co = coroutine.create(function ()
        for i=1,10 do
            print("co", i)
            coroutine.yield()
        end
    end)

从主线程调用
coroutine.resume(co)
会依次打印1到10

/* testcontext.c : demo of ucontex_t operations */
#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>

ucontext_t auc,buc,mainuc;

void a()
{
    int i;

    for (i = 0; i < 10; i++)
    {
        printf("a");
        swapcontext(&auc, &buc);        /* switch to thread B */
    }

    printf("\nswitching to main\n");
    swapcontext(&auc, &mainuc);         /* switch to main thread */
}

void b()
{
    int i;

    for (i = 0; i < 10; i++)
    {
        printf("b");
        swapcontext(&buc, &auc);        /* switch to thread A */
    }
}

int main(void)
{
    printf("start\n");                  /* main thread starts */

    /* Set up context for thread A (Unix code, see manpages) */
    getcontext(&auc);
    auc.uc_stack.ss_size = 16 * 1024;

    if ((auc.uc_stack.ss_sp = malloc(auc.uc_stack.ss_size)) == NULL)
        perror("malloc"), exit(1);

    auc.uc_stack.ss_flags = 0;
    makecontext(&auc, a, 0);

    /* Set up context for thread B */
    getcontext(&buc);
    buc.uc_stack.ss_size = 16 * 1024;

    if ((buc.uc_stack.ss_sp = malloc(buc.uc_stack.ss_size)) == NULL)
        perror("malloc"), exit(1);

    buc.uc_stack.ss_flags = 0;
    makecontext(&buc, b, 0);

    /* Switch to A */
    getcontext(&mainuc);           /* Save the context of main thread */
    swapcontext(&mainuc, &auc);    /* Switch to thread A */

    printf("\ndone\n");  /* Execution control returned to main thread */
    return 0;
}
运行结果
C:\>testcontext.exe
start
abababababababababab
switching to main

done
C:\>