How to store goto labels in an array and then jump to them?

Question

I want to declare an array of \"jumplabels\".

Then I want to jump to a \"jumplabel\" in this array.

But I have not any idea how to do this.

It should

Answer 1

You might want to look at setjmp/longjmp.

Answer 2

You can't do it with a goto - the labels have to be identifiers, not variables or constants. I can't see why you would not want to use a switch here - it will likely be just as efficient, if that is what is concerning you.

Answer 3

It is possible with GCC feature known as "labels as values".

void *s[3] = {&&s0, &&s1, &&s2};

if (n >= 0 && n <=2)
    goto *s[n];

s0:
...
s1:
...
s2:
...

It works only with GCC!

Answer 4

goto needs a compile-time label.

From this example it seems that you are implementing some kind of state machine. Most commonly they are implemented as a switch-case construct:

while (!finished) switch (state) {
  case s0:
  /* ... */
  state = newstate;
  break;

  /* ... */
}

If you need it to be more dynamic, use an array of function pointers.

Answer 5

Optimizing compilers (including GCC) will compile a switch statement into a jump table (making a switch statement exactly as fast as the thing you're trying to construct) IF the following conditions are met:

Your switch cases (state numbers) start at zero.

Your switch cases are strictly increasing.

You don't skip any integers in your switch cases.

There are enough cases that a jump table is actually faster (a couple dozen compare-and-gotos in the checking-each-case method of dealing with switch statements is actually faster than a jump table.)

This has the advantage of allowing you to write your code in standard C instead of relying on a compiler extension. It will work just as fast in GCC. It will also work just as fast in most optimizing compilers (I know the Intel compiler does it; not sure about Microsoft stuff). And it will work, although slower, on any compiler.