perliol - C API for Perl's implementation of IO in Layers.

    /* Defining a layer ... */
    #include <perliol.h>

This document describes the behavior and implementation of the PerlIO abstraction described in perlapio when CWUSE_PERLIO is defined (and CWUSE_SFIO is not).

The PerlIO abstraction was introduced in perl5.003_02 but languished as just an abstraction until perl5.7.0. However during that time a number of perl extensions switched to using it, so the API is mostly fixed to maintain (source) compatibility.

The aim of the implementation is to provide the PerlIO API in a flexible and platform neutral manner. It is also a trial of an Object Oriented C, with vtables approach which may be applied to perl6.

PerlIO is a stack of layers.

The low levels of the stack work with the low-level operating system calls (file descriptors in C) getting bytes in and out, the higher layers of the stack buffer, filter, and otherwise manipulate the I/O, and return characters (or bytes) to Perl. Terms above and below are used to refer to the relative positioning of the stack layers.

A layer contains a vtable, the table of I/O operations (at C level a table of function pointers), and status flags. The functions in the vtable implement operations like open, read, and write.

When I/O, for example read, is requested, the request goes from Perl first down the stack using read functions of each layer, then at the bottom the input is requested from the operating system services, then the result is returned up the stack, finally being interpreted as Perl data.

The requests do not necessarily go always all the way down to the operating system: that's where PerlIO buffering comes into play.

When you do an open() and specify extra PerlIO layers to be deployed, the layers you specify are pushed on top of the already existing default stack. One way to see it is that operating system is on the left and Perl is on the right.

What exact layers are in this default stack depends on a lot of things: your operating system, Perl version, Perl compile time configuration, and Perl runtime configuration. See PerlIO, PERLIO in perlrun, and open for more information.

binmode() operates similarly to open(): by default the specified layers are pushed on top of the existing stack.

However, note that even as the specified layers are pushed on top for open() and binmode(), this doesn't mean that the effects are limited to the top: PerlIO layers can be very 'active' and inspect and affect layers also deeper in the stack. As an example there is a layer called raw which repeatedly pops layers until it reaches the first layer that has declared itself capable of handling binary data. The pushed layers are processed in left-to-right order.

sysopen() operates (unsurprisingly) at a lower level in the stack than open(). For example in UNIX or UNIX-like systems sysopen() operates directly at the level of file descriptors: in the terms of PerlIO layers, it uses only the unix layer, which is a rather thin wrapper on top of the UNIX file descriptors.

Initial discussion of the ability to modify IO streams behaviour used the term discipline for the entities which were added. This came (I believe) from the use of the term in sfio, which in turn borrowed it from line disciplines on Unix terminals. However, this document (and the C code) uses the term layer.

This is, I hope, a natural term given the implementation, and should avoid connotations that are inherent in earlier uses of discipline for things which are rather different.

The basic data structure is a PerlIOl:

        typedef struct _PerlIO PerlIOl;
        typedef struct _PerlIO_funcs PerlIO_funcs;
        typedef PerlIOl *PerlIO;

        struct _PerlIO
        {
         PerlIOl *      next;       /* Lower layer */
         PerlIO_funcs * tab;        /* Functions for this layer */
         IV             flags;      /* Various flags for state */
        };

A CWPerlIOl * is a pointer to the struct, and the application level CWPerlIO * is a pointer to a CWPerlIOl * - i.e. a pointer to a pointer to the struct. This allows the application level CWPerlIO * to remain constant while the actual CWPerlIOl * underneath changes. (Compare perl's CWSV * which remains constant while its CWsv_any field changes as the scalar's type changes.) An IO stream is then in general represented as a pointer to this linked-list of layers.

It should be noted that because of the double indirection in a CWPerlIO *, a CW&(perlio->next) is a CWPerlIO *, and so to some degree at least one layer can use the standard API on the next layer down.

A layer is composed of two parts:

1.

The functions and attributes of the layer class.

2.

The per-instance data for a particular handle.

