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
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.
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.
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
Some returned values may be huge; they are not suitable for allocating memory.