Am 03.11.2024 um 00:52 schrieb Robert Elz: > Date: Sat, 2 Nov 2024 22:36:10 +0100 > From: Roland Illig <roland.il...@gmx.de> > > | Is the "- 0" in the first expression really necessary? If so, for which > | cases? > > When _POSIX_C_SOURCE is undefined (or defined to nothing). > That is the standard idiom for testing all of these feature macros.
When _POSIX_C_SOURCE (or any other identifier) is undefined, the preprocessor replaces it with 0. This has been spelled out in the C standards since C90, for example in C23 6.10.2p13, and I thought it would be common knowledge, that's why I didn't mention it in my original questions. From what I'm reading out of <sys/featuretest.h>, _POSIX_C_SOURCE is not defined to empty for any version of POSIX, thus making the "- 0" part superfluous. That part may be necessary for _XOPEN_SOURCE, but not for _POSIX_C_SOURCE. Roland
