Why could c++ include <stdio.h> and invoke printf method which is c

Question

c++ compiler could compile code like this, and it executed correctly

#include <stdio.h>

int main() {

    printf("test...\n");

    return 0;

}

I think printf.c will be compiled to printf.o with c compiler, I just checked the stdio.h, there is no extern "C" keyword, then how could c++ linker link printf in printf.o which is compiled with c compiler?(By the way, my platform is ubuntu 14.04, compiler is gcc 4.8.4)

Answer 1

You may not see an explicit extern "C" in stdio.h, but it is there. It's just hiding.

For example, on Linux, in stdio.h we see this:

#include <features.h>

__BEGIN_DECLS

In <features.h> you will find the following:

# ifndef _SYS_CDEFS_H
#  include <sys/cdefs.h>
# endif

And in <sys/cdefs.h> you finally see:

#ifdef  __cplusplus
# define __BEGIN_DECLS  extern "C" {
# define __END_DECLS    }
#else
# define __BEGIN_DECLS
# define __END_DECLS
#endif

So, via a fairly roundabout path, Linux header files have a __BEGIN_DECLS/__END_DECLS wrapper which, when compiled by a C++ compiler, end up wrapping the whole thing inside extern "C".