--- name: "sysconf" section: "3" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/sysconf/3" --- # sysconf(3) get configurable system variables ## NAME **sysconf** – get configurable system variables ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *long*\ **sysconf**(*int name*); ## DESCRIPTION This interface is defined by IEEE Std 1003.1-1988 (“POSIX.1”). A far more complete interface is available using [sysctl(3)](/sysctl/3.md). The **sysconf**() function provides a method for applications to determine the current value of a configurable system limit or option variable. The *name* argument specifies the system variable to be queried. Symbolic constants for each name value are found in the include file ⟨unistd.h⟩. Shell programmers who need access to these parameters should use the [getconf(1)](/getconf/1.md) utility. The available values are as follows: - \_SC\_ARG\_MAX The maximum bytes of argument to [execve(2)](/execve/2.md). - \_SC\_CHILD\_MAX The maximum number of simultaneous processes per user id. - \_SC\_CLK\_TCK The frequency of the statistics clock in ticks per second. - \_SC\_IOV\_MAX The maximum number of elements in the I/O vector used by [readv(2)](/readv/2.md), [writev(2)](/writev/2.md), [recvmsg(2)](/recvmsg/2.md), and [sendmsg(2)](/sendmsg/2.md). - \_SC\_NGROUPS\_MAX The maximum number of supplemental groups. - \_SC\_NPROCESSORS\_CONF The number of processors configured. - \_SC\_NPROCESSORS\_ONLN The number of processors currently online, taking into account current jail restrictions to report only the number of processors that are usable to the process. - \_SC\_OPEN\_MAX One more than the maximum value the system may assign to a new file descriptor. - \_SC\_PAGESIZE The size of a system page in bytes. - \_SC\_PAGE\_SIZE Equivalent to \_SC\_PAGESIZE. - \_SC\_STREAM\_MAX The minimum maximum number of streams that a process may have open at any one time. - \_SC\_TZNAME\_MAX The minimum maximum number of types supported for the name of a timezone. - \_SC\_JOB\_CONTROL Return 1 if job control is available on this system, otherwise -1. - \_SC\_SAVED\_IDS Returns 1 if saved set-group and saved set-user ID is available, otherwise -1. - \_SC\_VERSION The version of IEEE Std 1003.1 (“POSIX.1”) with which the system attempts to comply. - \_SC\_BC\_BASE\_MAX The maximum ibase/obase values in the [bc(1)](/bc/1.md) utility. - \_SC\_BC\_DIM\_MAX The maximum array size in the [bc(1)](/bc/1.md) utility. - \_SC\_BC\_SCALE\_MAX The maximum scale value in the [bc(1)](/bc/1.md) utility. - \_SC\_BC\_STRING\_MAX The maximum string length in the [bc(1)](/bc/1.md) utility. - \_SC\_COLL\_WEIGHTS\_MAX The maximum number of weights that can be assigned to any entry of the LC\_COLLATE order keyword in the locale definition file. - \_SC\_EXPR\_NEST\_MAX The maximum number of expressions that can be nested within parenthesis by the [expr(1)](/expr/1.md) utility. - \_SC\_LINE\_MAX The maximum length in bytes of a text-processing utility's input line. - \_SC\_RE\_DUP\_MAX The maximum number of repeated occurrences of a regular expression permitted when using interval notation. - \_SC\_2\_VERSION The version of IEEE Std 1003.2 (“POSIX.2”) with which the system attempts to comply. - \_SC\_2\_C\_BIND Return 1 if the system's C-language development facilities support the C-Language Bindings Option, otherwise -1. - \_SC\_2\_C\_DEV Return 1 if the system supports the C-Language Development Utilities Option, otherwise -1. - \_SC\_2\_CHAR\_TERM Return 1 if the system supports at least one terminal type capable of all operations described in IEEE Std 1003.2 (“POSIX.2”), otherwise -1. - \_SC\_2\_FORT\_DEV Return 1 if the system supports the FORTRAN Development Utilities Option, otherwise -1. - \_SC\_2\_FORT\_RUN Return 1 if the system supports