sysconf - get configuration information at run time
#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.
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.
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 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_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 cause of
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.1-2001, POSIX.1-2008.
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)