#include <unistd.h> long sysconf(int name);
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 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 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.
First, the POSIX.1 compatible values.
These values also exist, but may not be standard.
|sysconf()||Thread safety||MT-Safe env|