Simple, toy coroutine mechanism in C

coro_switch.c

/*
 * Simple coroutine mechanism in C
 *
 * Inspired by / stolen from:
 *
 *     http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html
 *
 * I found this very educational and wanted to publish a simple demonstration.
 *
 * Usage:
 *
 *     $> gcc -o coro_switch coro_switch.c
 *     $> ./coro_switch
 *
 * Explanation: A function in C can allocate so-called static values, whose
 * values are preserved between calls to the function.
 *
 * We initialize a static integer `__coro_state` to be 0 and use it as the
 * argument to a switch statement. To yield a value `__coro_state` is set to an
 * arbitrary but unique value (the source line number, provided by __LINE__)
 * and the immediate next statement is a case expecting *precisely* that value.
 *
 * The next time the function is called, `__coro_state` will be different and
 * we will switch directly to immediately after where we yielded.
 *
 * TODO:
 * 1. Make this threadsafe by making all static variables part of some context
 * struct.
 */

#include <stdio.h> /* For printf() */

#define coro_start static int __coro_state=0; switch(__coro_state) { case 0:
#define yield(X) do { __coro_state=__LINE__; return X; \
                      case __LINE__:; } while (0)
#define coro_finish }

int zero_to_nine(void) {
    static int i;
    coro_start;
    for (i = 0; i < 10; i++) {
        yield(i);
    }
    coro_finish;
}

int ten_to_nineteen(void) {
    static int i;
    coro_start;
    for (i = 10; i < 20; i++) {
        yield(i);
    }
    coro_finish;
}

int main(int argc, char **argv) {
    int i;
    int num_yielded;
    for (i = 0; i < 20; i++) {
        if (i % 2 == 0) {
            num_yielded = zero_to_nine();
        }
        else {
            num_yielded = ten_to_nineteen();
        }
        printf("Received: %d\n", num_yielded);
    }
    return 0;
}