C external declaration/definition

C extern is confusing. The C Programming Language gives a simple treatment for it. But its usage is quite convoluted in C99 standard ant various C implementations.For a complete understanding of extern keyword and external declaration/definition, we have to refer to C99 standard which is hard to read and comprehend. For details, refer to 6.2.2 Linkages of identifiers and 6.9 External definitions.

 

Here is a simple program for me to clarify my thoughts on this topic.

 

one.c

 

 

int extern_val = 100;

 

two.c

 

#include <stdio.h>

/*
 * extern keyword is option here since one.c contains the definition for extern_val.
 */
extern int extern_val;

/*
 * extern keyword can't be put here since no other files contain an external 
 * defition of def. 
 */
int def;

/*
 * It is legal according to C99 standard. GCC issues a warning for it.
 */
extern int def_with_extern = 77;

int extern_decl_many_times = 99;
extern int extern_decl_many_times;
int extern_decl_many_times;

/* extern is optional here */
extern void extern_func() 
{
}

int main(int argc, const char *argv[]) 
{
    printf("extern_val: %d\n", extern_val); 
    printf("def: %d\n", def);
    printf("def_with_extern: %d\n", def_with_extern);
    printf("extern_decl_many_times: %d\n", extern_decl_many_times); 
    extern_func();
    return 0;
}

 

Use gcc one.c two.c to try them.