Roman Shaposhnik's

Most of the time when you write your C or C++ code on a modern UNIX platform you don't give much thought to the strict compliance with various standards which used to rule the disjoint UNIX landscape some odd number of years ago: XOPEN, POSIX, BSD, SVID, etc. And if an application has only the very same platform you're developing on as a deployment platform all is well. That doesn't happen all that often though. And once you face the neccessity of making sure that your application behavior is preserved across the variety of UNIX-like OSs there the fun begins. The biggest problem is that one of the cornerstone pieces of functionality every application on UNIX depends upon has the most colorful history behind it. Of course I'm talking about libc and the number of (sometimes conflicting!) APIs it has to offer in order to comply with various standards.

Now, not a lot of folks know this, but libc is highly configurable at an application compile time. If you have a reasonable compiler and a reasonable libc implementation you can ask libc to behave like it used to during the XOPEN times, BSD times or SVID times. The way you do that is usually via specifying macros like: POSIX_SOURCE, etc. Here's a complete list of macros you can tweak from the latest revision of Glibc:

from /usr/include/features.h:

   _ISOC99_SOURCE       Extensions to ISO C89 from ISO C99.
   _POSIX_SOURCE        IEEE Std 1003.1.
   _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
   _XOPEN_SOURCE        Includes POSIX and XPG things.  Set to 500 if
                        Single Unix conformance is wanted, to 600 for the
                        upcoming sixth revision.
   _XOPEN_SOURCE_EXTENDED XPG things and X/Open Unix extensions.
   _LARGEFILE_SOURCE    Some more functions for correct standard I/O.
   _LARGEFILE64_SOURCE  Additional functionality from LFS for large files.
   _FILE_OFFSET_BITS=N  Select default filesystem interface.
   _BSD_SOURCE          ISO C, POSIX, and 4.3BSD things.
   _SVID_SOURCE         ISO C, POSIX, and SVID things.
   _GNU_SOURCE          All of the above, plus GNU extensions.

And it should be quite straightforward to realize that once you add something like this to your compilation line you're essentially changing behavior of quite a few APIs (and YES! things can break or stop working at all):

    $ cc -D_XOPEN_SOURCE=500 application.c

What is much less straightforward is the fact that vendors slice and dice what they give you by default at their pleasure. For example GCC always defines _GNU_SOURCE which means that by default you're getting much more non-standard functionality than would otherwise be available (and yes! sometimes getting more than you've asked for is a portability nightmare). And when you move your application to a place where the defaults are different you have to fish out for the interfaces you need and how to enable them. What's even more important is that you have to constantly be on a lookout for different standards implementing some of the interfaces a little bit differently. Which means that if vendor A decided to give you an API which happens to be part of the larger standard by default, there's no guarantee that when you go to a platform where that single API is not available unless you explicitly ask for the standard it is part of other thing won't break.

Now, personally, I got bitten by it when I was porting an application from Solaris to Linux and got bitten by the fact that a very simple and unassuming call to sigset(2) was, in fact, part of the larger XOPEN standard and was not available on Linux by default unless you ask for it. On Solaris, however it is. So the dilemma I was faced with was either to augment my compilation line with something like: -D_XOPEN_SOURCE=600 (which I knew would break a couple of other things) or fish for something else that would only enable the sigset.

I ended up doing -D_XOPEN_SOURCE=600 and fixing the other stuff.