context_hello_loop.c

#include <ucontext.h>
#include <sys/types.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

#define STACKSIZE 4096

void f(){
  printf("Hello World\n");
}

int main(int argc, char * argv[]){
  ucontext_t uc;   // Context for function `f`
  ucontext_t muc;  // Context for `main` function
  void * stack;

  getcontext(&uc);
  
  // This line isn't actually necessary. Can you figure out why?
  getcontext(&muc);

  stack = malloc(STACKSIZE);

  uc.uc_stack.ss_sp = stack;
  uc.uc_stack.ss_size = STACKSIZE;
  uc.uc_stack.ss_flags = 0;

  // Go back to the main context after the f context finishes
  uc.uc_link = &muc;

  sigemptyset(&(uc.uc_sigmask));

  makecontext(&uc, f, 0);

  while (1){
    swapcontext(&muc,&uc);
    printf("Hello Main\n");

    // !!!
    makecontext(&uc, f, 0);
  }

  //perror("setcontext"); //setcontext() doesn't return on success

  return 0;
}