sysconf —
get configurable system variables
Standard C Library (libc, -lc)
#include <unistd.h>
long
  
  sysconf(int
    name);
This interface is defined by IEEE Std 1003.1-1988
  (“POSIX.1”). A far more complete interface is available
  using sysctl(3).
The sysconf() function provides a method
    for applications to determine the current value of a configurable system
    limit or option variable. The name argument specifies
    the system variable to be queried. Symbolic constants for each name value
    are found in the include file
    <unistd.h>.
The available values are as follows:
  - _SC_ARG_MAX
- The maximum bytes of argument to
      execve(2).
- _SC_ATEXIT_MAX
- The maximum number of functions that may be registered with
      atexit(3).
- _SC_BARRIERS
- The version of IEEE Std 1003.1
      (“POSIX.1”) and its Barriers option to which the
      system attempts to conform, otherwise -1.
- _SC_CLOCK_SELECTION
- Return the POSIX version the implementation of the Clock Selection Option
      on this system conforms to, or -1 if unavailable.
- _SC_CHILD_MAX
- The maximum number of simultaneous processes per user id.
- _SC_CLK_TCK
- The number of clock ticks per second.
- _SC_FSYNC
- Return 1 if the File Synchronization Option is available on this system,
      otherwise -1.
- _SC_HOST_NAME_MAX
- The maximum size of a hostname, including
    NULL.
- _SC_IOV_MAX
- The maximum number of iovec structures that a
      process has available for use with
      preadv(2),
      pwritev(2),
      readv(2),
      recvmsg(2),
      sendmsg(2) or
      writev(2).
- _SC_JOB_CONTROL
- Return 1 if job control is available on this system, otherwise -1.
- _SC_LOGIN_NAME_MAX
- Returns the size of the storage required for a login name, in bytes,
      including the terminating NUL.
- _SC_MAPPED_FILES
- Return 1 if the Memory Mapped Files Option is available on this system,
      otherwise -1.
- _SC_MEMLOCK
- Return 1 if the Process Memory Locking Option is available on this system,
      otherwise -1.
- _SC_MEMLOCK_RANGE
- Return 1 if the Range Memory Locking Option is available on this system,
      otherwise -1.
- _SC_MEMORY_PROTECTION
- Return 1 if the Memory Protection Option is available on this system,
      otherwise -1.
- _SC_MONOTONIC_CLOCK
- Return the POSIX version the implementation of the Monotonic Clock Option
      on this system conforms to, or -1 if unavailable.
- _SC_NGROUPS_MAX
- The maximum number of supplemental groups.
- _SC_OPEN_MAX
- The maximum number of open files per process.
- _SC_PAGESIZE
- The size of a system page in bytes.
- _SC_PASS_MAX
- The maximum length of the password, not counting
      NULL.
- _SC_READER_WRITER_LOCKS
- The version of IEEE Std 1003.1
      (“POSIX.1”) and its Read-Write Locks option to which
      the system attempts to conform, otherwise -1.
- _SC_REGEXP
- Return 1 if POSIX regular expressions are available on this system,
      otherwise -1.
- _SC_SEMAPHORES
- The version of IEEE Std 1003.1
      (“POSIX.1”) and its Semaphores option to which the
      system attempts to conform, otherwise -1.
- _SC_SEM_NSEMS_MAX
- The maximum number of semaphores that one process can have open at a time,
      otherwise -1.
- _SC_SHELL
- Return 1 if POSIX shell is available on this system, otherwise -1.
- _SC_SPIN_LOCKS
- The version of IEEE Std 1003.1
      (“POSIX.1”) and its Spin Locks option to which the
      system attempts to conform, otherwise -1.
- _SC_STREAM_MAX
- The minimum maximum number of streams that a process may have open at any
      one time.
- _SC_SYMLOOP_MAX
- The maximum number of symbolic links that may be expanded in a path
    name.
- _SC_SYNCHRONIZED_IO
- Return 1 if the Synchronized I/O Option is available on this system,
      otherwise -1.
- _SC_THREADS
- The version of IEEE Std 1003.1
      (“POSIX.1”) and its Threads option to which the
      system attempts to conform, otherwise -1.
- _SC_THREAD_PRIO_PROTECT
- System supports the priority ceiling protocol for POSIX threads.
- _SC_TIMERS
- The version of IEEE Std 1003.1
      (“POSIX.1”) and its Timers option to which the system
      attempts to conform, otherwise -1.
- _SC_CPUTIME
- The clockID CLOCK_PROCESS_CPUTIME_ID is supported, otherwise -1.
