Provided by: manpages-posix-dev_2013a-1_all 
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 │
│_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
<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.1‐2008, 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().
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.1‐2008, <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 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 .