Uninitalised Variables and Optimisation

What is wrong with the following code

#include <stdio.h>

int main(void)
        char **blah;
        char *astring = "hello, world\n";

        *blah = astring;

        printf("%s\n", *blah);

        return 0;

Fairly obvious when it is layed out like this; that *blah = astring should be blah = &astring (this might be less obvious when it is buried deep within several functions :). blah is uninitalised, so you can't dereference it.

Unfortunatley, this code will compile with -Wall with no warnings. This is because of a little fact

           Warn if an automatic variable is used without first being
           initialized or if a variable may be clobbered by a "setjmp"

           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.

So always turn on at least -O to get the full checking gcc can give you, and you'll probably catch things like the above before they even segfault.