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

#define STACK_SIZE 8192

/*
 *   Structure of a process (a thread)
 */ 
struct proc {
	jmp_buf buf;
	char    stack[STACK_SIZE];
};

struct proc buff;      /* the "f" process */
struct proc bufg;      /* the "g" process */

char* stack;           /* lower adress of the stack (stack goes from stack to stack + STACK_SIZE */
int stack_index;       /* an index used when we copy the stack */
struct proc* current;  /* the current process */

void commut_to(struct proc* to) {
	/* save registers */
	if(setjmp(current->buf) == 0) {
		/* save stack */
		for(stack_index=0; stack_index<STACK_SIZE; stack_index++)
			current->stack[stack_index] = stack[stack_index];

		/* mark to as the new process */
		current = to;
		
		/* restore registers */
		longjmp(current->buf, 1);
	} else {
		/* be carreful: don't use any local variable to restore the stack of the new process */
		/* basically, we are restoring the stack of the new process */
		/* if we use a local variable, this variable will be in the stack (in the stack of the old process) */
		/* when we will restore the stack of the new process, we will overwrite this local variable, which will take a strange value! */
		/* => don't use local variable, and don't use memcpy as memcpy may use local variable */
		
		/* restore stack (stack_index and current are both global variables) */
		for(stack_index=0; stack_index<STACK_SIZE; stack_index++)
			stack[stack_index] = current->stack[stack_index];
	}	
}

void f() {
	int n = 0;
	
	for(int i=0; i<10; i++) {
		printf("f: %d\n", n++);
		/* f is running, jump to g */
		commut_to(&bufg);
	}
}

void g() {
	int m = 1000;

	for(int i=0; i<10; i++) {
		printf("g: %d\n", m++);
		/* g is running, jump to f */
		commut_to(&buff);
	}
}

int main(int argc, char** argv) {
	/* a random variable on the stack (in the frame of main) */
	char var;

	stack = &var - STACK_SIZE; /* the lower adress of the stack is at &var - STACK_SIZE */
	current = &buff;           /* f is running at the beginning */
	
	/* preserve the registers to start g */
	if(setjmp(bufg.buf) == 0) {
		f();
	} else {
		g();
	}
}