From Linux Man Pages
sysconf - Get configuration information at runtime
SYNOPSIS
#include <unistd.h>
long sysconf(int name);
DESCRIPTION
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(). On can ask for numerical
values that may depend on the filesystem a file is in using the calls 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 life-
time 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 runtime 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 sup-
port 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 more 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() parameter 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() family of functions. Must not be less than
_POSIX_ARG_MAX (4096).
CHILD_MAX - _SC_CHILD_MAX
The max number of simultaneous processes per user ID. Must not be less than _POSIX_CHILD_MAX(25).
HOST_NAME_MAX - _SC_HOST_NAME_MAX
Max 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).
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. (Some systems use PAGE_SIZE instead.)
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
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 length, either from standard input or from a file. This
includes length 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 runtime utilities are supported.
_POSIX2_LOCALEDEF - _SC_2_LOCALEDEF
indicates whether the POSIX.2 creation of locates 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_PAGE_SIZE to overflow.
- _SC_AVPHYS_PAGES
The number of currently available pages of physical memory.
RETURN VALUE
If name is invalid, -1 is returned, and errno is set to EINVAL. Otherwise, the value returned is the value of
the system resource and errno is not changed. In the case of options, a positive value is returned if a queried
option is available, and -1 if it is not. In the case of limits, -1 means that there is no definite limit.
CONFORMING TO
POSIX.1-2001.
BUGS
It is difficult to use ARG_MAX because it is not specified how much of the argument space for exec() is consumed
by the user's environment variables.
Some returned values may be huge; they are not suitable for allocating memory.
RELATED
bc(1), expr(1), locale(1), fpathconf(3), pathconf(3), posixoptions(7)
CATEGORY