#include <unistd.h>

long sysconf(int name);

The sysconf() function provides a method for an application to determine the current value of a configurable system limit or option (variable).

The name argument represents the system variable to be queried. The following table lists the minimal set of system variables from <limits.h> and <unistd.h> that can be returned by sysconf() and the symbolic constants defined in <unistd.h> that are the corresponding values used for name on the SPARC and x86 platforms.

 Name Return Value Meaning
_______________________________________________________________________
_SC_2_C_BIND _POSIX2_C_BIND Supports the C lang-
 uage binding option
_SC_2_C_DEV _POSIX2_C_DEV Supports the C lang-
 uage development
 utilities option
_SC_2_C_VERSION _POSIX2_C_VERSION Integer value
 indicates version
 of ISO POSIX-2
 standard (Commands)
_SC_2_CHAR_TERM _POSIX2_CHAR_TERM Supports at least
 one terminal
_SC_2_FORT_DEV _POSIX2_FORT_DEV Supports FORTRAN
 Development
 Utilities Option
_SC_2_FORT_RUN _POSIX2_FORT_RUN Supports FORTRAN
 Run-time Utilities
 Option
_SC_2_LOCALEDEF _POSIX2_LOCALEDEF Supports creation
 of locales by the
 localedef utility
_SC_2_SW_DEV _POSIX2_SW_DEV Supports Software
 Development Utility
 Option
_SC_2_UPE _POSIX2_UPE Supports User
 Portability
 Utilities Option
_SC_2_VERSION _POSIX2_VERSION Integer value
 indicates version
 of ISO POSIX-2
 standard (C language
 binding)
_SC_AIO_LISTIO_MAX AIO_LISTIO_MAX Max number of I/O
 operations in a
 single list I/O call
 supported
_SC_AIO_MAX AIO_MAX Max number of
 outstanding
 asynchronous I/O
 operations supported
_SC_AIO_PRIO_DELTA_MAX AIO_PRIO_DELTA_MAX Max amount by which
 process can decrease
 its asynchronous
 I/O priority level
 from its own
 scheduling priority
_SC_ARG_MAX ARG_MAX Max size of argv[]
 plus envp[]
_SC_ASYNCHRONOUS_IO _POSIX_ASYNCHRONOUS_IO Supports
 Asynchronous I/O
_SC_ATEXIT_MAX ATEXIT_MAX Max number of
 functions that can
 be registered with
 atexit()
_SC_AVPHYS_PAGES Number of physical
 memory pages not
 currently in use by
 system
_SC_BARRIERS _POSIX_BARRIERS Supports Barriers
 option
_SC_BC_BASE_MAX BC_BASE_MAX Maximum obase values
 allowed by bc
_SC_BC_DIM_MAX BC_DIM_MAX Max number of
 elements permitted
 in array by bc
_SC_BC_SCALE_MAX BC_SCALE_MAX Max scale value
 allowed by bc
_SC_BC_STRING_MAX BC_STRING_MAX Max length of string
 constant allowed by
 bc
_SC_CHILD_MAX CHILD_MAX Max processes
 allowed to a UID
_SC_CLK_TCK CLK_TCK Ticks per second
 (clock_t)
_SC_CLOCK_SELECTION _POSIX_CLOCK_SELECTION Supports Clock 
 Selection option
_SC_COLL_WEIGHTS_MAX COLL_WEIGHTS_MAX Max number of
 weights that can be
 assigned to entry of
 the LC_COLLATE order
 keyword in locale
 definition file
_SC_CPUID_MAX Max possible 
 processor ID
_SC_DELAYTIMER_MAX DELAYTIMER_MAX Max number of timer
 expiration overruns
_SC_EXPR_NEST_MAX EXPR_NEST_MAX Max number of
 parentheses by expr
_SC_FSYNC _POSIX_FSYNC Supports File
 Synchronization
_SC_GETGR_R_SIZE_MAX Max size of group
 entry buffer
_SC_GETPW_R_SIZE_MAX Max size of password
 entry buffer
_SC_HOST_NAME_MAX _POSIX_HOST_NAME_MAX Maximum length of a
 host name (excluding