The functions and attributes are accessed via the tab (for table) member of CWPerlIOl. The functions (methods of the layer class) are fixed, and are defined by the CWPerlIO_funcs type. They are broadly the same as the public CWPerlIO_xxxxx functions:

  struct _PerlIO_funcs
  {
   Size_t               fsize;
   char *               name;
   Size_t               size;
   IV           kind;
   IV           (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab);
   IV           (*Popped)(pTHX_ PerlIO *f);
   PerlIO *     (*Open)(pTHX_ PerlIO_funcs *tab,
                        AV *layers, IV n,
                        const char *mode,
                        int fd, int imode, int perm,
                        PerlIO *old,
                        int narg, SV **args);
   IV           (*Binmode)(pTHX_ PerlIO *f);
   SV *         (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
   IV           (*Fileno)(pTHX_ PerlIO *f);
   PerlIO *     (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags)
   /* Unix-like functions - cf sfio line disciplines */
   SSize_t      (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
   SSize_t      (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
   SSize_t      (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
   IV           (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
   Off_t        (*Tell)(pTHX_ PerlIO *f);
   IV           (*Close)(pTHX_ PerlIO *f);
   /* Stdio-like buffered IO functions */
   IV           (*Flush)(pTHX_ PerlIO *f);
   IV           (*Fill)(pTHX_ PerlIO *f);
   IV           (*Eof)(pTHX_ PerlIO *f);
   IV           (*Error)(pTHX_ PerlIO *f);
   void         (*Clearerr)(pTHX_ PerlIO *f);
   void         (*Setlinebuf)(pTHX_ PerlIO *f);
   /* Perl's snooping functions */
   STDCHAR *    (*Get_base)(pTHX_ PerlIO *f);
   Size_t       (*Get_bufsiz)(pTHX_ PerlIO *f);
   STDCHAR *    (*Get_ptr)(pTHX_ PerlIO *f);
   SSize_t      (*Get_cnt)(pTHX_ PerlIO *f);
   void         (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
  };

The first few members of the struct give a function table size for compatibility check name for the layer, the size to CWmalloc for the per-instance data, and some flags which are attributes of the class as whole (such as whether it is a buffering layer), then follow the functions which fall into four basic groups:

1.

Opening and setup functions

2.

Basic IO operations

3.

Stdio class buffering options.

4.

Functions to support Perl's traditional fast access to the buffer.

A layer does not have to implement all the functions, but the whole table has to be present. Unimplemented slots can be NULL (which will result in an error when called) or can be filled in with stubs to inherit behaviour from a base class. This inheritance is fixed for all instances of the layer, but as the layer chooses which stubs to populate the table, limited multiple inheritance is possible.

The per-instance data are held in memory beyond the basic PerlIOl struct, by making a PerlIOl the first member of the layer's struct thus:

        typedef struct
        {
         struct _PerlIO base;       /* Base "class" info */
         STDCHAR *      buf;        /* Start of buffer */
         STDCHAR *      end;        /* End of valid part of buffer */
         STDCHAR *      ptr;        /* Current position in buffer */
         Off_t          posn;       /* Offset of buf into the file */
         Size_t         bufsiz;     /* Real size of buffer */
         IV             oneword;    /* Emergency buffer */
        } PerlIOBuf;

In this way (as for perl's scalars) a pointer to a PerlIOBuf can be treated as a pointer to a PerlIOl.

                table           perlio          unix
            |           |
            +-----------+    +----------+    +--------+
   PerlIO ->|           |--->|  next    |--->|  NULL  |
            +-----------+    +----------+    +--------+
            |           |    |  buffer  |    |   fd   |
            +-----------+    |          |    +--------+
            |           |    +----------+

The above attempts to show how the layer scheme works in a simple case. The application's CWPerlIO * points to an entry in the table(s) representing open (allocated) handles. For example the first three slots in the table correspond to CWstdin,CWstdout and CWstderr. The table in turn points to the current top layer for the handle - in this case an instance of the generic buffering layer perlio. That layer in turn points to the next layer down - in this case the lowlevel unix layer.

The above is roughly equivalent to a stdio buffered stream, but with much more flexibility:

•

If Unix level CWread/CWwrite/CWlseek is not appropriate for (say) sockets then the unix layer can be replaced (at open time or even dynamically) with a socket layer.

