OFFSET

sysread FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using the system call read(2). It bypasses buffered IO, so mixing this with other kinds of reads, CWprint, CWwrite, CWseek, CWtell, or CWeof can cause confusion because the perlio or stdio layers usually buffers data. Returns the number of bytes actually read, CW0 at end of file, or undef if there was an error (in the latter case CW$! is also set). SCALAR will be grown or shrunk so that the last byte actually read is the last byte of the scalar after the read. An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with CW"\0" bytes before the result of the read is appended. There is no syseof() function, which is ok, since eof() doesn't work very well on device files (like ttys) anyway. Use sysread() and check for a return value for 0 to decide whether you're done. Note that if the filehandle has been marked as CW:utf8 Unicode characters are read instead of bytes (the LENGTH, OFFSET, and the return value of sysread() are in Unicode characters). The CW:encoding(...) layer implicitly introduces the CW:utf8 layer. See binmode, open, and the CWopen pragma, open.

sysseek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's system position in bytes using the system call lseek(2). FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are CW0 to set the new position to POSITION, CW1 to set the it to the current position plus POSITION, and CW2 to set it to EOF plus POSITION (typically negative). Note the in bytes: even if the filehandle has been set to operate on characters (for example by using the CW:utf8 I/O layer), tell() will return byte offsets, not character offsets (because implementing that would render sysseek() very slow). sysseek() bypasses normal buffered IO, so mixing this with reads (other than CWsysread, for example CW<> or read()) CWprint, CWwrite, CWseek, CWtell, or CWeof may cause confusion. For WHENCE, you may also use the constants CWSEEK_SET, CWSEEK_CUR, and CWSEEK_END (start of the file, current position, end of the file) from the Fcntl module. Use of the constants is also more portable than relying on 0, 1, and 2. For example to define a systell function:

        use Fcntl 'SEEK_CUR';
        sub systell { sysseek($_[0], 0, SEEK_CUR) }

Returns the new position, or the undefined value on failure. A position of zero is returned as the string CW"0 but true"; thus CWsysseek returns true on success and false on failure, yet you can still easily determine the new position.

system LIST

system PROGRAM LIST

Does exactly the same thing as CWexec LIST, except that a fork is done first, and the parent process waits for the child process to complete. Note that argument processing varies depending on the number of arguments. If there is more than one argument in LIST, or if LIST is an array with more than one value, starts the program given by the first element of the list with arguments given by the rest of the list. If there is only one scalar argument, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is CW/bin/sh -c on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to CWexecvp, which is more efficient. Beginning with v5.6.0, Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set CW$| ($AUTOFLUSH in English) or call the CWautoflush() method of CWIO::Handle on any open handles. The return value is the exit status of the program as returned by the CWwait call. To get the actual exit value shift right by eight (see below). See also exec. This is not what you want to use to capture the output from a command, for that you should use merely backticks or CWqx//, as described in `STRING` in perlop. Return value of -1 indicates a failure to start the program (inspect $! for the reason). Like CWexec, CWsystem allows you to lie to a program about its name if you use the CWsystem PROGRAM LIST syntax. Again, see exec. Since CWSIGINT and CWSIGQUIT are ignored during the execution of CWsystem, if you expect your program to terminate on receipt of these signals you will need to arrange to do so yourself based on the return value.

    @args = ("command", "arg1", "arg2");
    system(@args) == 0
         or die "system @args failed: $?"

You can check all the failure possibilities by inspecting CW$? like this:

    if ($? == -1) {
        print "failed to execute: $!\n";
    }
    elsif ($? & 127) {
        printf "child died with signal %d, %s coredump\n",
            ($? & 127),  ($? & 128) ? 'with' : 'without';
    }
    else {
        printf "child exited with value %d\n", $? >> 8;
    }

or more portably by using the W*() calls of the POSIX extension; see perlport for more information. When the arguments get executed via the system shell, results and return codes will be subject to its quirks and capabilities. See `STRING` in perlop and exec for details.

syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET

syswrite FILEHANDLE,SCALAR,LENGTH

syswrite FILEHANDLE,SCALAR

Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using the system call write(2). If LENGTH is not specified, writes whole SCALAR. It bypasses buffered IO, so mixing this with reads (other than CWsysread()), CWprint, CWwrite, CWseek, CWtell, or CWeof may cause confusion because the perlio and stdio layers usually buffers data. Returns the number of bytes actually written, or CWundef if there was an error (in this case the errno variable CW$! is also set). If the LENGTH is greater than the available data in the SCALAR after the OFFSET, only as much data as is available will be written. An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many characters counting backwards from the end of the string. In the case the SCALAR is empty you can use OFFSET but only zero offset. Note that if the filehandle has been marked as CW:utf8, Unicode characters are written instead of bytes (the LENGTH, OFFSET, and the return value of syswrite() are in UTF-8 encoded Unicode characters). The CW:encoding(...) layer implicitly introduces the CW:utf8 layer. See binmode, open, and the CWopen pragma, open.

