Is "unix" restricted keyword in C?
unix is not a identifier reserved by the Standard.
If you compile with -std=c89 or -std=c99 the gcc compiler will accept the program as you expected.
From gcc manual ( https://gcc.gnu.org/onlinedocs/cpp/System-specific-Predefined-Macros.html ), the emphasis is mine.
... However, historically system-specific macros have had names with no special prefix; for instance, it is common to find unix defined on Unix systems. For all such macros, GCC provides a parallel macro with two underscores added at the beginning and the end. If unix is defined, __unix__ will be defined too. There will never be more than two underscores; the parallel of _mips is __mips__.