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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(3POSIX)