sysconf(3)

✨ NAME ✨

sysconf — get configuration information at run time 🫶 (it's a Library Functions Manual situation, and yes we are in our libc era)

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📚 LIBRARY 📚

Standard C library (libc, -lc) 💜

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🌸 SYNOPSIS 🌸

#include <unistd.h>

long sysconf(int name);

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💅 DESCRIPTION 💅

okay so here's the tea, bestie ☕ POSIX lets your app do a lil vibe check — either at compile time or at run time — to find out whether certain options are supported, or what the actual value of certain configurable constants or limits is.

At compile time, you do that by including <unistd.h> and/or <limits.h> and testing the value of certain macros ✨

At run time, you can ask for numerical values using the present function, sysconf(). If you want numerical values that may depend on the filesystem where a file lives, that's fpathconf(3) and pathconf(3) territory 👀 And if you want string values, you go hit up confstr(3).

The values you get from these functions are system configuration constants. Lowkey important: they do not change during the lifetime of a process. no cap 📌

For options, typically there's a constant _POSIX_FOO that may be defined in <unistd.h>. Here's how to read it:

• if it's undefined, you should ask at run time 🙋
• if it's defined to -1, the option is not supported 💀
• if it's defined to 0, then the relevant functions and headers exist, but you still have to ask at run time what degree of support is actually available
• if it's defined to a value other than -1 or 0, then the option is supported 🎉

Usually that value (like 200112L) tells you 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 hasn't been published yet. In these cases the sysconf() argument will be _SC_FOO. For a list of options, go peek at posixoptions(7) 📖

For variables or limits, typically there's 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 👑 So if your app wants to take advantage of values that may change between systems, you call sysconf(). The sysconf() argument will be _SC_FOO.

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🌈 POSIX.1 variables 🌈

We give you the name of the variable, the name of the sysconf() argument used to inquire about its value, and a short description. First up, the POSIX.1 compatible values 💖

• ARG_MAX - _SC_ARG_MAX — the maximum length of the arguments to the exec(3) family of functions. Must not be less than _POSIX_ARG_MAX (4096).
• CHILD_MAX - _SC_CHILD_MAX — the maximum number of simultaneous processes per user ID. Must not be less than _POSIX_CHILD_MAX (25).
• HOST_NAME_MAX - _SC_HOST_NAME_MAX — 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).
• LOGIN_NAME_MAX - _SC_LOGIN_NAME_MAX — maximum length of a login name, including the terminating null byte. Must not be less than _POSIX_LOGIN_NAME_MAX (9).
• NGROUPS_MAX - _SC_NGROUPS_MAX — 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.)
• OPEN_MAX - _SC_OPEN_MAX — the maximum number of files that a process can have open at any time. Must not be less than _POSIX_OPEN_MAX (20).
• PAGESIZE - _SC_PAGESIZE — size of a page in bytes. Must not be less than 1.
• PAGE_SIZE - _SC_PAGE_SIZE — a synonym for PAGESIZE/_SC_PAGESIZE. (Both PAGESIZE and PAGE_SIZE are specified in POSIX.)
• RE_DUP_MAX - _SC_RE_DUP_MAX — 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).
• STREAM_MAX - _SC_STREAM_MAX — 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).
• SYMLOOP_MAX - _SC_SYMLOOP_MAX — the maximum number of symbolic links seen in a pathname before resolution returns ELOOP. Must not be less than _POSIX_SYMLOOP_MAX (8).
• TTY_NAME_MAX - _SC_TTY_NAME_MAX — the maximum length of terminal device name, including the terminating null byte. Must not be less than _POSIX_TTY_NAME_MAX (9).
• TZNAME_MAX - _SC_TZNAME_MAX — the maximum number of bytes in a timezone name. Must not be less than _POSIX_TZNAME_MAX (6).
• _POSIX_VERSION - _SC_VERSION — indicates the year and month the POSIX.1 standard was approved in the format YYYYMML; the value 199009L indicates the Sept. 1990 revision.

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🦋 POSIX.2 variables 🦋

Next up, the POSIX.2 values, which give limits for utilities 🧚

• BC_BASE_MAX - _SC_BC_BASE_MAX — indicates the maximum obase value accepted by the bc(1) utility.
• BC_DIM_MAX - _SC_BC_DIM_MAX — indicates the maximum value of elements permitted in an array by bc(1).
• BC_SCALE_MAX - _SC_BC_SCALE_MAX — indicates the maximum scale value allowed by bc(1).
• BC_STRING_MAX - _SC_BC_STRING_MAX — indicates the maximum length of a string accepted by bc(1).
• COLL_WEIGHTS_MAX - _SC_COLL_WEIGHTS_MAX — indicates the maximum numbers of weights that can be assigned to an entry of the LC_COLLATE order keyword in the locale definition file.
• EXPR_NEST_MAX - _SC_EXPR_NEST_MAX — is the maximum number of expressions which can be nested within parentheses by expr(1).
• LINE_MAX - _SC_LINE_MAX — the maximum length of a utility's input line, either from standard input or from a file. This includes space for a trailing newline.
• RE_DUP_MAX - _SC_RE_DUP_MAX — the maximum number of repeated occurrences of a regular expression when the interval notation \{m,n\} is used.
• POSIX2_VERSION - _SC_2_VERSION — indicates the version of the POSIX.2 standard in the format of YYYYMML.
• POSIX2_C_DEV - _SC_2_C_DEV — indicates whether the POSIX.2 C language development facilities are supported.
• POSIX2_FORT_DEV - _SC_2_FORT_DEV — indicates whether the POSIX.2 FORTRAN development utilities are supported.
• POSIX2_FORT_RUN - _SC_2_FORT_RUN — indicates whether the POSIX.2 FORTRAN run-time utilities are supported.
• _POSIX2_LOCALEDEF - _SC_2_LOCALEDEF — indicates whether the POSIX.2 creation of locales via localedef(1) is supported.
• POSIX2_SW_DEV - _SC_2_SW_DEV — indicates whether the POSIX.2 software development utilities option is supported.

These values also exist, but may not be standard 👀✨

• _SC_PHYS_PAGES — 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 😭
• _SC_AVPHYS_PAGES — the number of currently available pages of physical memory.
• _SC_NPROCESSORS_CONF — the number of processors configured. See also get_nprocs_conf(3).
• _SC_NPROCESSORS_ONLN — the number of processors currently online (available). See also get_nprocs_conf(3).

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🎀 RETURN VALUE 🎀

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

• On error, -1 is returned and errno is set to indicate the error (for example, EINVAL, indicating that name is invalid).
• If name corresponds to a maximum or minimum limit, and that limit is indeterminate, -1 is returned and errno is not changed. (To distinguish an indeterminate limit from an error, set errno to zero before the call, and then check whether errno is nonzero when -1 is returned.) 🕵️‍♀️
• If name corresponds to an option, a positive value is returned if the option is supported, and -1 is returned if the option is not supported.
• Otherwise, the current value of the option or limit is returned. This value will not be more restrictive than the corresponding value that was described to the application in <unistd.h> or <limits.h> when the application was compiled. 📌

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💀 ERRORS 💀

• EINVAL — name is invalid.

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🌟 ATTRIBUTES 🌟

For an explanation of the terms used in this section, see attributes(7). Here's the lil chart, translated to vibes:

• Interface: sysconf()
• Attribute: Thread safety
• Value: MT-Safe env

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👑 STANDARDS 👑

POSIX.1-2008. periodt 💅

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🕰️ HISTORY 🕰️

POSIX.1-2001.

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🙈 BUGS 🙈

• 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. 🚨

fr fr do not go malloc-ing based on these numbers, that's how you get got 💀

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🔥 SEE ALSO 🔥

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

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and that's the whole page, bestie 🫶 this was pulled straight from Linux man-pages 6.20, dated 2026-02-08 — the sysconf(3) era, slay 💖✨🎀