Question

When I call a function that expects a pointer, and I pass in a value, I get this warning, and I like that.

But when the value happens to be a literal ‘0’, I don’t get the warning. I think this is because C think it’s null-pointer, and not a value. Is there any way to still get warnings for 0-literals, because I already had some bugs because of it.

Answer 1

GCC supports a nonnull attribute on function parameters that can do what you want (as long as the -Wnonnull warning option is enabled):

void* foo( int* cannot_be_null)  __attribute((nonnull (1))) ;


int main(int argc, char *argv[])
{
    int x;
    foo(&x);

    foo(0);  // line 13 - generates a -Wnonnull warning

    return 0;
}

When compiled using gcc -c -Wnonnull test.c I get:

test.c: In function 'main':
test.c:13:5: warning: null argument where non-null required (argument 1) [-Wnonnull]

You can force this to be an error with -Werror=nonnull.

Note that this warning is only thrown when the null pointer literal (another name for 0) is used – the following code doesn’t trigger the warning:

int* p = NULL;
foo(p);