sysconf
SYSCONF(3) Linux Programmer’s Manual SYSCONF(3)
NAME
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 config-
urable 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 func-
tion sysconf(). On can ask for numerical values that may depend on
the filesystem a file is in using the calls fpathconf(3) and path-
conf(3). One can ask for string values using confstr(3).
The values obtained from these functions are system configuration con-
stants. 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 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 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 more might actu-
ally be supported. If an application wants to take advantage of val-
ues 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 final NUL, 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 final NUL. Must
not be less than _POSIX_LOGIN_NAME_MAX (9).
clock ticks - _SC_CLK_TCK
The number of clock ticks per second. The corresponding vari-
able 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 sys-
tems 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_SYM-
LOOP_MAX (8).
TTY_NAME_MAX - _SC_TTY_NAME_MAX
The maximum length of terminal device name, including final
NUL. 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) util-
ity.
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 expres-
sion 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_DEV
indicates whether the POSIX.2 C language development facilities
are supported.
POSIX2_FORT_RUN - _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 possi-
ble 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.
RETURN VALUE
If name is invalid, -1 is returned, and errno is set to EINVAL. Oth-
erwise, 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.
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.
SEE ALSO
bc(1), expr(1), locale(1), fpathconf(3), pathconf(3), posixoptions(7)
GNU 1993-04-18 SYSCONF(3)
