gistfile1.txt

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

/******************************************************************************/
/* The "library"                                                              */
/******************************************************************************/

#define STACK_SIZE 16384

struct cr_ {
    struct cr_ *next;
    jmp_buf ctx;
};

struct cr_ main_cr_ = {NULL};

struct cr_ *first_cr_ = &main_cr_;
struct cr_ *last_cr_ = &main_cr_;

#define set_sp_(ptr) \
    volatile int unoptimisable_ = 1;\
    int filler1_[unoptimisable_];\
    char filler2_[(char*)&filler1_ - (char*)(ptr)];

struct cr_ *go_prologue_(void) {
    if(setjmp(first_cr_->ctx))
        return NULL;
    char *stack = malloc(STACK_SIZE);
    assert(stack);
    struct cr_ *cr = (struct cr_*)(stack + STACK_SIZE - sizeof (struct cr_));
    cr->next = first_cr_;
    first_cr_ = cr;
    return cr;
}

void go_epilogue_(void)
{
    struct cr_ *cr = first_cr_;
    first_cr_ = cr->next;
    free(((char*)(cr + 1)) - STACK_SIZE);
    longjmp(first_cr_->ctx, 1);
}

#define go(fn) \
    {\
        struct cr_ *cr = go_prologue_();\
        if(cr) {\
            set_sp_(cr);\
            fn;\
            go_epilogue_();\
        }\
    }

void yield(void) {
    if(first_cr_ == last_cr_)
        return;
    if(setjmp(first_cr_->ctx))
        return;
    struct cr_ *cr = first_cr_;
    first_cr_ = cr->next;
    cr->next = NULL;
    last_cr_->next = cr;
    last_cr_ = cr;
    longjmp(first_cr_->ctx, 1);
}

/******************************************************************************/
/* Test program                                                               */
/******************************************************************************/

void printer(int count, const char *text) {
    int i;
    for(i = 0; i != count; ++i) {
        printf("%s\n", text);
        yield();
    }
}

int main() {
    go(printer(3, "a"));
    go(printer(2, "b"));
    printer(5, "c");
    return 0;
}