Question

According to this, the POSIX library does not include getopt.h. However, I found this in unistd.h:

#ifdef  __USE_POSIX2
/* Get definitions and prototypes for functions to process the
   arguments in ARGV (ARGC of them, minus the program name) for
   options given in OPTS.  */
# define __need_getopt
# include <getopt.h>
#endif

Does this mean that getopt.h is implicitly included when you include unistd.h? I mean, is the code above something I should expect from all implementations of the unistd header file, or is it just something that is in my particular version? Also, is the __USE_POSIX2 macro defined in POSIX.2 and onwards, or is it just for POSIX.2?

Answer 1

__USE_POSIX2 is an implementation detail of glibc; it corresponds to _POSIX_C_SOURCE >= 2 or _XOPEN_SOURCE being defined. These are also implied by _GNU_SOURCE, and are used implicitly unless strict ANSI mode is on. You are not supposed to define the __USE_ macros directly.

Since it corresponds to a value >= 2, it does apply to later versions. See the feature_test_macros manpage for further details.

Or, from the comments in features.h (the internal header – don’t include directly – that takes care of all this):

/* These are defined by the user (or the compiler)
   to specify the desired environment:
...
   _POSIX_C_SOURCE  If ==1, like _POSIX_SOURCE; if >=2 add IEEE Std 1003.2;
            if >=199309L, add IEEE Std 1003.1b-1993;
            if >=199506L, add IEEE Std 1003.1c-1995;
            if >=200112L, all of IEEE 1003.1-2004
            if >=200809L, all of IEEE 1003.1-2008
   _XOPEN_SOURCE    Includes POSIX and XPG things.  Set to 500 if
            Single Unix conformance is wanted, to 600 for the
            sixth revision, to 700 for the seventh revision.