#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <ucontext.h>

//
// Reference:	Original code was by Evan Jones as basic-uc.c.
//		[[ Mostly changed stdout messages to be more informative or
//		   accurate ]]
// Purpose:	Context switch between main thread and child thread using
//		swapcontext
// Usage:	ucontext-basic
// Calls:	makecontext, swapcontext
// Notes:
//   o A "fiber" is a user-level thread (i.e., no kernel thread support)
//   o ucontext makes it trivial to manipulate contexts
// Ideas:
//   o swapcontext(&from,&to) is equivalent to Yield(&from,&to) where the
//	context is stored in 'from' and the new context is in 'to'.
//   o Control is:
//		Parent			Child
//		------			-----
//		getcontext
//		makecontext
//		swapcontext ---------->	...
//		...	    <----------	swapcontext
//		swapcontext ---------->	...
//		...	    <----------	swapcontext
//		exit
//   o See end of file for ucontext
// History:
//   o 5 Jan 2007	kenw:  Modified from Evan Jones' basic-uc.c code
//

// 64kB stack
#define FIBER_STACK 1024*64

ucontext_t child, parent;

// The child thread will execute this function
void childFiber()
{
  int	x = 99;		// data on stack (use gdb to see it)

  printf( "\t+ BEGIN:  Child fiber with x = %d\n", x );
  printf( "\t+ Child fiber yielding to parent\n" );
  swapcontext( &child, &parent );
  printf( "\t+ Child thread to parent AGAIN\n" );
  swapcontext( &child, &parent );	// should never return from this
  printf( "\t+ EXIT:  Child fiber\n" );
}
  
int main() {
  int	x = 88;		// data on stack (use gdb to see it)

  printf( "\t+ BEGIN:  Parent fiber with x = %d\n", x );
  // Get the current execution context
  getcontext( &child );
  
  // Modify the context to a new stack
  child.uc_link = 0;
  child.uc_stack.ss_sp = malloc( FIBER_STACK );
  child.uc_stack.ss_size = FIBER_STACK;
  child.uc_stack.ss_flags = 0;        
  if ( child.uc_stack.ss_sp == 0 ) {
    perror( "malloc: Could not allocate stack" );
    exit( 1 );
  }
  
  // Create the new context
  printf( "Create child fiber\n" );
  makecontext( &child, &childFiber, 0 );
  
  // Execute the child context
  printf( "Switching to child fiber\n" );
  swapcontext( &parent, &child );
  printf( "Switching to child fiber AGAIN\n" );
  swapcontext( &parent, &child );
  
  // Free the stack
  free( child.uc_stack.ss_sp );
  
  printf( "Child fiber stack freed in parent and parent fiber exiting\n" );
  
  return 0;
}

//
// o Machine and User Context
//
//	typedef struct			// Machine context (processor state)
//	  {
//	    gregset_t gregs;			// general-purpose registers
//	    fpregset_t fpregs;			// floating point registers
//	    unsigned long int oldmask;		// old signal mask
//	    unsigned long int cr2;		// virtual memory related
//	  } mcontext_t;
//
//	typedef struct ucontext		// Userlevel context
//	  {
//	    unsigned long int uc_flags;		// 
//	    struct ucontext *uc_link;		// addr of next ucontext_t
//	    stack_t uc_stack;			// stack descriptor
//	    mcontext_t uc_mcontext;		// machine context (above)
//	    __sigset_t uc_sigmask;		// signal mask
//	    struct _libc_fpstate __fpregs_mem;	// floating point related
//	  } ucontext_t;
//