What does -D_DEFAULT_SOURCE do?

Question

Previously I was receiving warnings from gcc -std=c99 that usleep() was implicitly declared. Then I stumbled across this stackoverflow post, which led me to use -D_BSD_SOURCE. However, now gcc tells me that -D_BSD_SOURCE has been deprecated and I should use -D_DEFAULT_SOURCE instead.

#warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"

Why is -D_BSD_SOURCE deprecated? Why is -D_DEFAULT_SOURCE used instead? And what does it do?

I did some googling, and the results are just filled with people using it to shut gcc up. I couldn't find out why -D_BSD_SOURCE has been deprecated, just that it is.

Answer 1

The glibc manual describes each feature test macro (FTM) including _DEFAULT_SOURCE:

If you define this macro, most features are included apart from X/Open, LFS and GNU extensions: the effect is to enable features from the 2008 edition of POSIX, as well as certain BSD and SVID features without a separate feature test macro to control them. Defining this macro, on its own and without using compiler options such as -ansi or -std=c99, has the same effect as not defining any feature test macros; defining it together with other feature test macros, or when options such as -ansi are used, enables those features even when the other options would otherwise cause them to be disabled.

This LWN.net article about FTMs provides us with a rationale (among other perhaps interesting info):

The original intent seems to have been that, within each of the glibc header files that employs FTMs, only one of the __USE_* internal macros should govern the exposure of any particular definition. Additionally, the macros should not be used in nested #ifdef directives. An inspection of the glibc header files quickly shows that the reality is far from the intent, a situation that led Roland McGrath to suggest that it was time for a major cleanup to bring things back to the intended situation. Roland thought that task could be simplified by eliminating the _BSD_SOURCE and _SVID_SOURCE FTMs, which, although they had a purpose historically, have ceased to be useful these days. Anymore, he said, the only macros that are needed for modern source code are those that relate to formal standards plus _GNU_SOURCE.

Joseph Myers duly obliged with a series of patches to implement the first steps in this work. The conservative approach encouraged by Roland meant that the deprecation of the _BSD_SOURCE and _SVID_SOURCE FTMs is taking place across two glibc versions. Version 2.19 of glibc added a new FTM, _DEFAULT_SOURCE. Defining this macro causes the default definitions to be exposed even when the explicit definition of other macros would cause that not to happen. The effect of defining this macro is equivalent to the effect of explicitly defining three macros in earlier glibc versions:

cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809C

So if you need to define _BSD_SOURCE or _SVID_SOURCE, simply define _DEFAULT_SOURCE too. glibc versions <= 2.18 don't care about it and versions >= 2.19 don't warn if both or all three are defined.