longjmp.c

#include <setjmp.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static void first();
static void second();

/* Use a file scoped static variable for the exception stack so we can access
 * it anywhere within this translation unit. */
static jmp_buf exception_env;
static int exception_type;

int main(void) {
    char* volatile mem_buffer = NULL;

    if (setjmp(exception_env)) {
        // if we get here there was an exception
        printf("first failed, exception type: %d\n", exception_type);
    } else {
        // Run code that may signal failure via longjmp.
        puts("calling first");
        first();

        mem_buffer = malloc(300); // allocate a resource
        printf("%s\n", strcpy(mem_buffer, "first succeeded")); // not reached
    }

    free(mem_buffer); // NULL can be passed to free, no operation is performed

    return 0;
}

static void first() {
    jmp_buf my_env;

    puts("entering first"); // reached

    memcpy(my_env, exception_env, sizeof my_env);

    switch (setjmp(exception_env)) {
        case 3: // if we get here there was an exception.
            puts("second failed, exception type: 3; remapping to type 1");
            exception_type = 1;

        default: // fall through
            memcpy(exception_env, my_env, sizeof exception_env); // restore exception stack
            longjmp(exception_env, exception_type); // continue handling the exception

        case 0: // normal, desired operation
            puts("calling second"); // reached 
            second();
            puts("second succeeded"); // not reached
    }

    memcpy(exception_env, my_env, sizeof exception_env); // restore exception stack

    puts("leaving first"); // never reached
}

static void second() {
    puts("entering second" ); // reached

    exception_type = 3;
    longjmp(exception_env, exception_type); // declare that the program has failed

    puts("leaving second"); // not reached
}