•

Different handles can have different buffering schemes. The top layer could be the mmap layer if reading disk files was quicker using CWmmap than CWread. An unbuffered stream can be implemented simply by not having a buffer layer.

•

Extra layers can be inserted to process the data as it flows through. This was the driving need for including the scheme in perl 5.7.0+ - we needed a mechanism to allow data to be translated between perl's internal encoding (conceptually at least Unicode as UTF-8), and the native format used by the system. This is provided by the :encoding(xxxx) layer which typically sits above the buffering layer.

•

A layer can be added that does \n to CRLF translation. This layer can be used on any platform, not just those that normally do such things.

The generic flag bits are a hybrid of CWO_XXXXX style flags deduced from the mode string passed to CWPerlIO_open(), and state bits for typical buffer layers.

PERLIO_F_EOF

End of file.

PERLIO_F_CANWRITE

Writes are permitted, i.e. opened as w or r+ or a, etc.

PERLIO_F_CANREAD

Reads are permitted i.e. opened r or w+ (or even a+ - ick).

PERLIO_F_ERROR

An error has occurred (for CWPerlIO_error()).

PERLIO_F_TRUNCATE

Truncate file suggested by open mode.

PERLIO_F_APPEND

All writes should be appends.

PERLIO_F_CRLF

Layer is performing Win32-like \n mapped to CR,LF for output and CR,LF mapped to \n for input. Normally the provided crlf layer is the only layer that need bother about this. CWPerlIO_binmode() will mess with this flag rather than add/remove layers if the CWPERLIO_K_CANCRLF bit is set for the layers class.

PERLIO_F_UTF8

Data written to this layer should be UTF-8 encoded; data provided by this layer should be considered UTF-8 encoded. Can be set on any layer by :utf8 dummy layer. Also set on :encoding layer.

PERLIO_F_UNBUF

Layer is unbuffered - i.e. write to next layer down should occur for each write to this layer.

PERLIO_F_WRBUF

The buffer for this layer currently holds data written to it but not sent to next layer.

PERLIO_F_RDBUF

The buffer for this layer currently holds unconsumed data read from layer below.

PERLIO_F_LINEBUF

Layer is line buffered. Write data should be passed to next layer down whenever a \n is seen. Any data beyond the \n should then be processed.

PERLIO_F_TEMP

File has been CWunlink()ed, or should be deleted on CWclose().

PERLIO_F_OPEN

Handle is open.

PERLIO_F_FASTGETS

This instance of this layer supports the "fast CWgets" interface. Normally set based on CWPERLIO_K_FASTGETS for the class and by the existence of the function(s) in the table. However a class that normally provides that interface may need to avoid it on a particular instance. The pending layer needs to do this when it is pushed above a layer which does not support the interface. (Perl's CWsv_gets() does not expect the streams fast CWgets behaviour to change during one get.)

fsize

        Size_t fsize;

Size of the function table. This is compared against the value PerlIO code knows as a compatibility check. Future versions may be able to tolerate layers compiled against an old version of the headers.

name

        char * name;

The name of the layer whose open() method Perl should invoke on open(). For example if the layer is called APR, you will call:

  open $fh, ">:APR", ...

and Perl knows that it has to invoke the PerlIOAPR_open() method implemented by the APR layer.

size

        Size_t size;

The size of the per-instance data structure, e.g.:

  sizeof(PerlIOAPR)

If this field is zero then CWPerlIO_pushed does not malloc anything and assumes layer's Pushed function will do any required layer stack manipulation - used to avoid malloc/free overhead for dummy layers. If the field is non-zero it must be at least the size of CWPerlIOl, CWPerlIO_pushed will allocate memory for the layer's data structures and link new layer onto the stream's stack. (If the layer's Pushed method returns an error indication the layer is popped again.)

kind

        IV kind;

* PERLIO_K_BUFFERED

The layer is buffered.

* PERLIO_K_RAW

The layer is acceptable to have in a binmode(FH) stack - i.e. it does not (or will configure itself not to) transform bytes passing through it.

* PERLIO_K_CANCRLF

Layer can translate between \n and CRLF line ends.

* PERLIO_K_FASTGETS

Layer allows buffer snooping.

