"Local" labels in C and jump table implementation

Question

I'm trying to make a macro-based jump table in C.

Here's some example code:

#include "stdio.h"

#define GOTO(X) static void* caseArg[] = {&&I0, &&R0, &&S0, &&F0, &&G0, &&H0}; \
    goto *caseArg[X];

#define FINISH() goto caseEnd;

int main(int argc, char** argv) {

    GOTO(1);

    I0: printf("in I0\n"); FINISH();
    R0: printf("in R0\n"); FINISH();
    S0: printf("in R0\n"); FINISH();
    F0: printf("in R0\n"); FINISH();
    G0: printf("in R0\n"); FINISH();
    H0: printf("in R0\n"); FINISH();

    caseEnd:;

}

The possible labels (I0, R0, etc) have to be the same.

The problem is: I want to be able to use the same macro in different scoped parts of the same source file. However, the compiler complains that the labels are defined.

What I want to achieve:

int main(int argc, char** argv) {

     { // scope 1 

        GOTO(1);

        I0: printf("in I0\n"); FINISH();
        R0: printf("in R0\n"); FINISH();
        S0: printf("in R0\n"); FINISH();
        F0: printf("in R0\n"); FINISH();
        G0: printf("in R0\n"); FINISH();
        H0: printf("in R0\n"); FINISH();

        caseEnd:;

    }

    { // scope 2

        GOTO(4);

        I0: printf("in I0\n"); FINISH();
        R0: printf("in R0\n"); FINISH();
        S0: printf("in R0\n"); FINISH();
        F0: printf("in R0\n"); FINISH();
        G0: printf("in R0\n"); FINISH();
        H0: printf("in R0\n"); FINISH();

        caseEnd:;

    }

}

Any ideas? Any possible workaround?

Answer 1

You need the __label__ extension (at least in gcc, clang, and tinycc), which allows you to scope labels to a block.

The labels need to be declared at the very start of a block with

__label__ I0, R0, S0, F0, G0, H0;

(Contiguous __label__ I0; __label__ R0; ... or a mix of the two forms works as well.).

Unless declared scope-local with __label__, C labels are scoped to their enclosing function.

Your example with __label__:

#include "stdio.h"

#define GOTO(X) static void* const caseArg[] = {&&I0, &&R0, &&S0, &&F0, &&G0, &&H0}; \
    goto *caseArg[X];

#define FINISH() goto caseEnd;

#define DECL_LBLS() __label__ I0, R0, S0, F0, G0, H0, caseEnd

int main(int argc, char** argv) {

    {   DECL_LBLS();

        GOTO(2);

        I0: printf("in I0\n"); FINISH();
        R0: printf("in R0\n"); FINISH();
        S0: printf("in S0\n"); FINISH();
        F0: printf("in F0\n"); FINISH();
        G0: printf("in G0\n"); FINISH();
        H0: printf("in H0\n"); FINISH();
        caseEnd:;
    }

    {   DECL_LBLS();

        GOTO(1);

        I0: printf("in I0\n"); FINISH();
        R0: printf("in R0\n"); FINISH();
        S0: printf("in S0\n"); FINISH();
        F0: printf("in F0\n"); FINISH();
        G0: printf("in G0\n"); FINISH();
        H0: printf("in H0\n"); FINISH();
        caseEnd:;
    } 
}

https://gcc.godbolt.org/z/63YSkG

In this particular case, such a local-label based jumptable seems to buy little over a plain old switch.

Answer 2

This is pretty ugly solution, but if you are willing to add extra prefix definitions to the scopes, you can do this with concatenation

#include "stdio.h"

// Helpers
#define CONCAT(a, b) CONCAT2(a, b)
#define CONCAT2(a, b) a ## b

// Label redirection
#define I0 CONCAT(PREFIX, I0)
#define R0 CONCAT(PREFIX, R0)
#define S0 CONCAT(PREFIX, S0)
#define F0 CONCAT(PREFIX, F0)
#define G0 CONCAT(PREFIX, G0)
#define H0 CONCAT(PREFIX, H0)
#define caseEnd CONCAT(PREFIX, caseEnd)

#define GOTO(X) static void* caseArg[] = {&&I0, &&R0, &&S0, &&F0, &&G0, &&H0}; \
    goto *caseArg[X];

#define FINISH() goto caseEnd;

int main(int argc, char** argv) {

     { // scope 1 
        #define PREFIX SCOPE1
        GOTO(1);

        I0: printf("in I0\n"); FINISH();
        R0: printf("in R0\n"); FINISH();
        S0: printf("in R0\n"); FINISH();
        F0: printf("in R0\n"); FINISH();
        G0: printf("in R0\n"); FINISH();
        H0: printf("in R0\n"); FINISH();

        caseEnd:;
        #undef PREFIX

    }

    { // scope 2
        #define PREFIX SCOPE2
        GOTO(4);

        I0: printf("in I0\n"); FINISH();
        R0: printf("in R0\n"); FINISH();
        S0: printf("in R0\n"); FINISH();
        F0: printf("in R0\n"); FINISH();
        G0: printf("in R0\n"); FINISH();
        H0: printf("in R0\n"); FINISH();

        caseEnd:;
        #undef PREFIX
    }

}