SYSCONF
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
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 .