SYSCONF
Section: Linux Programmer's Manual (3)
Updated: 2017-07-13
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NAME
sysconf - get configuration information at run time
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().
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.
(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 - _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 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.
- - _SC_NPROCESSORS_ONLN
-
The number of processors currently online (available).
RETURN VALUE
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.
ERRORS
- EINVAL
-
name
is invalid.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
sysconf()
| Thread safety | MT-Safe env
|
CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
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.
SEE ALSO
bc(1),
expr(1),
getconf(1),
locale(1),
confstr(3),
fpathconf(3),
pathconf(3),
posixoptions(7)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- POSIX.1 variables
-
- POSIX.2 variables
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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