 terminating null)
_SC_IOV_MAX IOV_MAX Max number of iovec
 structures available
 to one process for
 use with readv()
 and writev()
_SC_JOB_CONTROL _POSIX_JOB_CONTROL Job control
 supported?
_SC_LINE_MAX LINE_MAX Max length of input
 line
_SC_LOGIN_NAME_MAX LOGNAME_MAX + 1 Max length of login
 name
_SC_LOGNAME_MAX LOGNAME_MAX
_SC_MAPPED_FILES _POSIX_MAPPED_FILES Supports Memory
 Mapped Files
_SC_MAXPID Max pid value
_SC_MEMLOCK _POSIX_MEMLOCK Supports Process
 Memory Locking
_SC_MEMLOCK_RANGE _POSIX_MEMLOCK_RANGE Supports Range
 Memory Locking
_SC_MEMORY_PROTECTION _POSIX_MEMORY_PROTECTION Supports Memory
 Protection
_SC_MESSAGE_PASSING _POSIX_MESSAGE_PASSING Supports Message
 Passing
_SC_MONOTONIC_CLOCK _POSIX_MONOTONIC_CLOCK Supports Monotonic
 Clock option
_SC_MQ_OPEN_MAX MQ_OPEN_MAX Max number of open
 message queues a
 process can hold
_SC_MQ_PRIO_MAX MQ_PRIO_MAX Max number of
 message priorities
 supported
_SC_NGROUPS_MAX NGROUPS_MAX Max simultaneous
 groups to which
 one can belong
_SC_NPROCESSORS_CONF Number of processors
 configured
_SC_NPROCESSORS_MAX Max number of
 processors supported
 by platform
_SC_NPROCESSORS_ONLN Number of processors
 online
_SC_OPEN_MAX OPEN_MAX Max open files per
 process
_SC_PAGESIZE PAGESIZE System memory page
 size
_SC_PAGE_SIZE PAGESIZE Same as _SC_PAGESIZE
_SC_PASS_MAX PASS_MAX Max number of
 significant bytes
 in a password
_SC_PHYS_PAGES Total number of
 pages of physical
 memory in system
_SC_PRIORITIZED_IO _POSIX_PRIORITIZED_IO Supports Prioritized
 I/O
_SC_PRIORITY_SCHEDULING _POSIX_PRIORITY_SCHEDULING Supports Process
 Scheduling
_SC_RAW_SOCKETS _POSIX_RAW_SOCKETS Supports Raw Sockets
 option
_SC_RE_DUP_MAX RE_DUP_MAX Max number of
 repeated occurrences
 of a regular
 expression permitted
 when using interval
 notation \e{m,n\e}
_SC_READER_WRITER_LOCKS _POSIX_READER_WRITER_LOCKS Supports IPV6 option
_SC_REALTIME_SIGNALS _POSIX_REALTIME_SIGNALS Supports Realtime
 Signals
_SC_REGEXP _POSIX_REGEXP Supports Regular
 Expression Handling
 option
_SC_RTSIG_MAX RTSIG_MAX Max number of
 realtime signals
 reserved for
 application use
_SC_SAVED_IDS _POSIX_SAVED_IDS Saved IDs
 (seteuid())
 supported?
_SC_SEM_NSEMS_MAX SEM_NSEMS_MAX Max number of POSIX
 semaphores a process
 can have
_SC_SEM_VALUE_MAX SEM_VALUE_MAX Max value a POSIX
 semaphore can have
_SC_SEMAPHORES _POSIX_SEMAPHORES Supports Semaphores
_SC_SHARED_MEMORY_ _POSIX_SHARED_MEMORY_ Supports Shared
 OBJECTS OBJECTS Memory Objects
_SC_SHELL _POSIX_SHELL Supports POSIX shell
_SC_SIGQUEUE_MAX SIGQUEUE_MAX Max number of queued
 signals that a
 process can send and
 have pending at
 receiver(s) at a
 time
_SC_SPAWN _POSIX_SPAWN Supports Spawn option
_SC_SPIN_LOCKS _POSIX_SPIN_LOCKS Supports Spin Locks
 option
_SC_STACK_PROT Default stack
 protection
_SC_STREAM_MAX STREAM_MAX Number of streams
 one process can
 have open at a time
_SC_SYMLOOP_MAX _POSIX_SYMLOOP_MAX Max number of symbolic
 links that can be
 reliably traversed in
 the resolution of a
 pathname in the absence
 of a loop
_SC_SYNCHRONIZED_IO _POSIX_SYNCHRONIZED_IO Supports
 Synchronized I/O
_SC_THREAD_ATTR_ _POSIX_THREAD_ATTR_ Supports Thread
