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
Maximum size of getgrgid_r() and	_SC_GETGR_R_SIZE_MAX
getgrnam_r() data buffers
Maximum 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_FILE_LOCKING	_SC_FILE_LOCKING
_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_MULTI_PROCESS	_SC_MULTI_PROCESS
_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_SYMLOOP_MAX	_SC_SYMLOOP_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_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_INHERIT	_SC_TRACE_INHERIT
_POSIX_TRACE_LOG	_SC_TRACE_LOG
_POSIX_TYPED_MEMORY_OBJECTS	_SC_TYPED_MEMORY_OBJECTS
_POSIX_VERSION	_SC_VERSION
_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_C_VERSION	_SC_2_C_VERSION
_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
_REGEX_VERSION	_SC_REGEX_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
_XBS5_ILP32_OFF32 (LEGACY)	_SC_XBS5_ILP32_OFF32 (LEGACY)
_XBS5_ILP32_OFFBIG (LEGACY)	_SC_XBS5_ILP32_OFFBIG (LEGACY)
_XBS5_LP64_OFF64 (LEGACY)	_SC_XBS5_LP64_OFF64 (LEGACY)
_XBS5_LPBIG_OFFBIG (LEGACY)	_SC_XBS5_LPBIG_OFFBIG (LEGACY)
_XOPEN_CRYPT	_SC_XOPEN_CRYPT
_XOPEN_ENH_I18N	_SC_XOPEN_ENH_I18N
_XOPEN_LEGACY	_SC_XOPEN_LEGACY
_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_VERSION	_SC_XOPEN_VERSION
_XOPEN_XCU_VERSION	_SC_XOPEN_XCU_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 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.

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.

If the value of sysconf(_SC_2_VERSION) is not equal to the value of the _POSIX2_VERSION symbolic constant, the utilities available via system() or popen() might not behave as described in the Shell and Utilities volume of IEEE Std 1003.1-2001. This would mean that the application is not running in an environment that conforms to the Shell and Utilities volume of IEEE Std 1003.1-2001. Some applications might be able to deal with this, others might not. However, the functions defined in this volume of IEEE Std 1003.1-2001 continue to operate as specified, even if sysconf(_SC_2_VERSION) reports that the utilities no longer perform as specified.

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 IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001.

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 writer.

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 IEEE Std 1003.1-2001 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 pathconf() .

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.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .