having trouble understanding include guards

[HyperFlexed]

I've gotten include guards to work, but now I'd like to understand why they work. The best way to illustrate is an example.

let's say I had main.c and something-else.c, and they both needed stdio.h

main.c

code:

#include <stdio.h> #include "something-else.h" // code here...

something-else.c

code:

#include "something-else.h" // code here...

something-else.h

code:

#ifndef SOMETHING_ELSE_H #define SOMETHING_ELSE_H #include <stdio.h> // code here... #endif

if I go "gcc main.c something-else.c -o main", the program will compile fine, but as I understand it, stdio.h will be included twice.

stdio.h is included in main.c, then something-else.h is included in main.c. Since SOMETHING_ELSE_H is not defined at this point, it will be defined, and then stdio.h will be included a second time. Why doesn't this cause an error? Is it because each C file is compiled independently, and then linked afterwards? I'm just guessing, so if anyone has the answer I'd be grateful.

[TheGuardian001]

stdio.h has it's own include guards inside.

Just like your header has

code:

#ifndef SOMETHING_ELSE_H #define SOMETHING_ELSE_H ... #endif

stdio.h will have something along the lines of

code:

#ifndef _STDIO_H #define _STDIO_H ... #endif

[chrisbrown]

Include guards are to prevent duplicate definitions, not duplicate includes. A properly guarded file can be included as many times as you want, because the definitions in that file will be ignored each time except the first.

Stdio.h - and all standard libraries - are properly guarded, so you need not and should not place it inside the #ifndef.

[HyperFlexed]

had a feeling it was simple. Thanks.