* PERLIO_K_MULTIARG

Used when the layer's open() accepts more arguments than usual. The extra arguments should come not before the CWMODE argument. When this flag is used it's up to the layer to validate the args.

Pushed

        IV      (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);

The only absolutely mandatory method. Called when the layer is pushed onto the stack. The CWmode argument may be NULL if this occurs post-open. The CWarg will be non-CWNULL if an argument string was passed. In most cases this should call CWPerlIOBase_pushed() to convert CWmode into the appropriate CWPERLIO_F_XXXXX flags in addition to any actions the layer itself takes. If a layer is not expecting an argument it need neither save the one passed to it, nor provide CWGetarg() (it could perhaps CWPerl_warn that the argument was un-expected). Returns 0 on success. On failure returns -1 and should set errno.

Popped

        IV      (*Popped)(pTHX_ PerlIO *f);

Called when the layer is popped from the stack. A layer will normally be popped after CWClose() is called. But a layer can be popped without being closed if the program is dynamically managing layers on the stream. In such cases CWPopped() should free any resources (buffers, translation tables, ...) not held directly in the layer's struct. It should also CWUnread() any unconsumed data that has been read and buffered from the layer below back to that layer, so that it can be re-provided to what ever is now above. Returns 0 on success and failure. If CWPopped() returns true then perlio.c assumes that either the layer has popped itself, or the layer is super special and needs to be retained for other reasons. In most cases it should return false.

Open

        PerlIO *        (*Open)(...);

The CWOpen() method has lots of arguments because it combines the functions of perl's CWopen, CWPerlIO_open, perl's CWsysopen, CWPerlIO_fdopen and CWPerlIO_reopen. The full prototype is as follows:

 PerlIO *       (*Open)(pTHX_ PerlIO_funcs *tab,
                        AV *layers, IV n,
                        const char *mode,
                        int fd, int imode, int perm,
                        PerlIO *old,
                        int narg, SV **args);

Open should (perhaps indirectly) call CWPerlIO_allocate() to allocate a slot in the table and associate it with the layers information for the opened file, by calling CWPerlIO_push. The layers AV is an array of all the layers destined for the CWPerlIO *, and any arguments passed to them, n is the index into that array of the layer being called. The macro CWPerlIOArg will return a (possibly CWNULL) SV * for the argument passed to the layer. The mode string is an "CWfopen()-like" string which would match the regular expression CW/^[I#]?[rwa]\+?[bt]?$/. The CW'I' prefix is used during creation of CWstdin..CWstderr via special CWPerlIO_fdopen calls; the CW'#' prefix means that this is CWsysopen and that imode and perm should be passed to CWPerlLIO_open3; CW'r' means read, CW'w' means write and CW'a' means append. The CW'+' suffix means that both reading and writing/appending are permitted. The CW'b' suffix means file should be binary, and CW't' means it is text. (Almost all layers should do the IO in binary mode, and ignore the b/t bits. The CW:crlf layer should be pushed to handle the distinction.) If old is not CWNULL then this is a CWPerlIO_reopen. Perl itself does not use this (yet?) and semantics are a little vague. If fd not negative then it is the numeric file descriptor fd, which will be open in a manner compatible with the supplied mode string, the call is thus equivalent to CWPerlIO_fdopen. In this case nargs will be zero. If nargs is greater than zero then it gives the number of arguments passed to CWopen, otherwise it will be 1 if for example CWPerlIO_open was called. In simple cases SvPV_nolen(*args) is the pathname to open. Having said all that translation-only layers do not need to provide CWOpen() at all, but rather leave the opening to a lower level layer and wait to be pushed. If a layer does provide CWOpen() it should normally call the CWOpen() method of next layer down (if any) and then push itself on top if that succeeds. If CWPerlIO_push was performed and open has failed, it must CWPerlIO_pop itself, since if it's not, the layer won't be removed and may cause bad problems. Returns CWNULL on failure.

Binmode

        IV        (*Binmode)(pTHX_ PerlIO *f);

Optional. Used when CW:raw layer is pushed (explicitly or as a result of binmode(FH)). If not present layer will be popped. If present should configure layer as binary (or pop itself) and return 0. If it returns -1 for error CWbinmode will fail with layer still on the stack.

