Tony Finch - Coroutines in less than 20 lines of standard C

coroutine2.c

/**
 * From
 * http://fanf.livejournal.com/105413.html
 **/

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

#define MAXTHREAD 5

static void *coarg;

void *coto(jmp_buf here, jmp_buf there, void *arg) {
    coarg = arg;
    if (setjmp(here)) return(coarg);
    longjmp(there, 1);
}

#define STACKDIR - // set to + for upwards and - for downwards
#define STACKSIZE (1<<12)

static char *tos; // top of stack

void *cogo(jmp_buf here, void (*fun)(void*), void *arg) {
    if (tos == NULL) tos = (char*)&arg;
    tos += STACKDIR STACKSIZE;
    char n[STACKDIR (tos - (char*)&arg)];
    coarg = n; // ensure optimizer keeps n
    if (setjmp(here)) return(coarg);
    fun(arg);
    abort();
}

static jmp_buf thread[MAXTHREAD];
static int count = 0;

static void comain(void *arg) {
    int *p = arg, i = *p;
    for (;;) {
        printf("coroutine %d at %p arg %p\n", i, (void*)&i, arg);
        int n = arc4random() % count;
        printf("jumping to %d\n", n);
        arg = coto(thread[i], thread[n], (char*)arg + 1);
    }
}

int main(void) {
    while (++count < MAXTHREAD) {
        printf("spawning %d\n", count);
        cogo(thread[0], comain, &count);
    }
    return 0;
}

Result:

/Users/linjinhui/Library/Caches/clion10/cmake/generated/e2a0a57b/e2a0a57b/Debug/Coroutine
spawning 1
coroutine 1 at 0x7fff5f675b4c arg 0x10058a060
jumping to 0
spawning 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a060
jumping to 1
coroutine 1 at 0x7fff5f675b4c arg 0x10058a061
jumping to 0
spawning 3
coroutine 3 at 0x7fff5f673b4c arg 0x10058a060
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a061
jumping to 1
coroutine 1 at 0x7fff5f675b4c arg 0x10058a062
jumping to 1
coroutine 1 at 0x7fff5f675b4c arg 0x10058a063
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a064
jumping to 1
coroutine 1 at 0x7fff5f675b4c arg 0x10058a065
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a066
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a067
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a068
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a069
jumping to 1
coroutine 1 at 0x7fff5f675b4c arg 0x10058a06a
jumping to 0
spawning 4
coroutine 4 at 0x7fff5f672b4c arg 0x10058a060
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a061
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a062
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a063
jumping to 2
coroutine 2 at 0x7fff5f674b4c arg 0x10058a064
jumping to 1
coroutine 1 at 0x7fff5f675b4c arg 0x10058a065
jumping to 3
coroutine 3 at 0x7fff5f673b4c arg 0x10058a066
jumping to 0

Process finished with exit code 0