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.
NAMEsysconf — get configurable system variables
SYNOPSIS
#include <unistd.h>
long sysconf(int name);
DESCRIPTION
The sysconf() function provides a method for the application to deter‐
mine the current value of a configurable system limit or option (vari‐
able). 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 <lim‐
its.h> or <unistd.h> that can be returned by sysconf(), and the sym‐
bolic 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 │
└────────────────────────────────┴─────────────────────────────────┘
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 <lim‐
its.h>.
Otherwise, sysconf() shall return the current variable value on the
system. The value returned shall not be more restrictive than the cor‐
responding 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 sys‐
tem configurations. It is closely related to pathconf() and fpath‐
conf().
Although a conforming application can run on all systems by never
demanding more resources than the minimum values published in this vol‐
ume 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 applica‐
tion 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 par‐
tially 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 associ‐
ated 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 configura‐
tion 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().
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 ALSOconfstr(), fpathconf()
The Base Definitions volume of POSIX.1‐2008, <limits.h>, <unistd.h>
The Shell and Utilities volume of POSIX.1‐2008, getconf
COPYRIGHT
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 Electri‐
cal 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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SYSCONF(3P)
