sysconf()

Arguments:

name

The name of the system limit or option that you want to get; see below.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The sysconf() function returns the value of a system variable (a configurable limit or option) indicated by name.

System variables are defined in <confname.h>, and include at least the following values:

_SC_AIO_PRIO_DELTA_MAX

The maximum amount by which a process can decrease its asynchronous I/O priority level from its own scheduling priority.

_SC_ARG_MAX

The maximum length of arguments for the exec*() functions, in bytes, including environment data.

_SC_CHILD_MAX

The maximum number of simultaneous processes per real user ID.

_SC_CLK_TCK

The number of intervals per second used to express the value in type clock_t.

_SC_DELAYTIMER_MAX

The maximum number of times a timer can overrun and you can still detect it.

_SC_GETGR_R_SIZE_MAX

If defined (not -1), the maximum size of buffer that you need to supply to getgrgid_r() for any memory that it needs to allocate.

_SC_GETPW_R_SIZE_MAX

If defined (not -1), the maximum size of buffer that you need to supply to getpwent_r(), getpwuid_r(), getspent_r(), or getspnam_r() for any memory that they need to allocate.

_SC_JOB_CONTROL

If this variable is defined, then job control is supported.

_SC_LOGNAME_MAX

The maximum length of a login name.

_SC_MQ_OPEN_MAX

The maximum number of open message queue descriptors a process may hold. This variable is set only while mqueue is running.

_SC_MQ_PRIO_MAX

The maximum number of message priorities. This variable is set only while mqueue is running.

_SC_NGROUPS_MAX

The maximum number of simultaneous supplementary group IDs per process.

_SC_OPEN_MAX

The maximum number of files that one process can have open at any given time.

_SC_PAGESIZE or _SC_PAGE_SIZE

The size in bytes of a page.

_SC_RCT_MEM

The minimum amount of memory that the system retains as a resource constraint threshold. Only an unconstrained process (i.e., one with the PROCMGR_AID_RCONSTRAINT ability enabled—see procmgr_ability()) can allocate memory if there's less than this amount left.

_SC_RCT_SCOID

The minimum number of connection IDs that a server retains as a resource constraint threshold. Only an unconstrained process (i.e., one with the PROCMGR_AID_RCONSTRAINT ability enabled—see procmgr_ability()) can create a connection if there are fewer than this number left.

_SC_RTSIG_MAX

The maximum number of realtime signals reserved for application use in this implementation.

_SC_SAVED_IDS

If this variable is defined, then each process has a saved set-user ID and a saved set-group ID.

_SC_SEM_NSEMS_MAX

The maximum number of semaphores that one process can have open at a time. The sysconf() function returns -1 to indicate that this value is indeterminate because that limit applies to both named and unnamed semaphores. The kernel allows an arbitrary number of unnamed semaphores (they're kernel synchronization objects, so the number of them is limited only by the amount of available kernel memory).

_SC_SEM_VALUE_MAX

The maximum value a semaphore may have.

_SC_SIGQUEUE_MAX

The maximum number of outstanding realtime signals sent per process.

_SC_THREAD_STACK_MIN

The minimum stack size for a thread.

_SC_TZNAME_MAX

The maximum length of the names for time zones. The value is the same as TZNAME_MAX.

_SC_VERSION

The current POSIX version that is currently supported. A value of 198808L indicates the August (08) 1988 standard, as approved by the IEEE Standards Board.

_SC_XOPEN_VERSION

An integer value indicating the version of the X/Open Portability Guide to which the implementation conforms.

Returns:

The requested configurable system limit or option. If name isn't defined for the system, -1 is returned and errno is set to EINVAL.

Examples:

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <unistd.h>

int main( void )
   {
    printf( "_SC_ARG_MAX = %ld\n",
         sysconf( _SC_ARG_MAX ) );
    return EXIT_SUCCESS;
  }

Classification:

POSIX 1003.1