On Sun, Nov 03, 2024 at 02:02:59 +0100, Roland Illig wrote: > 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.
The idiom (though I learned it as FOO + 0, not minus) is intended to protect you from contexts where you have in scope plain "#define FOO" with an empty replacement list (NB: -DFOO on the command line is different as it means -DFOO=1). In that case "after all replacements due to macro expansion" the FOO is no longer there and is not one of "remaining identifiers" that "are replaced with the pp-number 0". Now, we can argue whether _POSIX_C_SOURCE can be defined that way, but that's how idioms are, you use them as a canned recipe so to say, so I don't think that would be a tangential discussion at best. -uwe
