Originální popis anglicky:
sysconf - Get configuration information at runtime
Návod, kniha: Linux Programmer's Manual
#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(). 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 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 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() 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
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 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.
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_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
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.
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.
POSIX.1.
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.
bc(1),
expr(1),
locale(1),
fpathconf(3),
pathconf(3),
posixoptions(7)
