C programming/Coroutines

A little known fact is that most C implementations have built-in primitives that can be used for cooperative multitasking / coroutines. They are setcontext and setjmp.

setjmp

The function setjmp is used in a pair with longjmp to transfer execution to a different point in the code. It relies on an existing jmp_buf declaration.

#include <setjmp.h>

int main (void)
{
  jmp_buf buf;
  
  if (setjmp(buf) == 0)
  {
    /* This code is executed on the first call to setjmp. */
    longjmp(buf, 1);
  } else {
    /* This code is executed once longjmp is called. */
  }
  return 0; 
}

setjmp() stores the current execution point in memory, which remains valid as long as the containing function doesn't return. It initially returns 0. Control is returned to setjmp once longjmp is called with the original jmp_buf and the replacement return value.

Note that jmp_buf is passed to setjmp without using the address-of operator.

The easiest way to understand setjmp and longjmp, is that setjmp stores the state of the CPU which includes program counter, stack pointer, all the registers, including the bits of the flags register, at the location pointed to by jmp_buf, which is defined some LEN+1, which is enough bytes to store the registers of whatever CPU is involved. The longjmp(buf), never returns, because it restores the execution context from the contents of struct jmp_buf buf previously set by the call to setjmp, so execution continues after setjmp was called, but the return value of setjmp is not 0, but with the value passed as the second parameter to longjmp. This is somewhat similar to the fork() system call, which returns 0 to the child process, and the PID of the child process to the parent.