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 file system 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 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
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 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.
- _SC_NPROCESSORS_CONF
The number of processors configured.
- _SC_NPROCESSORS_ONLN
The number of processors currently online (available).
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(3)
is consumed by the user's environment variables.
Some returned values may be huge; they are not suitable for allocating
memory.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