tell FILEHANDLE

tell

Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read. Note the in bytes: even if the filehandle has been set to operate on characters (for example by using the CW:utf8 open layer), tell() will return byte offsets, not character offsets (because that would render seek() and tell() rather slow). The return value of tell() for the standard streams like the STDIN depends on the operating system: it may return -1 or something else. tell() on pipes, fifos, and sockets usually returns -1. There is no CWsystell function. Use CWsysseek(FH, 0, 1) for that. Do not use tell() (or other buffered I/O operations) on a file handle that has been manipulated by sysread(), syswrite() or sysseek(). Those functions ignore the buffering, while tell() does not.

telldir DIRHANDLE

Returns the current position of the CWreaddir routines on DIRHANDLE. Value may be given to CWseekdir to access a particular location in a directory. Has the same caveats about possible directory compaction as the corresponding system library routine.

tie VARIABLE,CLASSNAME,LIST

This function binds a variable to a package class that will provide the implementation for the variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a class implementing objects of correct type. Any additional arguments are passed to the CWnew method of the class (meaning CWTIESCALAR, CWTIEHANDLE, CWTIEARRAY, or CWTIEHASH). Typically these are arguments such as might be passed to the CWdbm_open() function of C. The object returned by the CWnew method is also returned by the CWtie function, which would be useful if you want to access other methods in CLASSNAME. Note that functions such as CWkeys and CWvalues may return huge lists when used on large objects, like DBM files. You may prefer to use the CWeach function to iterate over such. Example:

    # print out history file offsets
    use NDBM_File;
    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);

A class implementing a hash should have the following methods:

    TIEHASH classname, LIST
    FETCH this, key
    STORE this, key, value
    DELETE this, key
    CLEAR this
    EXISTS this, key
    FIRSTKEY this
    NEXTKEY this, lastkey
    SCALAR this
    DESTROY this
    UNTIE this

A class implementing an ordinary array should have the following methods:

    TIEARRAY classname, LIST
    FETCH this, key
    STORE this, key, value
    FETCHSIZE this
    STORESIZE this, count
    CLEAR this
    PUSH this, LIST
    POP this
    SHIFT this
    UNSHIFT this, LIST
    SPLICE this, offset, length, LIST
    EXTEND this, count
    DESTROY this
    UNTIE this

A class implementing a file handle should have the following methods:

    TIEHANDLE classname, LIST
    READ this, scalar, length, offset
    READLINE this
    GETC this
    WRITE this, scalar, length, offset
    PRINT this, LIST
    PRINTF this, format, LIST
    BINMODE this
    EOF this
    FILENO this
    SEEK this, position, whence
    TELL this
    OPEN this, mode, LIST
    CLOSE this
    DESTROY this
    UNTIE this

A class implementing a scalar should have the following methods:

    TIESCALAR classname, LIST
    FETCH this,
    STORE this, value
    DESTROY this
    UNTIE this

Not all methods indicated above need be implemented. See perltie, Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle. Unlike CWdbmopen, the CWtie function will not use or require a module for youyou need to do that explicitly yourself. See DB_File or the Config module for interesting CWtie implementations. For further details see perltie, tied VARIABLE.

tied VARIABLE

Returns a reference to the object underlying VARIABLE (the same value that was originally returned by the CWtie call that bound the variable to a package.) Returns the undefined value if VARIABLE isn't tied to a package.

time

Returns the number of non-leap seconds since whatever time the system considers to be the epoch, suitable for feeding to CWgmtime and CWlocaltime. On most systems the epoch is 00:00:00 UTC, January 1, 1970; a prominent exception being Mac OS Classic which uses 00:00:00, January 1, 1904 in the current local time zone for its epoch. For measuring time in better granularity than one second, you may use either the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution), or if you have gettimeofday(2), you may be able to use the CWsyscall interface of Perl. See perlfaq8 for details.

times

Returns a four-element list giving the user and system times, in seconds, for this process and the children of this process.

