c header-guards are ignored? - c

Question

Why does gcc ignore these header guards in this simple test program?
The header file is:
#ifndef MYHEADER_H
#define MYHEADER_H
#warning "header declared"
int some_int=0;
#endif
And the two .c files are:
main.c:
#include "header.h"
int main ()
{
return some_int;
}
source.c:
#include "header.h"
int get_int()
{
return some_int;
}
When compiling with:
gcc -o out main.c source.c
I get the following output:
In file included from main.c:1:
header.h:4:2: warning: #warning "header declared" [-Wcpp]
4 | #warning "header declared"
| ^~~~~~~
In file included from source.c:1:
header.h:4:2: warning: #warning "header declared" [-Wcpp]
4 | #warning "header declared"
| ^~~~~~~
/usr/bin/ld: /tmp/ccmAbN1J.o:(.bss+0x0): multiple definition of `some_int'; /tmp/ccEd5PwN.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status
As expected, the warning shows up, when the compiler includes the header file for the first time. But why wont the header guards stop the second inclusion?
The gcc version is:
gcc version 9.2.1 20200130 (Arch Linux 9.2.1+20200130-2)

Answer 1

Header guards guard against multiple inclusion in a single translation unit (usually a .c file and everything it includes, directly or indirectly).
You have two translation units, main.c and source.c, and they're compiled independently (even if you use a single command line line gcc main.c source.c). That's why you're getting an error message from the linker, not from the compiler.
If you want to define an object, you should do it in a .c file and declare it as extern in the corresponding .h file. The .c file defining the object is compiled just once, and multiple other .c files can see the declaration in the .h file.

Answer 2

It's not ignoring them. You have two separate compilation units there and each one needs header.h. Header guards are intended to prevent a single compilation unit from including the same header twice. For example. If main.c included header.h' directly, but also includedfoo.hwhich also includedheader.h, when all the includes are expanded the header guard ensures thatheader.h`'s contents only appear once.

Answer 3

There are 2 compilation units : 1 for main.c and 1 for source.c. These are compiled separately (and completely independently) by the compiler, resulting in 2 object files (eg. main.o resp. source.o). Each of these prints the warning :
In file included from main.c:1:
header.h:4:2: warning: #warning "header declared" [-Wcpp]
4 | #warning "header declared"
| ^~~~~~~
These object files are then linked into an executable out by the linker.
But the linker discovers that both object files define the same symbol some_int, and thus fails to generate the executable, resulting in :
/usr/bin/ld: /tmp/ccmAbN1J.o:(.bss+0x0): multiple definition of `some_int'; /tmp/ccEd5PwN.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status