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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | BUGS | SEE ALSO |
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sysconf(3) Library Functions Manual sysconf(3)
sysconf - 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.
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.
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).
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).
The return value of sysconf() is one of the following:
• 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.
EINVAL name is invalid.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────┬───────────────┬─────────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────┼───────────────┼─────────────┤
│ sysconf() │ Thread safety │ MT-Safe env │
└─────────────────────────────────┴───────────────┴─────────────┘
POSIX.1-2008.
POSIX.1-2001.
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.
bc(1), expr(1), getconf(1), locale(1), confstr(3), fpathconf(3),
pathconf(3), posixoptions(7)
Linux man-pages (unreleased) (date) 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)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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