sysconf

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



PROLOG
       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
       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(3P)