_GNU_SOURCE and __USE_GNU
c, c++, glibc, gnu, linux
Solution
`_GNU_SOURCE` is the only one you should ever define yourself. `__USE_GNU` is defined internally through a mechanism in `features.h` (which is included by all other glibc headers) when `_GNU_SOURCE` is defined, and possibly under other conditions. Defining or undefining `__USE_GNU` yourself will badly break the glibc headers.
Problem
I want to use `CPU_SET`, which is a glibc linux-specific macro that should be defined in `sched.h` The manpage clearly states that `_GNU_SOURCE` must be defined so that the macro is defined. However, looking at the header, `CPU_SET` is defined only if `__USE_GNU` is defined (there is an `#ifdef` guard). I seem to remember a few years ago that `_GNU_SOURCE` was needed. Questions: 1) Clearly the manpage is off. How do I notify the maintainer that the manpage is incorrect? 2) When did the transition from `_GNU_SOURCE` to `__USE_GNU` happen (either in terms of version or time) 3) Are there circumstances where newer versions of glibc still use `_GNU_SOURCE`? Or can I safely assume that defining `__USE_GNU` is sufficient?