c - -D_DEFAULT_SOURCE 是做什么的？

Question

之前我收到来自 gcc -std=c99 的警告，即 usleep() 被隐式声明。然后我偶然发现了this stackoverflow post ，这导致我使用 -D_BSD_SOURCE。但是，现在 gcc 告诉我 -D_BSD_SOURCE 已被弃用，我应该改用 -D_DEFAULT_SOURCE。

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

为什么不推荐使用 -D_BSD_SOURCE？为什么使用 -D_DEFAULT_SOURCE 代替？它有什么作用？

我做了some googling ，而结果只是充满了使用它来关闭 gcc 的人。我无法找出为什么 -D_BSD_SOURCE 已被弃用，只是事实如此。

Answer 1

glibc manual描述每个功能测试宏 (FTM)，包括 _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关于 FTM 为我们提供了一个基本原理(以及其他可能有趣的信息):

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

因此，如果您需要定义 _BSD_SOURCE 或 _SVID_SOURCE，也只需定义 _DEFAULT_SOURCE。 glibc 版本 <= 2.18 不关心它，版本 >= 2.19 如果同时定义或全部定义三个则不发出警告。