Is it possible to store the address of a label in a variable and use goto to jump to it?

Question

I know everyone hates gotos. In my code, for reasons I have considered and am comfortable with, they provide an effective solution (ie I\'m not looking for \"don\'t do that\

Answer 1

You can do something similar with setjmp/longjmp.

int main (void)
{
    jmp_buf buf;
    int i=1;

    // this acts sort of like a dynamic label
    setjmp(buf);

    if( i-- )
        // and this effectively does a goto to the dynamic label
        longjmp(buf, 1);

    return 0;
}

Answer 2

According to the C99 standard, § 6.8.6, the syntax for a goto is:

    goto identifier ;

So, even if you could take the address of a label, you couldn't use it with goto.

You could combine a goto with a switch, which is like a computed goto, for a similar effect:

int foo() {
    static int i=0;
    return i++;
}

int main(void) {
    enum {
        skip=-1,
        run,
        jump,
        scamper
    } label = skip; 

#define STATE(lbl) case lbl: puts(#lbl); break
    computeGoto:
    switch (label) {
    case skip: break;
        STATE(run);
        STATE(jump);
        STATE(scamper);
    default:
        printf("Unknown state: %d\n", label);
        exit(0);
    }
#undef STATE
    label = foo();
    goto computeGoto;
}

If you use this for anything other than an obfuscated C contest, I will hunt you down and hurt you.

Answer 3

The only officially supported thing that you can do with a label in C is goto it. As you've noticed, you can't take the address of it or store it in a variable or anything else. So instead of saying "don't do that", I'm going to say "you can't do that".

Looks like you will have to find a different solution. Perhaps assembly language, if this is performance-critical?

Answer 4

The C and C++ standards do not support this feature. However, the GNU Compiler Collection (GCC) includes a non-standard extension for doing this as described in this article. Essentially, they have added a special operator "&&" that reports the address of the label as type "void*". See the article for details.

P.S. In other words, just use "&&" instead of "&" in your example, and it will work on GCC.
P.P.S. I know you don't want me to say it, but I'll say it anyway,... DON'T DO THAT!!!

Answer 5

Use function pointers and a while loop. Don't make a piece of code someone else will have to regret fixing for you.

I presume you're trying to change the address of the label somehow externally. Function pointers will work.

Answer 6

According to this thread, label points are not a standard, so whether they work or not would depend on the compiler you're using.