rolf.c

#include <assert.h>
#include <setjmp.h>
#include <signal.h>
#include <stdio.h>

static jmp_buf _strlen_jump;
static void* _strlen_label;
static void* _strlen_done_label;
static void _strlen_divzero(int signal) {
    _strlen_label = _strlen_done_label;
    longjmp(_strlen_jump, 1);
}

int unconditional_strlen(char* s) {
    int ret = 0;
    volatile int junk;  // prevent optimizing the division away

    _strlen_label = &&_strlen_redo;
    _strlen_done_label = &&_strlen_done;

    signal(SIGFPE, _strlen_divzero);

    setjmp(_strlen_jump);
    goto *_strlen_label;

_strlen_redo:
    junk = s[0] / s[0];
    ret++;
    s++;
    goto *_strlen_label;

_strlen_done:
    signal(SIGFPE, SIG_IGN);
    return ret;
}

int main(int argc, char** argv) {
    assert(argc > 1);

    printf("%d\n", unconditional_strlen(argv[1]));
    return 0;
}