sysconf(3p) — Linux manual page


SYSCONF(3P)             POSIX Programmer's Manual            SYSCONF(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       sysconf — get configurable system variables

SYNOPSIS         top

       #include <unistd.h>

       long sysconf(int name);

DESCRIPTION         top

       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.
    │             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                   │
    │ Initial size of getgrgid_r() and │_SC_GETGR_R_SIZE_MAX              │
    │ getgrnam_r() data buffers        │                                  │
    │ Initial 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                      │
    │ {PAGE_SIZE}                      │_SC_PAGE_SIZE                     │
    │ {PAGESIZE}                       │_SC_PAGESIZE                      │
    │ {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                    │
    │ _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_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_MESSAGE_PASSING           │_SC_MESSAGE_PASSING               │
    │ _POSIX_MONOTONIC_CLOCK           │_SC_MONOTONIC_CLOCK               │
    │ _POSIX_PRIORITIZED_IO            │_SC_PRIORITIZED_IO                │
    │ _POSIX_RAW_SOCKETS               │_SC_RAW_SOCKETS                   │
    │ _POSIX_REALTIME_SIGNALS          │_SC_REALTIME_SIGNALS              │
    │ _POSIX_REGEXP                    │_SC_REGEXP                        │
    │ _POSIX_SAVED_IDS                 │_SC_SAVED_IDS                     │
    │ _POSIX_SEMAPHORES                │_SC_SEMAPHORES                    │
    │ _POSIX_SHELL                     │_SC_SHELL                         │
    │ _POSIX_SPAWN                     │_SC_SPAWN                         │
    │ _POSIX_SPIN_LOCKS                │_SC_SPIN_LOCKS                    │
    │ _POSIX_SPORADIC_SERVER           │_SC_SPORADIC_SERVER               │
    │ _POSIX_SS_REPL_MAX               │_SC_SS_REPL_MAX                   │
    │ _POSIX_SYNCHRONIZED_IO           │_SC_SYNCHRONIZED_IO               │
    │ _POSIX_THREAD_CPUTIME            │_SC_THREAD_CPUTIME                │
    │ _POSIX_THREADS                   │_SC_THREADS                       │
    │ _POSIX_TIMEOUTS                  │_SC_TIMEOUTS                      │
        │           Variable           Value of Name         │
        │ _POSIX_TIMERS                │_SC_TIMERS                    │
        │ _POSIX_TRACE                 │_SC_TRACE                     │
        │ _POSIX_TRACE_INHERIT         │_SC_TRACE_INHERIT             │
        │ _POSIX_TRACE_LOG             │_SC_TRACE_LOG                 │
        │ _POSIX_TRACE_NAME_MAX        │_SC_TRACE_NAME_MAX            │
        │ _POSIX_TRACE_SYS_MAX         │_SC_TRACE_SYS_MAX             │
        │ _POSIX_VERSION               │_SC_VERSION                   │
        │ _POSIX_V7_ILP32_OFF32        │_SC_V7_ILP32_OFF32            │
        │ _POSIX_V7_ILP32_OFFBIG       │_SC_V7_ILP32_OFFBIG           │
        │ _POSIX_V7_LP64_OFF64         │_SC_V7_LP64_OFF64             │
        │ _POSIX_V7_LPBIG_OFFBIG       │_SC_V7_LPBIG_OFFBIG           │
        │ _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_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                 │
        │ _XOPEN_CRYPT                 │_SC_XOPEN_CRYPT               │
        │ _XOPEN_ENH_I18N              │_SC_XOPEN_ENH_I18N            │
        │ _XOPEN_REALTIME              │_SC_XOPEN_REALTIME            │
        │ _XOPEN_SHM                   │_SC_XOPEN_SHM                 │
        │ _XOPEN_STREAMS               │_SC_XOPEN_STREAMS             │
        │ _XOPEN_UNIX                  │_SC_XOPEN_UNIX                │
        │ _XOPEN_UUCP                  │_SC_XOPEN_UUCP                │
        │ _XOPEN_VERSION               │_SC_XOPEN_VERSION             │

RETURN VALUE         top

       If name is an invalid value, sysconf() shall return -1 and set
       errno to indicate the error. If the variable corresponding to
       name is described in <limits.h> as a maximum or minimum value and
       the variable 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.

       If the variable corresponding to name is dependent on an
       unsupported option, the results are unspecified.

ERRORS         top

       The sysconf() function shall fail if:

       EINVAL The value of the name argument is invalid.

       The following sections are informative.

EXAMPLES         top



       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.

       Application developers should check whether an option, such as
       _POSIX_TRACE, is supported prior to obtaining and using values
       for related variables, such as _POSIX_TRACE_NAME_MAX.

RATIONALE         top

       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 POSIX.1‐2017, 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

       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 POSIX.1‐2017.

        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

       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 developer.

       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
       POSIX.1‐2017 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

       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.



SEE ALSO         top

       confstr(3p), fpathconf(3p)

       The Base Definitions volume of POSIX.1‐2017, limits.h(0p),

       The Shell and Utilities volume of POSIX.1‐2017, getconf(1p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see .

IEEE/The Open Group               2017                       SYSCONF(3P)

Pages that refer to this page: limits.h(0p)time.h(0p)unistd.h(0p)c99(1p)getconf(1p)atexit(3p)confstr(3p)fpathconf(3p)getgrgid(3p)getgrnam(3p)getpwnam(3p)getpwuid(3p)getrlimit(3p)mmap(3p)mprotect(3p)msync(3p)munmap(3p)popen(3p)posix_madvise(3p)realpath(3p)times(3p)