- _SC_THREAD_CPUTIME
- The clockID CLOCK_THREAD_CPUTIME_ID is supported, otherwise -1.
- _SC_DELAYTIMER_MAX
- The maximum number of overrun for a specific timer, otherwise -1.
- _SC_TZNAME_MAX
- The minimum maximum number of types supported for the name of a
    timezone.
- _SC_SAVED_IDS
- Returns 1 if saved set-group and saved set-user ID is available, otherwise
      -1.
- _SC_VERSION
- The version of ISO/IEC 9945 (POSIX 1003.1) with which the system attempts
      to comply.
- _SC_XOPEN_SHM
- Return 1 if the X/Open Portability Guide Issue 4,
      Version 2 (“XPG4.2”) Shared Memory option is
      available on this system, otherwise -1.
    Availability of the Shared Memory option depends on the
        SYSVSHMkernel option.
 
- _SC_BC_BASE_MAX
- The maximum ibase/obase values in the
      bc(1) utility.
- _SC_BC_DIM_MAX
- The maximum array size in the
      bc(1) utility.
- _SC_BC_SCALE_MAX
- The maximum scale value in the
      bc(1) utility.
- _SC_BC_STRING_MAX
- The maximum string length in the
      bc(1) utility.
- _SC_COLL_WEIGHTS_MAX
- The maximum number of weights that can be assigned to any entry of the
      LC_COLLATE order keyword in the locale definition file.
- _SC_EXPR_NEST_MAX
- The maximum number of expressions that can be nested within parenthesis by
      the expr(1) utility.
- _SC_LINE_MAX
- The maximum length in bytes of a text-processing utility's input
    line.
- _SC_RE_DUP_MAX
- The maximum number of repeated occurrences of a regular expression
      permitted when using interval notation.
- _SC_2_VERSION
- The version of POSIX 1003.2 with which the system attempts to comply.
- _SC_2_C_BIND
- Return 1 if the system's C-language development facilities support the
      C-Language Bindings Option, otherwise -1.
- _SC_2_C_DEV
- Return 1 if the system supports the C-Language Development Utilities
      Option, otherwise -1.
- _SC_2_CHAR_TERM
- Return 1 if the system supports at least one terminal type capable of all
      operations described in POSIX 1003.2, otherwise -1.
- _SC_2_FORT_DEV
- Return 1 if the system supports the FORTRAN Development Utilities Option,
      otherwise -1.
- _SC_2_FORT_RUN
- Return 1 if the system supports the FORTRAN Runtime Utilities Option,
      otherwise -1.
- _SC_2_LOCALEDEF
- Return 1 if the system supports the creation of locales, otherwise
    -1.
- _SC_2_SW_DEV
- Return 1 if the system supports the Software Development Utilities Option,
      otherwise -1.
- _SC_2_UPE
- Return 1 if the system supports the User Portability Utilities Option,
      otherwise -1.
- _SC_GETGR_R_SIZE_MAX
- The minimum size of the buffer passed to
      getgrgid_r(3) and
      getgrnam_r(3).
- _SC_GETPW_R_SIZE_MAX
- The minimum size of the buffer passed to
      getpwnam_r(3) and
      getpwuid_r(3).
- _SC_NPROCESSORS_CONF
- The number of processors configured.
- _SC_NPROCESSORS_ONLN
- The number of processors online (capable of running processes).
- _SC_PHYS_PAGES
- The amount of physical memory on the system in
      _SC_PAGESIZEbytes.
- _SC_TIMER_MAX
- The number of timers available for
      timer_create(2). This
      is also known as _POSIX_TIMER_MAX.
If the call tosysconf is not successful, -1 is returned
  and errno is set appropriately. Otherwise, if the
  variable is associated with functionality that is not supported, -1 is
  returned and errno is not modified. Otherwise, the
  current variable value is returned.
The sysconf() function may fail and set
  errno for any of the errors specified for the library
  functions sysctl(3). In
  addition, the following error may be reported:
  - [EINVAL]
- The value of the name argument is invalid.
Thesysconf() function conforms to IEEE
  Std 1003.1-1990 (“POSIX.1”). The constants
  _SC_NPROCESSORS_CONF and
  _SC_NPROCESSORS_ONLN are not part of the standard, but
  are provided by many systems.
The sysconf function first appeared in
  4.4BSD.
The value for _SC_STREAM_MAX is a minimum maximum, and required to be the same
  as ANSI C's FOPEN_MAX, so the returned value is a ridiculously small and
  misleading number.