How to store goto labels in an array and then jump

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 look like the following code:

function()
{
    "gotolabel" s[3];
    s[0] = s0;
    s[1] = s1;
    s[2] = s2;

    s0:
    ....
    goto s[v];

    s1:
    ....
    goto s[v];

    s2:
    ....
    goto s[v];
}

Does anyone have a idea how to perform this?

Answer 1

You might want to look at setjmp/longjmp.

Answer 2

That's what switch statements are for.

switch (var)
{
case 0:
    /* ... */
    break;
case 1:
    /* ... */
    break;
default:
    /* ... */
    break;  /* not necessary here */
}

Note that it's not necessarily translated into a jump table by the compiler.

If you really want to build the jump table yourself, you could use a function pointers array.

Answer 3

There's no direct way to store code addresses to jump to in C. How about using switch.

#define jump(x)  do{ label=x; goto jump_target; }while(0)
int label=START;
jump_target:
switch(label)
{
    case START:
        /* ... */
    case LABEL_A:
        /* ... */
}

You can find similar code produced by every stack-less parser / state machine generator. Such code is not easy to follow so unless it is generated code or your problem is most easily described by state machine I would recommend not do this.

Answer 4

Tokenizer? This looks like what gperf was made for. No really, take a look at it.

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.