sysconf() -- System Call (libc)

Get configurable system variables
#include <unistd.h>
long sysconf(name)
int name;

sysconf() returns  the value  of the system  limit or option  identified by
name.

In the following table, the left  column gives a symbolic constant to which
name  can be  set,  and the  right  column gives  the corresponding  system
variable  (as  defined   in  <limits.h>  and  <unistd.h>)  that
sysconf() reads and returns:

     Name                     Variable
     _SC_ARG_MAX              ARG_MAX
     _SC_CHILD_MAX            CHILD_MAX
     _SC_CLK_TCK              CLK_TCK
     _SC_NGROUPS_MAX          NGROUPS_MAX
     _SC_OPEN_MAX             OPEN_MAX
     _SC_PASS_MAX             PASS_MAX
     _SC_JOB_CONTROL          _POSIX_JOB_CONTROL
     _SC_SAVED_IDS            _POSIX_SAVED_IDS
     _SC_VERSION              _POSIX_VERSION

The following describes the values returned in more detail:

ARG_MAX
     Maximum number  of bytes that can be occupied  by a process's argument
     list and environment.

CHILD_MAX
     Number of processes a user can run simultaneously.

CLK_TCK
     Length of a clock tick, in microseconds.

NGROUPS_MAX
     The maximum number  of groups to which a user  can belong, in addition
     to her primary group.

OPEN_MAX
     The number of files a process can have open simultaneously.

PASS_MAX
     The  maximum length  of  a password.   Please note  that the  constant
     _SC_PASS_MAX is  defined only for programs compiled  for UNIX System V
     release 4.

_POSIX_JOB_CONTROL
     This is  a Boolean  flag that  indicates whether the  operating system
     supports the POSIX job-control functions.

_POSIX_SAVED_IDS
     This is  a Boolean  flag that  indicates whether the  operating system
     permits each process to have a saved set-user ID and a saved set-group
     ID.

_POSIX_VERSION
     This is a long integer  that encodes the four-digit year and two-digit
     month of  approval for the version of the  POSIX standard supported by
     the  operating system.   For  example, 199009L  indicates the  version
     approved in September of 1990.

The value of variable CLK_TCK can  vary; you should not assume that it is a
compile-time constant.

If  name is  an invalid  value, sysconf()  returns -1 and  set errno  to an
appropriate value.  If  sysconf() fails due to a value  of name that is not
defined on the system, it returns -1 without setting errno.

Example

At the time of this writing (August 1994), the program

#include <unistd.h>
main()
{
    printf("_SC_ARG_MAX: %d\n", sysconf(_SC_ARG_MAX));
    printf("_SC_CHILD_MAX: %d\n", sysconf(_SC_CHILD_MAX));
    printf("_SC_CLK_TCK: %d\n", sysconf(_SC_CLK_TCK));
    printf("_SC_NGROUPS_MAX: %d\n", sysconf(_SC_NGROUPS_MAX));
    printf("_SC_OPEN_MAX: %d\n", sysconf(_SC_OPEN_MAX));
    printf("_SC_JOB_CONTROL: %d\n", sysconf(_SC_JOB_CONTROL));
    printf("_SC_SAVED_IDS: %d\n", sysconf(_SC_SAVED_IDS));
    printf("_SC_VERSION: %d\n", sysconf(_SC_VERSION));
}

returns the following values:

    _SC_ARG_MAX: 5120
    _SC_CHILD_MAX: 25
    _SC_CLK_TCK: 100
    _SC_NGROUPS_MAX: 32
    _SC_OPEN_MAX: 60
    _SC_JOB_CONTROL: 0
    _SC_SAVED_IDS: 1
    _SC_VERSION: 199009

See Also

libc,
unistd.h
POSIX Standard, §4.8.1

Notes

Programs can use the appropriate #ifndef guards to control whether they use
sysconf() or  a symbol from  <limits.h> for each kind  of limit.  For
example:

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

    #ifdef  _SC_OPEN_MAX
        max = sysconf (_SC_OPEN_MAX);
    #elif   defined (OPEN_MAX)
        max = OPEN_MAX;
    #else
    /* either complain, or make some rational assumption, e.g. */
    #error  Open file descriptor limits cannot be determined
    #endif

© 2012 by Stephen A. Ness