#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int flag = 1;

void f(int signo)
{
  /* nop (juste pour reveiller le processus) */
  flag = 0;
}

main(int argc, char *argv[])
{
  char c;
  int pid;
  int fd;

  struct sigaction action;
  sigset_t signaux;

  action.sa_handler = f;
  sigemptyset(&action.sa_mask);
  action.sa_flags = 0;
  sigaction(SIGUSR1,&action,NULL);
  sigemptyset(&signaux);

  fd = open(argv[1],O_WRONLY | O_CREAT | O_TRUNC, 0666);

  pid = fork();
  if(pid == 0) { /* code du processus P1 */

    for (c ='a';c<='z';c++)
      {
        write(fd,&c,1);
        kill(getppid(),SIGUSR1);
	if(flag) sigsuspend(&signaux);
        flag = 1;
      }

    close(fd);
    exit(0);
  }

  /* code du processus P2 */

    for (c ='A';c<='Z';c++)
      {
	if(flag) sigsuspend(&signaux);
        flag = 1;
        write(fd,&c,1);
        kill(pid,SIGUSR1);
     }

  wait(NULL);
  close(fd);
  exit(0);
}