Getarg

        SV *      (*Getarg)(pTHX_ PerlIO *f,
                            CLONE_PARAMS *param, int flags);

Optional. If present should return an SV * representing the string argument passed to the layer when it was pushed. e.g. :encoding(ascii) would return an SvPV with value ascii. (param and flags arguments can be ignored in most cases) CWDup uses CWGetarg to retrieve the argument originally passed to CWPushed, so you must implement this function if your layer has an extra argument to CWPushed and will ever be CWDuped.

Fileno

        IV        (*Fileno)(pTHX_ PerlIO *f);

Returns the Unix/Posix numeric file descriptor for the handle. Normally CWPerlIOBase_fileno() (which just asks next layer down) will suffice for this. Returns -1 on error, which is considered to include the case where the layer cannot provide such a file descriptor.

Dup

        PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
                        CLONE_PARAMS *param, int flags);

XXX: Needs more docs. Used as part of the clone process when a thread is spawned (in which case param will be non-NULL) and when a stream is being duplicated via '&' in the CWopen. Similar to CWOpen, returns PerlIO* on success, CWNULL on failure.

Read

        SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);

Basic read operation. Typically will call CWFill and manipulate pointers (possibly via the API). CWPerlIOBuf_read() may be suitable for derived classes which provide fast gets methods. Returns actual bytes read, or -1 on an error.

Unread

        SSize_t (*Unread)(pTHX_ PerlIO *f,
                          const void *vbuf, Size_t count);

A superset of stdio's CWungetc(). Should arrange for future reads to see the bytes in CWvbuf. If there is no obviously better implementation then CWPerlIOBase_unread() provides the function by pushing a fake pending layer above the calling layer. Returns the number of unread chars.

Write

        SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);

Basic write operation. Returns bytes written or -1 on an error.

Seek

        IV      (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);

Position the file pointer. Should normally call its own CWFlush method and then the CWSeek method of next layer down. Returns 0 on success, -1 on failure.

Tell

        Off_t   (*Tell)(pTHX_ PerlIO *f);

Return the file pointer. May be based on layers cached concept of position to avoid overhead. Returns -1 on failure to get the file pointer.

Close

        IV      (*Close)(pTHX_ PerlIO *f);

Close the stream. Should normally call CWPerlIOBase_close() to flush itself and close layers below, and then deallocate any data structures (buffers, translation tables, ...) not held directly in the data structure. Returns 0 on success, -1 on failure.

Flush

        IV      (*Flush)(pTHX_ PerlIO *f);

Should make stream's state consistent with layers below. That is, any buffered write data should be written, and file position of lower layers adjusted for data read from below but not actually consumed. (Should perhaps CWUnread() such data to the lower layer.) Returns 0 on success, -1 on failure.

Fill

        IV      (*Fill)(pTHX_ PerlIO *f);

The buffer for this layer should be filled (for read) from layer below. When you subclass PerlIOBuf layer, you want to use its _read method and to supply your own fill method, which fills the PerlIOBuf's buffer. Returns 0 on success, -1 on failure.

Eof

        IV      (*Eof)(pTHX_ PerlIO *f);

Return end-of-file indicator. CWPerlIOBase_eof() is normally sufficient. Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.

Error

        IV      (*Error)(pTHX_ PerlIO *f);

Return error indicator. CWPerlIOBase_error() is normally sufficient. Returns 1 if there is an error (usually when CWPERLIO_F_ERROR is set, 0 otherwise.

Clearerr

        void    (*Clearerr)(pTHX_ PerlIO *f);

Clear end-of-file and error indicators. Should call CWPerlIOBase_clearerr() to set the CWPERLIO_F_XXXXX flags, which may suffice.

Setlinebuf

        void    (*Setlinebuf)(pTHX_ PerlIO *f);

Mark the stream as line buffered. CWPerlIOBase_setlinebuf() sets the PERLIO_F_LINEBUF flag and is normally sufficient.

Get_base

        STDCHAR *       (*Get_base)(pTHX_ PerlIO *f);

Allocate (if not already done so) the read buffer for this layer and return pointer to it. Return NULL on failure.

