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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 │
│_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 │
│_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 │
└─────────────────────────────────┴──────────────────────────────────┘
┌────────────────────────────────┬─────────────────────────────────┐
│ 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_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 │
│_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.1‐2008, 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
behavior.
None.
confstr(3p), fpathconf(3p)
The Base Definitions volume of POSIX.1‐2008, limits.h(0p),
unistd.h(0p)
The Shell and Utilities volume of POSIX.1‐2008, getconf(1p)
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 http://www.unix.org/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 2013 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)