context switching with setjmp

From: aditya (adityavasishth_at_gmail.com)
Date: 09/22/04 

Date: 21 Sep 2004 20:50:39 -0700

hi all,
cananybody please tell me that why the control transfers to f() and
then g() and then f() and so on,when longjmp(jbuf[0],1) is called in
main.
Here is the complete code.

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

#define SECOND 1000000
 
char stack1[4096];
char stack2[4096];

jmp_buf jbuf[2];

f()
{
  int i=0;
  while(1){
    printf("in f (%d)\n",i++);
    usleep(SECOND);
  }
}

g()
{
  int i=0;
  while(1){
    printf("in g (%d)\n",i++);
    usleep(SECOND);
  }
}

setup()
{
  sigsetjmp(jbuf[0],1);
  jbuf[0][0].__jmpbuf[JB_SP] = (unsigned) stack1 + 4096;
  jbuf[0][0].__jmpbuf[JB_PC] = (unsigned) f;
  
  sigsetjmp(jbuf[1],1);
  jbuf[1][0].__jmpbuf[JB_SP] = (unsigned) stack2 + 4096;
  jbuf[1][0].__jmpbuf[JB_PC] = (unsigned) g;

}

static toggle = 0;

void dispatch(int sig)
{
  puts("in dispatch");
  if (sigsetjmp(jbuf[toggle],1) == 1)
    return;

  puts("DISPATCH");
  toggle = 1 - toggle;
  siglongjmp(jbuf[toggle],1);
}

main()
{
  signal(SIGALRM, dispatch);
  setup();
  ualarm(5*SECOND, 5*SECOND);
  longjmp(jbuf[0],1);
 }

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