sysconf - get configurable system variables

#include <unistd.h>

long sysconf(int name);

The sysconf() function provides a method for the application to determine the current value of a configurable system limit or option ( variable). The implementation shall support all of the variables listed in the following table and may support others.

The name argument represents the system variable to be queried. The following table lists the minimal set of system variables from <limits.h> or <unistd.h> that can be returned by sysconf(), and the symbolic constants defined in <unistd.h> that are the corresponding values used for name. center; l1 l. Variable Value of Name {AIO_LISTIO_MAX} _SC_AIO_LISTIO_MAX {AIO_MAX} _SC_AIO_MAX {AIO_PRIO_DELTA_MAX} _SC_AIO_PRIO_DELTA_MAX {ARG_MAX} _SC_ARG_MAX {ATEXIT_MAX} _SC_ATEXIT_MAX {BC_BASE_MAX} _SC_BC_BASE_MAX {BC_DIM_MAX} _SC_BC_DIM_MAX {BC_SCALE_MAX} _SC_BC_SCALE_MAX {BC_STRING_MAX} _SC_BC_STRING_MAX {CHILD_MAX} _SC_CHILD_MAX Clock ticks/second _SC_CLK_TCK {COLL_WEIGHTS_MAX} _SC_COLL_WEIGHTS_MAX {DELAYTIMER_MAX} _SC_DELAYTIMER_MAX {EXPR_NEST_MAX} _SC_EXPR_NEST_MAX {HOST_NAME_MAX} _SC_HOST_NAME_MAX {IOV_MAX} _SC_IOV_MAX {LINE_MAX} _SC_LINE_MAX {LOGIN_NAME_MAX} _SC_LOGIN_NAME_MAX {NGROUPS_MAX} _SC_NGROUPS_MAX Maximum size of getgrgid_r() and _SC_GETGR_R_SIZE_MAX getgrnam_r() data buffers Maximum size of getpwuid_r() and _SC_GETPW_R_SIZE_MAX getpwnam_r() data buffers {MQ_OPEN_MAX} _SC_MQ_OPEN_MAX {MQ_PRIO_MAX} _SC_MQ_PRIO_MAX {OPEN_MAX} _SC_OPEN_MAX _POSIX_ADVISORY_INFO _SC_ADVISORY_INFO _POSIX_BARRIERS _SC_BARRIERS _POSIX_ASYNCHRONOUS_IO _SC_ASYNCHRONOUS_IO _POSIX_CLOCK_SELECTION _SC_CLOCK_SELECTION _POSIX_CPUTIME _SC_CPUTIME _POSIX_FILE_LOCKING _SC_FILE_LOCKING _POSIX_FSYNC _SC_FSYNC _POSIX_IPV6 _SC_IPV6 _POSIX_JOB_CONTROL _SC_JOB_CONTROL _POSIX_MAPPED_FILES _SC_MAPPED_FILES _POSIX_MEMLOCK _SC_MEMLOCK _POSIX_MEMLOCK_RANGE _SC_MEMLOCK_RANGE _POSIX_MEMORY_PROTECTION _SC_MEMORY_PROTECTION _POSIX_MESSAGE_PASSING _SC_MESSAGE_PASSING _POSIX_MONOTONIC_CLOCK _SC_MONOTONIC_CLOCK _POSIX_MULTI_PROCESS _SC_MULTI_PROCESS _POSIX_PRIORITIZED_IO _SC_PRIORITIZED_IO _POSIX_PRIORITY_SCHEDULING _SC_PRIORITY_SCHEDULING _POSIX_RAW_SOCKETS _SC_RAW_SOCKETS _POSIX_READER_WRITER_LOCKS _SC_READER_WRITER_LOCKS _POSIX_REALTIME_SIGNALS _SC_REALTIME_SIGNALS _POSIX_REGEXP _SC_REGEXP _POSIX_SAVED_IDS _SC_SAVED_IDS _POSIX_SEMAPHORES _SC_SEMAPHORES _POSIX_SHARED_MEMORY_OBJECTS _SC_SHARED_MEMORY_OBJECTS _POSIX_SHELL _SC_SHELL _POSIX_SPAWN _SC_SPAWN _POSIX_SPIN_LOCKS _SC_SPIN_LOCKS _POSIX_SPORADIC_SERVER _SC_SPORADIC_SERVER _POSIX_SYMLOOP_MAX _SC_SYMLOOP_MAX _POSIX_SYNCHRONIZED_IO _SC_SYNCHRONIZED_IO _POSIX_THREAD_ATTR_STACKADDR _SC_THREAD_ATTR_STACKADDR _POSIX_THREAD_ATTR_STACKSIZE _SC_THREAD_ATTR_STACKSIZE _POSIX_THREAD_CPUTIME _SC_THREAD_CPUTIME _POSIX_THREAD_PRIO_INHERIT _SC_THREAD_PRIO_INHERIT _POSIX_THREAD_PRIO_PROTECT _SC_THREAD_PRIO_PROTECT _POSIX_THREAD_PRIORITY_SCHEDULING _SC_THREAD_PRIORITY_SCHEDULING _POSIX_THREAD_PROCESS_SHARED _SC_THREAD_PROCESS_SHARED _POSIX_THREAD_SAFE_FUNCTIONS _SC_THREAD_SAFE_FUNCTIONS _POSIX_THREAD_SPORADIC_SERVER _SC_THREAD_SPORADIC_SERVER _POSIX_THREADS _SC_THREADS _POSIX_TIMEOUTS _SC_TIMEOUTS _POSIX_TIMERS _SC_TIMERS _POSIX_TRACE _SC_TRACE _POSIX_TRACE_EVENT_FILTER _SC_TRACE_EVENT_FILTER _POSIX_TRACE_INHERIT _SC_TRACE_INHERIT _POSIX_TRACE_LOG _SC_TRACE_LOG _POSIX_TYPED_MEMORY_OBJECTS _SC_TYPED_MEMORY_OBJECTS _POSIX_VERSION _SC_VERSION _POSIX_V6_ILP32_OFF32 _SC_V6_ILP32_OFF32 _POSIX_V6_ILP32_OFFBIG _SC_V6_ILP32_OFFBIG _POSIX_V6_LP64_OFF64 _SC_V6_LP64_OFF64 _POSIX_V6_LPBIG_OFFBIG _SC_V6_LPBIG_OFFBIG _POSIX2_C_BIND _SC_2_C_BIND _POSIX2_C_DEV _SC_2_C_DEV _POSIX2_C_VERSION _SC_2_C_VERSION _POSIX2_CHAR_TERM _SC_2_CHAR_TERM _POSIX2_FORT_DEV _SC_2_FORT_DEV _POSIX2_FORT_RUN _SC_2_FORT_RUN _POSIX2_LOCALEDEF _SC_2_LOCALEDEF _POSIX2_PBS _SC_2_PBS _POSIX2_PBS_ACCOUNTING _SC_2_PBS_ACCOUNTING _POSIX2_PBS_CHECKPOINT _SC_2_PBS_CHECKPOINT _POSIX2_PBS_LOCATE _SC_2_PBS_LOCATE _POSIX2_PBS_MESSAGE _SC_2_PBS_MESSAGE _POSIX2_PBS_TRACK _SC_2_PBS_TRACK _POSIX2_SW_DEV _SC_2_SW_DEV _POSIX2_UPE _SC_2_UPE _POSIX2_VERSION _SC_2_VERSION _REGEX_VERSION _SC_REGEX_VERSION {PAGE_SIZE} _SC_PAGE_SIZE {PAGESIZE} _SC_PAGESIZE {PTHREAD_DESTRUCTOR_ITERATIONS} _SC_THREAD_DESTRUCTOR_ITERATIONS {PTHREAD_KEYS_MAX} _SC_THREAD_KEYS_MAX {PTHREAD_STACK_MIN} _SC_THREAD_STACK_MIN {PTHREAD_THREADS_MAX} _SC_THREAD_THREADS_MAX {RE_DUP_MAX} _SC_RE_DUP_MAX {RTSIG_MAX} _SC_RTSIG_MAX {SEM_NSEMS_MAX} _SC_SEM_NSEMS_MAX {SEM_VALUE_MAX} _SC_SEM_VALUE_MAX {SIGQUEUE_MAX} _SC_SIGQUEUE_MAX {STREAM_MAX} _SC_STREAM_MAX {SYMLOOP_MAX} _SC_SYMLOOP_MAX {TIMER_MAX} _SC_TIMER_MAX {TTY_NAME_MAX} _SC_TTY_NAME_MAX {TZNAME_MAX} _SC_TZNAME_MAX _XBS5_ILP32_OFF32 (LEGACY) _SC_XBS5_ILP32_OFF32 (LEGACY) _XBS5_ILP32_OFFBIG (LEGACY) _SC_XBS5_ILP32_OFFBIG (LEGACY) _XBS5_LP64_OFF64 (LEGACY) _SC_XBS5_LP64_OFF64 (LEGACY) _XBS5_LPBIG_OFFBIG (LEGACY) _SC_XBS5_LPBIG_OFFBIG (LEGACY) _XOPEN_CRYPT _SC_XOPEN_CRYPT _XOPEN_ENH_I18N _SC_XOPEN_ENH_I18N _XOPEN_LEGACY _SC_XOPEN_LEGACY _XOPEN_REALTIME _SC_XOPEN_REALTIME _XOPEN_REALTIME_THREADS _SC_XOPEN_REALTIME_THREADS _XOPEN_SHM _SC_XOPEN_SHM _XOPEN_STREAMS _SC_XOPEN_STREAMS _XOPEN_UNIX _SC_XOPEN_UNIX _XOPEN_VERSION _SC_XOPEN_VERSION _XOPEN_XCU_VERSION _SC_XOPEN_XCU_VERSION

