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sysconf - Man Page

get configuration information at run time


Standard C library (libc, -lc)


#include <unistd.h>

long sysconf(int name);


POSIX allows an application to test at compile or run time whether certain options are supported, or what the value is of certain configurable constants or limits.

At compile time this is done by including <unistd.h> and/or <limits.h> and testing the value of certain macros.

At run time, one can ask for numerical values using the present function sysconf(). One can ask for numerical values that may depend on the filesystem in which a file resides using fpathconf(3) and pathconf(3). One can ask for string values using confstr(3).

The values obtained from these functions are system configuration constants. They do not change during the lifetime of a process.

For options, typically, there is a constant _POSIX_FOO that may be defined in <unistd.h>. If it is undefined, one should ask at run time. If it is defined to -1, then the option is not supported. If it is defined to 0, then relevant functions and headers exist, but one has to ask at run time what degree of support is available. If it is defined to a value other than -1 or 0, then the option is supported. Usually the value (such as 200112L) indicates the year and month of the POSIX revision describing the option. glibc uses the value 1 to indicate support as long as the POSIX revision has not been published yet. The sysconf() argument will be _SC_FOO. For a list of options, see posixoptions(7).

For variables or limits, typically, there is a constant _FOO, maybe defined in <limits.h>, or _POSIX_FOO, maybe defined in <unistd.h>. The constant will not be defined if the limit is unspecified. If the constant is defined, it gives a guaranteed value, and a greater value might actually be supported. If an application wants to take advantage of values which may change between systems, a call to sysconf() can be made. The sysconf() argument will be _SC_FOO.

POSIX.1 variables

We give the name of the variable, the name of the sysconf() argument used to inquire about its value, and a short description.

First, the POSIX.1 compatible values.


The maximum length of the arguments to the exec(3) family of functions. Must not be less than _POSIX_ARG_MAX (4096).


The maximum number of simultaneous processes per user ID. Must not be less than _POSIX_CHILD_MAX (25).


Maximum length of a hostname, not including the terminating null byte, as returned by gethostname(2). Must not be less than _POSIX_HOST_NAME_MAX (255).


Maximum length of a login name, including the terminating null byte. Must not be less than _POSIX_LOGIN_NAME_MAX (9).


Maximum number of supplementary group IDs.

clock ticks - _SC_CLK_TCK

The number of clock ticks per second. The corresponding variable is obsolete. It was of course called CLK_TCK. (Note: the macro CLOCKS_PER_SEC does not give information: it must equal 1000000.)


The maximum number of files that a process can have open at any time. Must not be less than _POSIX_OPEN_MAX (20).


Size of a page in bytes. Must not be less than 1.


A synonym for PAGESIZE/_SC_PAGESIZE. (Both PAGESIZE and PAGE_SIZE are specified in POSIX.)


The number of repeated occurrences of a BRE permitted by regexec(3) and regcomp(3). Must not be less than _POSIX2_RE_DUP_MAX (255).


The maximum number of streams that a process can have open at any time. If defined, it has the same value as the standard C macro FOPEN_MAX. Must not be less than _POSIX_STREAM_MAX (8).


The maximum number of symbolic links seen in a pathname before resolution returns ELOOP. Must not be less than _POSIX_SYMLOOP_MAX (8).


The maximum length of terminal device name, including the terminating null byte. Must not be less than _POSIX_TTY_NAME_MAX (9).


The maximum number of bytes in a timezone name. Must not be less than _POSIX_TZNAME_MAX (6).


indicates the year and month the POSIX.1 standard was approved in the format YYYYMML; the value 199009L indicates the Sept. 1990 revision.

POSIX.2 variables

Next, the POSIX.2 values, giving limits for utilities.


indicates the maximum obase value accepted by the bc(1) utility.


indicates the maximum value of elements permitted in an array by bc(1).


indicates the maximum scale value allowed by bc(1).


indicates the maximum length of a string accepted by bc(1).


indicates the maximum numbers of weights that can be assigned to an entry of the LC_COLLATE order keyword in the locale definition file.


is the maximum number of expressions which can be nested within parentheses by expr(1).


The maximum length of a utility's input line, either from standard input or from a file. This includes space for a trailing newline.


The maximum number of repeated occurrences of a regular expression when the interval notation \{m,n\} is used.


indicates the version of the POSIX.2 standard in the format of YYYYMML.


indicates whether the POSIX.2 C language development facilities are supported.


indicates whether the POSIX.2 FORTRAN development utilities are supported.


indicates whether the POSIX.2 FORTRAN run-time utilities are supported.


indicates whether the POSIX.2 creation of locales via localedef(1) is supported.


indicates whether the POSIX.2 software development utilities option is supported.

These values also exist, but may not be standard.


The number of pages of physical memory. Note that it is possible for the product of this value and the value of _SC_PAGESIZE to overflow.


The number of currently available pages of physical memory.


The number of processors configured. See also get_nprocs_conf(3).


The number of processors currently online (available). See also get_nprocs_conf(3).

Return Value

The return value of sysconf() is one of the following:



name is invalid.


For an explanation of the terms used in this section, see attributes(7).

sysconf()Thread safetyMT-Safe env






It is difficult to use ARG_MAX because it is not specified how much of the argument space for exec(3) is consumed by the user's environment variables.

Some returned values may be huge; they are not suitable for allocating memory.

See Also

bc(1), expr(1), getconf(1), locale(1), confstr(3), fpathconf(3), pathconf(3), posixoptions(7)

Referenced By

atexit(3), clock_getres(2), confstr(3), execve(2), fpathconf(3), fsync(2), getgroups(2), get_nprocs(3), getpagesize(2), get_phys_pages(3), ksh93(1), mlock(2), mmap(2), mprotect(2), msync(2), pmem2_config_set_length(3), pmem2_config_set_offset(3), pmem2_config_set_sharing(3), poolset(5), posixoptions(7), procenv(1), realpath(3), sched_setaffinity(2), signal-safety(7), standards(7), systemctl(1), times(2), ulimit(3).

2024-06-15 Linux man-pages 6.9.1