Get_bufsiz

        Size_t  (*Get_bufsiz)(pTHX_ PerlIO *f);

Return the number of bytes that last CWFill() put in the buffer.

Get_ptr

        STDCHAR *       (*Get_ptr)(pTHX_ PerlIO *f);

Return the current read pointer relative to this layer's buffer.

Get_cnt

        SSize_t (*Get_cnt)(pTHX_ PerlIO *f);

Return the number of bytes left to be read in the current buffer.

Set_ptrcnt

        void    (*Set_ptrcnt)(pTHX_ PerlIO *f,
                              STDCHAR *ptr, SSize_t cnt);

Adjust the read pointer and count of bytes to match CWptr and/or CWcnt. The application (or layer above) must ensure they are consistent. (Checking is allowed by the paranoid.)

To ask for the next layer down use PerlIONext(PerlIO *f).

To check that a PerlIO* is valid use PerlIOValid(PerlIO *f). (All this does is really just to check that the pointer is non-NULL and that the pointer behind that is non-NULL.)

PerlIOBase(PerlIO *f) returns the Base pointer, or in other words, the CWPerlIOl* pointer.

PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.

Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either calls the callback from the functions of the layer f (just by the name of the IO function, like Read) with the args, or if there is no such callback, calls the base version of the callback with the same args, or if the f is invalid, set errno to EBADF and return failure.

Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls the callback of the functions of the layer f with the args, or if there is no such callback, set errno to EINVAL. Or if the f is invalid, set errno to EBADF and return failure.

Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls the callback of the functions of the layer f with the args, or if there is no such callback, calls the base version of the callback with the same args, or if the f is invalid, set errno to EBADF.

Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the callback of the functions of the layer f with the args, or if there is no such callback, set errno to EINVAL. Or if the f is invalid, set errno to EBADF.

If you find the implementation document unclear or not sufficient, look at the existing PerlIO layer implementations, which include:

* C implementations

The perlio.c and perliol.h in the Perl core implement the unix, perlio, stdio, crlf, utf8, byte, raw, pending layers, and also the mmap and win32 layers if applicable. (The win32 is currently unfinished and unused, to see what is used instead in Win32, see Querying the layers of filehandles in PerlIO .) PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core. PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.

* Perl implementations

PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.

If you are creating a PerlIO layer, you may want to be lazy, in other words, implement only the methods that interest you. The other methods you can either replace with the blank methods

    PerlIOBase_noop_ok
    PerlIOBase_noop_fail

(which do nothing, and return zero and -1, respectively) or for certain methods you may assume a default behaviour by using a NULL method. The Open method looks for help in the 'parent' layer. The following table summarizes the behaviour:

    method      behaviour with NULL

    Clearerr    PerlIOBase_clearerr
    Close       PerlIOBase_close
    Dup         PerlIOBase_dup
    Eof         PerlIOBase_eof
    Error       PerlIOBase_error
    Fileno      PerlIOBase_fileno
    Fill        FAILURE
    Flush       SUCCESS
    Getarg      SUCCESS
    Get_base    FAILURE
    Get_bufsiz  FAILURE
    Get_cnt     FAILURE
    Get_ptr     FAILURE
    Open        INHERITED
    Popped      SUCCESS
    Pushed      SUCCESS
    Read        PerlIOBase_read
    Seek        FAILURE
    Set_cnt     FAILURE
    Set_ptrcnt  FAILURE
    Setlinebuf  PerlIOBase_setlinebuf
    Tell        FAILURE
    Unread      PerlIOBase_unread
    Write       FAILURE

 FAILURE        Set errno (to EINVAL in UNIXish, to LIB$_INVARG in VMS) and
                return -1 (for numeric return values) or NULL (for pointers)
 INHERITED      Inherited from the layer below
 SUCCESS        Return 0 (for numeric return values) or a pointer