 STACKADDR STACKADDR Stack Address
 Attribute option
_SC_THREAD_ATTR_ _POSIX_THREAD_ATTR_ Supports Thread
 STACKSIZE STACKSIZE Stack Size
 Attribute option
_SC_THREAD_DESTRUCTOR_ PTHREAD_DESTRUCTOR_ Number attempts made
 ITERATIONS ITERATIONS to destroy thread-
 specific data on
 thread exit
_SC_THREAD_KEYS_MAX PTHREAD_KEYS_MAX Max number of data
 keys per process
_SC_THREAD_PRIO_ _POSIX_THREAD_PRIO_ Supports Priority
 INHERIT INHERIT Inheritance option
_SC_THREAD_PRIO_ _POSIX_THREAD_PRIO_ Supports Priority
 PROTECT PROTECT Protection option
_SC_THREAD_PRIORITY_ _POSIX_THREAD_PRIORITY_ Supports Thread
 SCHEDULING SCHEDULING Execution
 Scheduling option
_SC_THREAD_PROCESS_ _POSIX_THREAD_PROCESS_ Supports
 SHARED SHARED Process-Shared
 Synchronization
 option
_SC_THREAD_SAFE_ _POSIX_THREAD_SAFE_ Supports Thread-Safe
 FUNCTIONS FUNCTIONS Functions option
_SC_THREAD_STACK_MIN PTHREAD_STACK_MIN Min byte size of
 thread stack storage
_SC_THREAD_THREADS_MAX PTHREAD_THREADS_MAX Max number of
 threads per process
_SC_THREADS _POSIX_THREADS Supports Threads
 option
_SC_TIMEOUTS _POSIX_TIMEOUTS Supports Timeouts
 option
_SC_TIMER_MAX TIMER_MAX Max number of timer
 per process
 supported
_SC_TIMERS _POSIX_TIMERS Supports Timers
_SC_TTY_NAME_MAX TTYNAME_MAX Max length of tty
 device name
_SC_TZNAME_MAX TZNAME_MAX Max number of bytes
 supported for name
 of a time zone
_SC_V6_ILP32_OFF32 _POSIX_V6_ILP32_OFF32 Supports X/Open
 ILP32 w/32-bit
 offset build 
 environment
_SC_V6_ILP32_OFFBIG _POSIX_V6_ILP32_OFFBIG Supports X/Open
 ILP32 w/64-bit
 offset build 
 environment
_SC_V6_LP64_OFF64 _POSIX_V6_LP64_OFF64 Supports X/Open
 LP64 w/64-bit
 offset build 
 environment
_SC_V6_LPBIG_OFFBIG _POSIX_V6_LPBIG_OFFBIG Same as 
 _SC_V6_LP64_OFF64
_SC_VERSION _POSIX_VERSION POSIX.1 version
 supported
_SC_XBS5_ILP32_OFF32 _XBS_ILP32_OFF32 Indicates support
 for X/Open ILP32
 w/32-bit offset
 build environment
_SC_XBS5_ILP32_OFFBIG _XBS5_ILP32_OFFBIG Indicates support
 for X/Open ILP32
 w/64-bit offset
 build environment
_SC_XBS5_LP64_OFF64 _XBS5_LP64_OFF64 Indicates support of
 X/Open LP64,
 64-bit offset
 build environment
_SC_XBS5_LPBIG_OFFBIG _XBS5_LP64_OFF64 Same as
 _SC_XBS5_LP64_OFF64
_SC_XOPEN_CRYPT _XOPEN_CRYPT Supports X/Open
 Encryption Feature
 Group
_SC_XOPEN_ENH_I18N _XOPEN_ENH_I18N Supports X/Open
 Enhanced
 Internationalization
 Feature Group
_SC_XOPEN_LEGACY _XOPEN_LEGACY Supports X/Open
 Legacy Feature Group
_SC_XOPEN_REALTIME _XOPEN_REALTIME Supports X/Open
 POSIX Realtime
 Feature Group
_SC_XOPEN_REALTIME_ _XOPEN_REALTIME_THREADS Supports X/Open
 THREADS POSIX Reatime
 Threads Feature
 Group
_SC_XOPEN_SHM _XOPEN_SHM Supports X/Open
 Shared Memory
 Feature Group
_SC_XOPEN_STREAMS _POSIX_XOPEN_STREAMS Supports XSI Streams
 option group
_SC_XOPEN_UNIX _XOPEN_UNIX Supports X/Open CAE
 Specification,
 August 1994, System
 Interfaces and
 Headers, Issue 4,
 Version 2
_SC_XOPEN_VERSION _XOPEN_VERSION Integer value
 indicates version of
 X/Open Portability
 Guide to which
 implementation
 conforms
_SC_XOPEN_XCU_VERSION _XOPEN_XCU_VERSION Integer value
 indicates version of
 XCU specification to
 which implementation
 conforms