If name is an invalid value, sysconf() shall return -1 and set errno to indicate the error. If the variable corresponding to name has no limit, sysconf() shall return -1 without changing the value of errno. Note that indefinite limits do not imply infinite limits; see <limits.h>.

Otherwise, sysconf() shall return the current variable value on the system. The value returned shall not be more restrictive than the corresponding value described to the application when it was compiled with the implementation's <limits.h> or <unistd.h>. The value shall not change during the lifetime of the calling process, except that sysconf(_SC_OPEN_MAX) may return different values before and after a call to setrlimit() which changes the RLIMIT_NOFILE soft limit.

The sysconf() function shall fail if:

EINVAL

The value of the name argument is invalid.

The following sections are informative.

None.

As -1 is a permissible return value in a successful situation, an application wishing to check for error situations should set errno to 0, then call sysconf(), and, if it returns -1, check to see if errno is non-zero.

If the value of sysconf(_SC_2_VERSION) is not equal to the value of the _POSIX2_VERSION symbolic constant, the utilities available via system() or popen() might not behave as described in the Shell and Utilities volume of IEEE Std 1003.1-2001. This would mean that the application is not running in an environment that conforms to the Shell and Utilities volume of IEEE Std 1003.1-2001. Some applications might be able to deal with this, others might not. However, the functions defined in this volume of IEEE Std 1003.1-2001 continue to operate as specified, even if sysconf(_SC_2_VERSION) reports that the utilities no longer perform as specified.

This functionality was added in response to requirements of application developers and of system vendors who deal with many international system configurations. It is closely related to pathconf() and fpathconf().

Although a conforming application can run on all systems by never demanding more resources than the minimum values published in this volume of IEEE Std 1003.1-2001, it is useful for that application to be able to use the actual value for the quantity of a resource available on any given system. To do this, the application makes use of the value of a symbolic constant in <limits.h> or <unistd.h>.

However, once compiled, the application must still be able to cope if the amount of resource available is increased. To that end, an application may need a means of determining the quantity of a resource, or the presence of an option, at execution time.

Two examples are offered:

1.

Applications may wish to act differently on systems with or without job control. Applications vendors who wish to distribute only a single binary package to all instances of a computer architecture would be forced to assume job control is never available if it were to rely solely on the <unistd.h> value published in this volume of IEEE Std 1003.1-2001.

2.

International applications vendors occasionally require knowledge of the number of clock ticks per second. Without these facilities, they would be required to either distribute their applications partially in source form or to have 50 Hz and 60 Hz versions for the various countries in which they operate.

It is the knowledge that many applications are actually distributed widely in executable form that leads to this facility. If limited to the most restrictive values in the headers, such applications would have to be prepared to accept the most limited environments offered by the smallest microcomputers. Although this is entirely portable, there was a consensus that they should be able to take advantage of the facilities offered by large systems, without the restrictions associated with source and object distributions.

During the discussions of this feature, it was pointed out that it is almost always possible for an application to discern what a value might be at runtime by suitably testing the various functions themselves. And, in any event, it could always be written to adequately deal with error returns from the various functions. In the end, it was felt that this imposed an unreasonable level of complication and sophistication on the application writer.

This runtime facility is not meant to provide ever-changing values that applications have to check multiple times. The values are seen as changing no more frequently than once per system initialization, such as by a system administrator or operator with an automatic configuration program. This volume of IEEE Std 1003.1-2001 specifies that they shall not change within the lifetime of the process.

Some values apply to the system overall and others vary at the file system or directory level. The latter are described in pathconf() .

Note that all values returned must be expressible as integers. String values were considered, but the additional flexibility of this approach was rejected due to its added complexity of implementation and use.

Some values, such as {PATH_MAX}, are sometimes so large that they must not be used to, say, allocate arrays. The sysconf() function returns a negative value to show that this symbolic constant is not even defined in this case.

Similar to pathconf(), this permits the implementation not to have a limit. When one resource is infinite, returning an error indicating that some other resource limit has been reached is conforming behavior.

None.

confstr() , pathconf() , the Base Definitions volume of IEEE Std 1003.1-2001, <limits.h>, <unistd.h>, the Shell and Utilities volume of IEEE Std 1003.1-2001, getconf

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .