sysconf(3) - Linux manual page (original) (raw)
sysconf(3) Library Functions Manual sysconf(3)
NAME top
sysconf - get configuration information at run timeLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <unistd.h>**
**long sysconf(int** _name_**);**DESCRIPTION top
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)](../man3/fpathconf.3.html) and [pathconf(3)](../man3/pathconf.3.html). One can ask for string values
using [confstr(3)](../man3/confstr.3.html).
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)](../man7/posixoptions.7.html).
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.
**ARG_MAX** - **_SC_ARG_MAX**
The maximum length of the arguments to the [exec(3)](../man3/exec.3.html) 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)](../man2/gethostname.2.html).
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)](../man3/regexec.3.html) and [regcomp(3)](../man3/regcomp.3.html). 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.POSIX.2 variables Next, the POSIX.2 values, giving 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)](../man1/expr.1.html).
**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)](../man1/localedef.1.html) 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)](../man3/get%5Fnprocs%5Fconf.3.html).
- **_SC_NPROCESSORS_ONLN**
The number of processors currently online (available).
See also [get_nprocs_conf(3)](../man3/get%5Fnprocs%5Fconf.3.html).RETURN VALUE top
The return value of **sysconf**() is one of the following:
• On error, -1 is returned and _[errno](../man3/errno.3.html)_ 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](../man3/errno.3.html)_ is not
changed. (To distinguish an indeterminate limit from an
error, set _[errno](../man3/errno.3.html)_ to zero before the call, and then check
whether _[errno](../man3/errno.3.html)_ 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.ERRORS top
**EINVAL** _name_ is invalid.ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌─────────────────────────────────┬───────────────┬─────────────┐
│ **Interface** │ **Attribute** │ **Value** │
├─────────────────────────────────┼───────────────┼─────────────┤
│ **sysconf**() │ Thread safety │ MT-Safe env │
└─────────────────────────────────┴───────────────┴─────────────┘STANDARDS top
POSIX.1-2008.HISTORY top
POSIX.1-2001.BUGS top
It is difficult to use **ARG_MAX** because it is not specified how
much of the argument space for [exec(3)](../man3/exec.3.html) is consumed by the user's
environment variables.
Some returned values may be huge; they are not suitable for
allocating memory.SEE ALSO top
**bc**(1), [expr(1)](../man1/expr.1.html), **getconf**(1), [locale(1)](../man1/locale.1.html), [confstr(3)](../man3/confstr.3.html), [fpathconf(3)](../man3/fpathconf.3.html),
[pathconf(3)](../man3/pathconf.3.html), [posixoptions(7)](../man7/posixoptions.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.9.1 2024-06-15 sysconf(3)
Pages that refer to this page:systemctl(1), clock_getres(2), execve(2), fsync(2), getgroups(2), getpagesize(2), mlock(2), mmap(2), mprotect(2), msync(2), sched_setaffinity(2), times(2), atexit(3), confstr(3), crypt(3), fpathconf(3), get_nprocs(3), get_phys_pages(3), realpath(3), ulimit(3), posixoptions(7), signal-safety(7), standards(7)