It isn't specific to GCC at all. It's widespread among unixes historically (and they generally have their own C compiler which is not GCC), and it was mentione

It isn't specific to GCC at all. It's widespread among unixes historically (and they generally have their own C compiler which is not GCC), and it was mentioned in the C89 standard. However, it is not part of standard C or POSIX/SUS. https://stackoverflow.com/a/10321474 https://stackoverflow.com/a/10321474