https://www.cprogramming.com/tutorial/shared-libraries-linux-gcc.html from the link I saw a head foo.h #ifndef foo_h__ #define foo_h__ extern void foo(void); #endif // foo_h__ What is the usage of "foo_h__" here?
header guard. if means if the header is not defined,define it. That way, compiler saves time from go overthe same files. Another benefit is to make sure the notwo ambiguous, duplicate headers are being compiled.