The following options are not supported and return -1:

Upon successful completion, sysconf() returns the current variable value on the system. The value returned will not be more restrictive than the corresponding value described to the application when it was compiled with the implementation's <limits.h>, <unistd.h> or <time.h>. With only a few obvious exceptions such as _SC_AVPHYS_PAGES and _SC_NPROCESSORS_ONLN, the value will not change during the lifetime of the calling process.

If name is an invalid value, sysconf() returns -1 and sets errno to indicate the error. If the variable corresponding to name is associated with functionality that is not supported by the system, sysconf() returns -1 without changing the value of errno.

Calling sysconf() with the following returns -1 without setting errno, because no maximum limit can be determined. The system supports at least the minimum values and can support higher values depending upon system resources.

Variable Minimum supported value
_SC_AIO_MAX _POSIX_AIO_MAX
_SC_ATEXIT_MAX 32
_SC_MQ_OPEN_MAX 32
_SC_THREAD_THREADS_MAX _POSIX_THREAD_THREADS_MAX
_SC_THREAD_KEYS_MAX _POSIX_THREAD_KEYS_MAX
_SC_THREAD_DESTRUCTOR_ITERATIONS _POSIX_THREAD_DESTRUCTOR_ITERATIONS

The following SPARC and x86 platform variables return EINVAL:

_SC_COHER_BLKSZ _SC_DCACHE_ASSOC
_SC_DCACHE_BLKSZ _SC_DCACHE_LINESZ
_SC_DCACHE_SZ _SC_DCACHE_TBLKSZ
_SC_ICACHE_ASSOC _SC_ICACHE_BLKSZ
_SC_ICACHE_LINESZ _SC_ICACHE_SZ
_SC_SPLIT_CACHE

The sysconf() function will fail if:

EINVAL

See attributes(5) for descriptions of the following attributes:

pooladm(1M), zoneadm(1M), fpathconf(2), seteuid(2), setrlimit(2), confstr(3C), attributes(5), standards(5)

A call to setrlimit() can cause the value of OPEN_MAX to change.

Multiplying sysconf(_SC_PHYS_PAGES) or sysconf(_SC_AVPHYS_PAGES) by sysconf(_SC_PAGESIZE) to determine memory amount in bytes can exceed the maximum values representable in a 32-bit signed or unsigned integer.

The value of CLK_TCK can be variable and it should not be assumed that CLK_TCK is a compile-time constant.

If the caller is in a non-global zone and the pools facility is active, sysconf(_SC_NPROCESSORS_CONF) and sysconf(_SC_NPROCESSORS_ONLN) return the number of processors in the processor set of the pool to which the zone is bound.