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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SYSCONF(3P) POSIX Programmer's Manual SYSCONF(3P)
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.
sysconf — get configurable system variables
#include <unistd.h>
long sysconf(int name);
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 │
└──────────────────────────────┴─────────────────────────────┘
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.
The sysconf() function shall fail if:
EINVAL The value of the name argument is invalid.
The following sections are informative.
None.
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.
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(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 behavior.
None.
confstr(3p), fpathconf(3p)
The Base Definitions volume of POSIX.1‐2017, limits.h(0p),
unistd.h(0p)
The Shell and Utilities volume of POSIX.1‐2017, getconf(1p)
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)
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)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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