-W flag for gcc to detect taking const pointer of uninitialised variable

Question

I have the following code, saved on Coding Ground here:

#include <stdio.h>

void foo(int const *x)
{
    printf("Hello, %d!\n", *x);
}

int main()
{
    int y;

    foo(&y);
    y = 3;
    printf("Hello, World %d!\n", y);
    return 0;
}

I compile with:

gcc -std=c99 -Wall -Wextra -Wuninitialized -o main *.c

However, I get no warning about taking a const pointer of an uninitialised variable, and I cannot find a suitable flag.

Note: pasting the code into Gimpel's Online Lint gives me the expected:

test.c 12 Warning 603: Symbol 'y' (line 10) not initialized

Answer 1

-Wuninitialized Warn whenever an automatic variable is used without first being initialized.

These warnings are possible only in optimizing compilation, because they require data-flow information that is computed only when optimizing. If you don't specify `-O', you simply won't get these warnings.

If you try to compile with:

gcc -std=c99 -Wall -Wextra -Wuninitialized -O2 -o main *.c

The warning will be:

pippo.c: In function ‘main’:
pippo.c:56:5: warning: ‘y’ is used uninitialized in this function [-Wuninitialized]
     printf("Hello, %d!\n", *x);
     ^
pippo.c:61:9: note: ‘y’ was declared here
     int y;
         ^