What is the effect of extern “C”?

I often see the following code in C header files:

#ifdef __cplusplus
extern "C" {
#endif

......

#ifdef __cplusplus
}
#endif

What does it mean? Since there is __cplusplus marco, it must be related to C++ compilation. Let’s see a simple program (print.c):

$ cat print.c
#include <stdio.h>

void print(void)
{
        printf("Hello world!\n");
}

Use gcc to generate object file:

$ gcc -c print.c
$ 

Then create a main.cpp file which calls print() in its main() function:

$ cat main.cpp
extern void print(void);

int main(void)
{
        print();
        return 0;
}

Compile main.cpp and link with print.o:

$ g++ main.cpp print.o
/tmp/cc60fu19.o: In function `main':
main.cpp:(.text+0x5): undefined reference to `print()'
collect2: error: ld returned 1 exit status

It is weird, right? the print() function must be defined in print.o, why can’t g++ find it? Let’s do a simple magic, add "C" in extern void print(void);:

$ cat main.cpp
extern "C" void print(void);

int main(void)
{
        print();
        return 0;
}

Try compile main.cpp again:

$ g++ main.cpp print.o
$ ./a.out
Hello world!

It is OK now! The root cause is related to name mangling. To be simplified, when compile C++ code, the names of functions, global variables, etc will be modified, not the same as original format. While compile C code, this won’t happen. The function of extern "C" is to tell C++ compiler search the original name, not the mangled ones. To get a sense of name mangling, you can check the print() name in object file:

$ readelf -s print.o | grep print
 1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS print.c
 9: 0000000000000000    17 FUNC    GLOBAL DEFAULT    1 print

Then use g++ to compile print.c, and check function name again:

$ g++ -c print.c
$ readelf -s print.o | grep print
 1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS print.c
 9: 0000000000000000    17 FUNC    GLOBAL DEFAULT    1 _Z5printv

You can see the print() function name is actually _Z5printv when use g++ to generate the object file.

References:
Why do we use extern “C”?;
In C++ source, what is the effect of extern “C”?.