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



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

NAME
       sysconf -- get configurable system variables

SYNOPSIS
       #include <unistd.h>

       long sysconf(int name);

DESCRIPTION
       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_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                    |
        |_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_MEMORY_PROTECTION         |_SC_MEMORY_PROTECTION             |
        |_POSIX_MESSAGE_PASSING           |_SC_MESSAGE_PASSING               |
        |_POSIX_MONOTONIC_CLOCK           |_SC_MONOTONIC_CLOCK               |
        |_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_SS_REPL_MAX               |_SC_SS_REPL_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_ROBUST_PRIO_INHERIT|_SC_THREAD_ROBUST_PRIO_INHERIT    |
        |_POSIX_THREAD_ROBUST_PRIO_PROTECT|_SC_THREAD_ROBUST_PRIO_PROTECT    |
        |_POSIX_THREAD_SAFE_FUNCTIONS     |_SC_THREAD_SAFE_FUNCTIONS         |
        |_POSIX_THREAD_SPORADIC_SERVER    |_SC_THREAD_SPORADIC_SERVER        |
        |_POSIX_THREADS                   |_SC_THREADS                       |
        |_POSIX_TIMEOUTS                  |_SC_TIMEOUTS                      |
        +---------------------------------+----------------------------------+
            +-----------------------------+-----------------------------+
            |          Variable           |       Value of Name         |
            +-----------------------------+-----------------------------+
            |_POSIX_TIMERS                |_SC_TIMERS                    |
            |_POSIX_TRACE                 |_SC_TRACE                     |
            |_POSIX_TRACE_EVENT_FILTER    |_SC_TRACE_EVENT_FILTER        |
            |_POSIX_TRACE_EVENT_NAME_MAX  |_SC_TRACE_EVENT_NAME_MAX      |
            |_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_TRACE_USER_EVENT_MAX  |_SC_TRACE_USER_EVENT_MAX      |
            |_POSIX_TYPED_MEMORY_OBJECTS  |_SC_TYPED_MEMORY_OBJECTS      |
            |_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_REALTIME_THREADS      |_SC_XOPEN_REALTIME_THREADS    |
            |_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
       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
       The sysconf() function shall fail if:

       EINVAL The value of the name argument is invalid.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       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
       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 <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
           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 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 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 fpathconf().

       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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       confstr(), fpathconf()

       The Base Definitions volume of POSIX.1-2017, <limits.h>, <unistd.h>

       The Shell and Utilities volume of POSIX.1-2017, getconf

COPYRIGHT
       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 http://www.opengroup.org/unix/online.html .

       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           SYSCONF(3)
