sysconf man page

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.

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

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2013 IEEE/The Open Group POSIX Programmer's Manual