    ($user,$system,$cuser,$csystem) = times;

In scalar context, CWtimes returns CW$user.

tr///

The transliteration operator. Same as CWy///. See perlop.

truncate FILEHANDLE,LENGTH

truncate EXPR,LENGTH

Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a fatal error if truncate isn't implemented on your system. Returns true if successful, the undefined value otherwise. The behavior is undefined if LENGTH is greater than the length of the file.

uc EXPR

uc

Returns an uppercased version of EXPR. This is the internal function implementing the CW\U escape in double-quoted strings. Respects current LC_CTYPE locale if CWuse locale in force. See perllocale and perlunicode for more details about locale and Unicode support. It does not attempt to do titlecase mapping on initial letters. See CWucfirst for that. If EXPR is omitted, uses CW$_.

ucfirst EXPR

ucfirst

Returns the value of EXPR with the first character in uppercase (titlecase in Unicode). This is the internal function implementing the CW\u escape in double-quoted strings. Respects current LC_CTYPE locale if CWuse locale in force. See perllocale and perlunicode for more details about locale and Unicode support. If EXPR is omitted, uses CW$_.

umask EXPR

umask

Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask. The Unix permission CWrwxr-x--- is represented as three sets of three bits, or three octal digits: CW0750 (the leading 0 indicates octal and isn't one of the digits). The CWumask value is such a number representing disabled permissions bits. The permission (or mode) values you pass CWmkdir or CWsysopen are modified by your umask, so even if you tell CWsysopen to create a file with permissions CW0777, if your umask is CW0022 then the file will actually be created with permissions CW0755. If your CWumask were CW0027 (group can't write; others can't read, write, or execute), then passing CWsysopen CW0666 would create a file with mode CW0640 (CW0666 &~ 027 is CW0640). Here's some advice: supply a creation mode of CW0666 for regular files (in CWsysopen) and one of CW0777 for directories (in CWmkdir) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of CW022, CW027, or even the particularly antisocial mask of CW077. Programs should rarely if ever make policy decisions better left to the user. The exception to this is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on. If umask(2) is not implemented on your system and you are trying to restrict access for yourself (i.e., (EXPR & 0700) > 0), produces a fatal error at run time. If umask(2) is not implemented and you are not trying to restrict access for yourself, returns CWundef. Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also oct, if all you have is a string.

undef EXPR

undef

Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an array (using CW@), a hash (using CW%), a subroutine (using CW&), or a typeglob (using CW*). (Saying CWundef $hash{$key} will probably not do what you expect on most predefined variables or DBM list values, so don't do that; see delete.) Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine, assign to a variable or pass as a parameter. Examples:

    undef $foo;
    undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
    undef @ary;
    undef %hash;
    undef &mysub;
    undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
    return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
    select undef, undef, undef, 0.25;
    ($a, $b, undef, $c) = &foo;       # Ignore third value returned

Note that this is a unary operator, not a list operator.

unlink LIST

unlink

Deletes a list of files. Returns the number of files successfully deleted.

    $cnt = unlink 'a', 'b', 'c';
    unlink @goners;
    unlink <*.bak>;

Note: CWunlink will not delete directories unless you are superuser and the -U flag is supplied to Perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Use CWrmdir instead. If LIST is omitted, uses CW$_.

unpack TEMPLATE,EXPR

CWunpack does the reverse of CWpack: it takes a string and expands it out into a list of values. (In scalar context, it returns merely the first value produced.) The string is broken into chunks described by the TEMPLATE. Each chunk is converted separately to a value. Typically, either the string is a result of CWpack, or the bytes of the string represent a C structure of some kind. The TEMPLATE has the same format as in the CWpack function. Here's a subroutine that does substring:

    sub substr {
        my($what,$where,$howmuch) = @_;
        unpack("x$where a$howmuch", $what);
    }

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition to fields allowed in pack(), you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. Checksum is calculated by summing numeric values of expanded values (for string fields the sum of CWord($char) is taken, for bit fields the sum of zeroes and ones). For example, the following computes the same number as the System V sum program:

    $checksum = do {
        local $/;  # slurp!
        unpack("%32C*",<>) % 65535;
    };

The following efficiently counts the number of set bits in a bit vector:

    $setbits = unpack("%32b*", $selectmask);

The CWp and CWP formats should be used with care. Since Perl has no way of checking whether the value passed to CWunpack() corresponds to a valid memory location, passing a pointer value that's not known to be valid is likely to have disastrous consequences. If there are more pack codes or if the repeat count of a field or a group is larger than what the remainder of the input string allows, the result is not well defined: in some cases, the repeat count is decreased, or CWunpack() will produce null strings or zeroes, or terminate with an error. If the input string is longer than one described by the TEMPLATE, the rest is ignored. See pack for more examples and notes.

untie VARIABLE

Breaks the binding between a variable and a package. (See CWtie.) Has no effect if the variable is not tied.

unshift ARRAY,LIST

Does the opposite of a CWshift. Or the opposite of a CWpush, depending on how you look at it. Prepends list to the front of the array, and returns the new number of elements in the array.

    unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;

