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                      │
       │_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                      │
       │_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               │
        │            Variable            Value of Name           │
        │_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                    │
        │{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                   │
        │_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.1‐2008, 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.1‐2008.

        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.1‐2008 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(3p).

       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



SEE ALSO         top

       confstr(3p), fpathconf(3p)

       The Base Definitions volume of POSIX.1‐2008, limits.h(0p),

       The Shell and Utilities volume of POSIX.1‐2008, getconf(1p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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                 2013                         SYSCONF(3P)

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