the FORTRAN Runtime Utilities Option, otherwise -1. - \_SC\_2\_LOCALEDEF Return 1 if the system supports the creation of locales, otherwise -1. - \_SC\_2\_SW\_DEV Return 1 if the system supports the Software Development Utilities Option, otherwise -1. - \_SC\_2\_UPE Return 1 if the system supports the User Portability Utilities Option, otherwise -1. - \_SC\_AIO\_LISTIO\_MAX Maximum number of I/O operations in a single list I/O call supported. - \_SC\_AIO\_MAX Maximum number of outstanding asynchronous I/O operations supported. - \_SC\_AIO\_PRIO\_DELTA\_MAX The maximum amount by which a process can decrease its asynchronous I/O priority level from its own scheduling priority. - \_SC\_DELAYTIMER\_MAX Maximum number of timer expiration overruns. - \_SC\_MQ\_OPEN\_MAX The maximum number of open message queue descriptors a process may hold. - \_SC\_RTSIG\_MAX Maximum number of realtime signals reserved for application use. - \_SC\_SEM\_NSEMS\_MAX Maximum number of semaphores that a process may have. - \_SC\_SEM\_VALUE\_MAX The maximum value a semaphore may have. - \_SC\_SIGQUEUE\_MAX Maximum number of queued signals that a process may send and have pending at the receiver(s) at any time. - \_SC\_TIMER\_MAX Maximum number of timers per process supported. - \_SC\_GETGR\_R\_SIZE\_MAX Suggested initial value for the size of the group entry buffer. - \_SC\_GETPW\_R\_SIZE\_MAX Suggested initial value for the size of the password entry buffer. - \_SC\_HOST\_NAME\_MAX Maximum length of a host name (not including the terminating null) as returned from the **gethostname**() function. - \_SC\_LOGIN\_NAME\_MAX Maximum length of a login name. - \_SC\_THREAD\_STACK\_MIN Minimum size in bytes of thread stack storage. - \_SC\_THREAD\_THREADS\_MAX Maximum number of threads that can be created per process. - \_SC\_TTY\_NAME\_MAX Maximum length of terminal device name. - \_SC\_SYMLOOP\_MAX Maximum number of symbolic links that can be reliably traversed in the resolution of a pathname in the absence of a loop. - \_SC\_ATEXIT\_MAX Maximum number of functions that may be registered with **atexit**(). - \_SC\_XOPEN\_VERSION An integer value greater than or equal to 4, indicating the version of the X/Open Portability Guide to which this system conforms. - \_SC\_XOPEN\_XCU\_VERSION An integer value indicating the version of the XCU Specification to which this system conforms. These values also exist, but may not be standard: - \_SC\_CPUSET\_SIZE Size of the kernel cpuset. - \_SC\_PHYS\_PAGES The number of pages of physical memory. Note that it is possible that the product of this value and the value of \_SC\_PAGESIZE will overflow a *long* in some configurations on a 32bit machine. ## RETURN VALUES If the call to **sysconf**() is not successful, -1 is returned and *errno* is set appropriately. Otherwise, if the variable is associated with functionality that is not supported, -1 is returned and *errno* is not modified. Otherwise, the current variable value is returned. ## ERRORS The **sysconf**() function may fail and set *errno* for any of the errors specified for the library function [sysctl(3)](/sysctl/3.md). In addition, the following error may be reported: - \[EINVAL\] The value of the *name* argument is invalid. ## SEE ALSO [getconf(1)](/getconf/1.md), [pathconf(2)](/pathconf/2.md), [confstr(3)](/confstr/3.md), [sysctl(3)](/sysctl/3.md) ## STANDARDS Except for the fact that values returned by **sysconf**() may change over the lifetime of the calling process, this function conforms to IEEE Std 1003.1-1988 (“POSIX.1”). ## HISTORY The **sysconf**() function first appeared in 4.4BSD. ## BUGS The value for \_SC\_STREAM\_MAX is a minimum maximum, and required to be the same as ANSI C's FOPEN\_MAX, so the returned value is a ridiculously small and misleading number.