GOSUB in C

GOSUB is a keyword in certain BASIC dialects which allows you to jump to a label, then return from it later on. This is a convenient way to incorporate a call stack in a language which lacks functions.

I wrote it in C because someone asked if it was possible.

You probably shouldn't actually use this library, if you can even call it that. Use functions, please.

Compilation

This library is a header-only library, meaning the only thing you need to do is include it into your program.

Usage

Here is an example use case of this library.

#include <stdio.h>
#include "gosub.h"

int main()
{
    GOSUB(subroutine_1);
    GOSUB(subroutine_2);
    GOSUB(subroutine_2);

    printf("This should be called last.\n");
    return 0;

subroutine_1:
    printf("This should be called first.\n");
    RETURN;

subroutine_2:
    printf("This should be called twice.\n");
    RETURN;
}

This results in the following output:

This should be called first.
This should be called twice.
This should be called twice.
This should be called last.

Limitations

To maintain portability across different platforms, no inline assembly is used. The downside is that the && operator is used from the GNU compiler extensions to get the address that a label references. Therefore, this library must be used only with a GNU C compiler (or other compilers which support this use-case).

This library creates dummy labels in order to return to the callsite. To eliminate label reusage, labels are created based on the current line number. Therefore, you can only have one GOSUB call per line in your source file.

Finally, the stack size is currently fixed at 256. Compile with -DGOSUB_STACK_SIZE to change.