Should I expect POSIX to include getopt.h?

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 macr开发者_运维问答o defined in POSIX.2 and onwards, or is it just for POSIX.2?

__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.

getopt.h is not mentioned in the getopt(3) man page. If you need getopt, you should include unistd.h and define (assuming GLIBC) _XOPEN_SOURCE or _POSIX_C_SOURCE=something_greater_than_2 and not worry about the implementation details of the C library. Other environments might have different ways of turning on/off POSIX features.

Note that your tags imply the use of getopt_long. This is a GNU extension, and thus not portable.