The file CWperlio.c provides the following layers: A basic non-buffered layer which calls Unix/POSIX CWread(), CWwrite(), CWlseek(), CWclose(). No buffering. Even on platforms that distinguish between O_TEXT and O_BINARY this layer is always O_BINARY. A very complete generic buffering layer which provides the whole of PerlIO API. It is also intended to be used as a base class for other layers. (For example its CWRead() method is implemented in terms of the CWGet_cnt()/CWGet_ptr()/CWSet_ptrcnt() methods). perlio over unix provides a complete replacement for stdio as seen via PerlIO API. This is the default for USE_PERLIO when system's stdio does not permit perl's fast gets access, and which do not distinguish between CWO_TEXT and CWO_BINARY. A layer which provides the PerlIO API via the layer scheme, but implements it by calling system's stdio. This is (currently) the default if system's stdio provides sufficient access to allow perl's fast gets access and which do not distinguish between CWO_TEXT and CWO_BINARY. A layer derived using perlio as a base class. It provides Win32-like \n to CR,LF translation. Can either be applied above perlio or serve as the buffer layer itself. crlf over unix is the default if system distinguishes between CWO_TEXT and CWO_BINARY opens. (At some point unix will be replaced by a native Win32 IO layer on that platform, as Win32's read/write layer has various drawbacks.) The crlf layer is a reasonable model for a layer which transforms data in some way. If Configure detects CWmmap() functions this layer is provided (with perlio as a base) which does read operations by mmap()ing the file. Performance improvement is marginal on modern systems, so it is mainly there as a proof of concept. It is likely to be unbundled from the core at some point. The mmap layer is a reasonable model for a minimalist derived layer. An internal derivative of perlio which can be used to provide Unread() function for layers which have no buffer or cannot be bothered. (Basically this layer's CWFill() pops itself off the stack and so resumes reading from layer below.) A dummy layer which never exists on the layer stack. Instead when pushed it actually pops the stack removing itself, it then calls Binmode function table entry on all the layers in the stack - normally this (via PerlIOBase_binmode) removes any layers which do not have CWPERLIO_K_RAW bit set. Layers can modify that behaviour by defining their own Binmode entry. Another dummy layer. When pushed it pops itself and sets the CWPERLIO_F_UTF8 flag on the layer which was (and now is once more) the top of the stack.

In addition perlio.c also provides a number of CWPerlIOBase_xxxx() functions which are intended to be used in the table slots of classes which do not need to do anything special for a particular method.

Layers can made available by extension modules. When an unknown layer is encountered the PerlIO code will perform the equivalent of :

   use PerlIO 'layer';

Where layer is the unknown layer. PerlIO.pm will then attempt to:

   require PerlIO::layer;

If after that process the layer is still not defined then the CWopen will fail.

The following extension layers are bundled with perl:

   use Encoding;

   open( $fh, "<:encoding(iso-8859-7)", $pathname );

   open( $fh, "+<:scalar", \$scalar );

   open( $fh, "+<", \$scalar );

   use PerlIO::via::StripHTML;
   open( my $fh, "<:via(StripHTML)", "index.html" );

Things that need to be done to improve this document.

•

Explain how to make a valid fh without going through open()(i.e. apply a layer). For example if the file is not opened through perl, but we want to get back a fh, like it was opened by Perl. How PerlIO_apply_layera fits in, where its docs, was it made public? Currently the example could be something like this:

  PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
  {
      char *mode; /* "w", "r", etc */
      const char *layers = ":APR"; /* the layer name */
      PerlIO *f = PerlIO_allocate(aTHX);
      if (!f) {
          return NULL;
      }

      PerlIO_apply_layers(aTHX_ f, mode, layers);

      if (f) {
          PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
          /* fill in the st struct, as in _open() */
          st->file = file;
          PerlIOBase(f)->flags |= PERLIO_F_OPEN;

          return f;
      }
      return NULL;
  }

•

fix/add the documentation in places marked as XXX.

•

The handling of errors by the layer is not specified. e.g. when $! should be set explicitly, when the error handling should be just delegated to the top layer. Probably give some hints on using SETERRNO() or pointers to where they can be found.

•

I think it would help to give some concrete examples to make it easier to understand the API. Of course I agree that the API has to be concise, but since there is no second document that is more of a guide, I think that it'd make it easier to start with the doc which is an API, but has examples in it in places where things are unclear, to a person who is not a PerlIO guru (yet).