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SYSCONF(3) Library Functions Manual SYSCONF(3)

sysconf — get configurable system variables

library “libc”

#include <unistd.h>

long
sysconf(int name);

This interface is defined by IEEE Std 1003.1-1988 (“POSIX.1”). A far more complete interface is available using sysctl(3).

The () 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) utility.

The available values are as follows:

The maximum bytes of argument to execve(2).
The maximum number of simultaneous processes per user id.
The frequency of the statistics clock in ticks per second.
The maximum number of elements in the I/O vector used by readv(2), writev(2), recvmsg(2), and sendmsg(2).
The maximum number of supplemental groups.
The number of processors configured.
The number of processors currently online.
One more than the maximum value the system may assign to a new file descriptor.
The size of a system page in bytes.
Equivalent to _SC_PAGESIZE.
The minimum maximum number of streams that a process may have open at any one time.
The minimum maximum number of types supported for the name of a timezone.
Return 1 if job control is available on this system, otherwise -1.
Returns 1 if saved set-group and saved set-user ID is available, otherwise -1.
The version of IEEE Std 1003.1 (“POSIX.1”) with which the system attempts to comply.
The maximum ibase/obase values in the bc(1) utility.
The maximum array size in the bc(1) utility.
The maximum scale value in the bc(1) utility.
The maximum string length in the bc(1) utility.
The maximum number of weights that can be assigned to any entry of the LC_COLLATE order keyword in the locale definition file.
The maximum number of expressions that can be nested within parenthesis by the expr(1) utility.
The maximum length in bytes of a text-processing utility's input line.
The maximum number of repeated occurrences of a regular expression permitted when using interval notation.
The version of IEEE Std 1003.2 (“POSIX.2”) with which the system attempts to comply.
Return 1 if the system's C-language development facilities support the C-Language Bindings Option, otherwise -1.
Return 1 if the system supports the C-Language Development Utilities Option, otherwise -1.
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.
Return 1 if the system supports the FORTRAN Development Utilities Option, otherwise -1.
Return 1 if the system supports the FORTRAN Runtime Utilities Option, otherwise -1.
Return 1 if the system supports the creation of locales, otherwise -1.
Return 1 if the system supports the Software Development Utilities Option, otherwise -1.
Return 1 if the system supports the User Portability Utilities Option, otherwise -1.
Maximum number of I/O operations in a single list I/O call supported.
Maximum number of outstanding asynchronous I/O operations supported.
The maximum amount by which a process can decrease its asynchronous I/O priority level from its own scheduling priority.
Maximum number of timer expiration overruns.
The maximum number of open message queue descriptors a process may hold.
Maximum number of realtime signals reserved for application use.
Maximum number of semaphores that a process may have.
The maximum value a semaphore may have.
Maximum number of queued signals that a process may send and have pending at the receiver(s) at any time.
Maximum number of timers per process supported.
Suggested initial value for the size of the group entry buffer.
Suggested initial value for the size of the password entry buffer.
Maximum length of a host name (not including the terminating null) as returned from the () function.
Maximum length of a login name.
Minimum size in bytes of thread stack storage.
Maximum number of threads that can be created per process.
Maximum length of terminal device name.
Maximum number of symbolic links that can be reliably traversed in the resolution of a pathname in the absence of a loop.
Maximum number of functions that may be registered with ().
An integer value greater than or equal to 4, indicating the version of the X/Open Portability Guide to which this system conforms.
An integer value indicating the version of the XCU Specification to which this system conforms.

These values also exist, but may not be standard:

Size of the kernel cpuset.
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.

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.

The sysconf() function may fail and set errno for any of the errors specified for the library function sysctl(3). In addition, the following error may be reported:

[]
The value of the name argument is invalid.

getconf(1), pathconf(2), confstr(3), sysctl(3)

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”).

The sysconf() function first appeared in 4.4BSD.

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.

April 26, 2013 FreeBSD-12.0