Coroutines

Coroutines allow us to exit and later continue functions. This can be useful when we have a yielder and consumer function. They are not threads, but can be used to implement something similar

To implement coroutines, we can use the setjmp and longjmp functions from <setjmp.h>. setjmpstores the state of a function in a buffer and returns 0. longjmp then continues execution at that state. (To us, it looks like the original call to setjmp just returned again, but now with the value given to it by longjmp.) A very minimal example could look like this.

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

static jmp_buf buffer;

void other() {
    longjmp(buffer, 1);
    printf("Hello from other");
    return;
}

int main() {
    if(!setjmp(buffer))
        other();
    else
        printf("Hello from me");
    return 0;
}

Here, the first call to setjmp in main will return 0 and main will call other normally. other will then longjmp, which means that setjmp will return again with 1. At this point main prints and then the program ends. The print statement it other is never executed.