Note the LIST is prepended whole, not one element at a time, so the prepended elements stay in the same order. Use CWreverse to do the reverse.

use Module VERSION LIST

use Module VERSION

use Module LIST

use Module

use VERSION

Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to

    BEGIN { require Module; import Module LIST; }

except that Module must be a bareword. VERSION may be either a numeric argument such as 5.006, which will be compared to CW$], or a literal of the form v5.6.1, which will be compared to CW$^V (aka CW$PERL_VERSION. A fatal error is produced if VERSION is greater than the version of the current Perl interpreter; Perl will not attempt to parse the rest of the file. Compare with require, which can do a similar check at run time. Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads to misleading error messages under earlier versions of Perl which do not support this syntax. The equivalent numeric version should be used instead.

    use v5.6.1;         # compile time version check
    use 5.6.1;          # ditto
    use 5.006_001;      # ditto; preferred for backwards compatibility

This is often useful if you need to check the current Perl version before CWuseing library modules that have changed in incompatible ways from older versions of Perl. (We try not to do this more than we have to.) The CWBEGIN forces the CWrequire and CWimport to happen at compile time. The CWrequire makes sure the module is loaded into memory if it hasn't been yet. The CWimport is not a builtinit's just an ordinary static method call into the CWModule package to tell the module to import the list of features back into the current package. The module can implement its CWimport method any way it likes, though most modules just choose to derive their CWimport method via inheritance from the CWExporter class that is defined in the CWExporter module. See Exporter. If no CWimport method can be found then the call is skipped. If you do not want to call the package's CWimport method (for instance, to stop your namespace from being altered), explicitly supply the empty list:

    use Module ();

That is exactly equivalent to

    BEGIN { require Module }

If the VERSION argument is present between Module and LIST, then the CWuse will call the VERSION method in class Module with the given version as an argument. The default VERSION method, inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the variable CW$Module::VERSION. Again, there is a distinction between omitting LIST (CWimport called with no arguments) and an explicit empty LIST CW() (CWimport not called). Note that there is no comma after VERSION! Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Currently implemented pragmas are:

    use constant;
    use diagnostics;
    use integer;
    use sigtrap  qw(SEGV BUS);
    use strict   qw(subs vars refs);
    use subs     qw(afunc blurfl);
    use warnings qw(all);
    use sort     qw(stable _quicksort _mergesort);

Some of these pseudo-modules import semantics into the current block scope (like CWstrict or CWinteger, unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file). There's a corresponding CWno command that unimports meanings imported by CWuse, i.e., it calls CWunimport Module LIST instead of CWimport.

    no integer;
    no strict 'refs';
    no warnings;

See perlmodlib for a list of standard modules and pragmas. See perlrun for the CW-M and CW-m command-line options to perl that give CWuse functionality from the command-line.

utime LIST

Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERICAL access and modification times, in that order. Returns the number of files successfully changed. The inode change time of each file is set to the current time. For example, this code has the same effect as the Unix touch(1) command when the files already exist and belong to the user running the program:

    #!/usr/bin/perl
    $atime = $mtime = time;
    utime $atime, $mtime, @ARGV;

Since perl 5.7.2, if the first two elements of the list are CWundef, then the utime(2) function in the C library will be called with a null second argument. On most systems, this will set the file's access and modification times to the current time (i.e. equivalent to the example above) and will even work on other users' files where you have write permission:

    utime undef, undef, @ARGV;

Under NFS this will use the time of the NFS server, not the time of the local machine. If there is a time synchronization problem, the NFS server and local machine will have different times. The Unix touch(1) command will in fact normally use this form instead of the one shown in the first example. Note that only passing one of the first two elements as CWundef will be equivalent of passing it as 0 and will not have the same effect as described when they are both CWundef. This case will also trigger an uninitialized warning.

values HASH

Returns a list consisting of all the values of the named hash. (In a scalar context, returns the number of values.) The values are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be the same order as either the CWkeys or CWeach function would produce on the same (unmodified) hash. Since Perl 5.8.1 the ordering is different even between different runs of Perl for security reasons (see Algorithmic Complexity Attacks in perlsec). As a side effect, calling values() resets the HASH's internal iterator, see each. (In particular, calling values() in void context resets the iterator with no other overhead.) Note that the values are not copied, which means modifying them will modify the contents of the hash:

    for (values %hash)      { s/foo/bar/g }   # modifies %hash values
    for (@hash{keys %hash}) { s/foo/bar/g }   # same

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cd ~tigrou/pwet.fr/man/linux/Commandes/perlfunc

man perlfunc (Commandes) - Perl builtin functions

NAME

DESCRIPTION

Perl Functions by Category

Portability

Alphabetical Listing of Perl Functions

Sections du manuel

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