Bug 1310929

Description of problem:
It's a common pattern in large software projects to define a single header file shared by many smaller object files.

This leads to gcc displaying hundreds of unused const variable warnings where a const defined in the .h may be used in 2 or 3 source, files, but not all. In any file it is not used, the warning is omitted.

For example:

I0> cat test_b.c 

#include "test.h"


void do_nothing() {
    return;
}

I0> cat test_a.c 

#include "test.h"

int main(int argc, char **argv) {
    do_nothing();
    printf("%s\n", hello_world);
}

I0> cat test.h 
#include <stdio.h>

void do_nothing();

static char* const hello_world     = "Hello World";

I0> cat Makefile 

all: test_a

test_a: test_b
	gcc -Wall -o test test_b.o test_a.c

test_b:
	gcc -Wall -c -fpic -o test_b.o test_b.c


Running this yields:

gcc -Wall -c -fpic -o test_b.o test_b.c
In file included from test_b.c:2:0:
test.h:5:20: warning: ‘hello_world’ defined but not used [-Wunused-const-variable]
 static char* const hello_world     = "Hello World";
                    ^~~~~~~~~~~
gcc -Wall -o test test_b.o test_a.c


Which is incorrect: the const variable *is* used, but just not in this file yet.

I *like* the idea of the unused-const-variable check, but too many false positives will lead to it being disabled.

I am not sure what the correct solution is here: Perhaps const variables in .h files are ignored? Perhaps there needs to be a way to track these over man runs of gcc through a cache / check file?

Until this is fixed, I do not think that it is wise to include unused-const-variable in -Wall, as it will create excessive noise and could diminish the value of the feature.