#define _GNU_SOURCE
#define _LARGEFILE_SOURCE
#define _FILE_OFFSET_BITS 64

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <errno.h>

#define FNAME		"kill-locks.tmp"
#define BUFSIZE		16384
#define DEATHSIG	SIGINT

void sighandler(int signum)
{
  if (0) signum = 0;
}

int
main()
{
  int			successcount = 0;
  struct sigaction	sa;
  sigset_t		blockset, origset, waitset;
  /**/

  sa.sa_handler = &sighandler;
  sa.sa_flags	= 0;
  sigemptyset(&sa.sa_mask);
  if (sigaction(SIGUSR1, &sa, NULL) == -1)
    perror("sigaction(SIGUSR1)"), exit(1);
  if (sigaction(SIGCHLD, &sa, NULL) == -1)
    perror("sigaction(SIGCHLD)"), exit(1);

  sigemptyset(&blockset);
  sigaddset(&blockset, SIGUSR1);
  sigaddset(&blockset, SIGCHLD);
  if (sigprocmask(SIG_BLOCK, &blockset, &origset) == -1)
    perror("sigprocmask"), exit(1);
  waitset = origset;
  sigdelset(&waitset, SIGUSR1);
  sigdelset(&waitset, SIGCHLD);
  sigaddset(&waitset, DEATHSIG);

  while (1)
    {
      pid_t	childpid = fork();
      int	status;
      /**/

      if (childpid == (pid_t) -1)
	perror("fork()"), exit(1);
      if (childpid == 0)
	{
	  /* Child */
	  int		fd;
	  struct flock	lck;
	  char		buf[BUFSIZE];
	  /**/

	  if (sigprocmask(SIG_SETMASK, &origset, NULL) == -1)
	    perror("[child] sigprocmask"), exit(1);

	  fd = open(FNAME, O_RDWR|O_CREAT, 0666);
	  if (fd == -1)
	    perror("[child] open()"), exit(1);

	  lck.l_type   = F_WRLCK;
	  lck.l_whence = SEEK_SET;
	  lck.l_start  = (off_t)0;
	  lck.l_len    = (off_t)0;
	  if (fcntl(fd, F_SETLK, &lck) == -1)
	    perror("[child] fcntl(F_SETLK)"), exit(1);
	  memset(buf, 0, sizeof(buf));
	  kill(getppid(), SIGUSR1);
	  while(1)
	    write(fd, buf, sizeof(buf));
	}

      if ( ! (sigsuspend(&waitset) == -1 && errno == EINTR))
	perror("sigsuspend"), exit(1);
      usleep(rand()%1000);
      kill(childpid, DEATHSIG);
      if (waitpid(childpid, &status, 0) != childpid)
	perror("waitpid"), exit(1);
      if ( ! (WIFSIGNALED(status) && WTERMSIG(status) == DEATHSIG))
	{
	  fprintf(stderr,
		  "unexpected status from child %08X\n"
		  "successful locking attempts: %d\n",
		  status, successcount);
	  exit(1);
	}
      ++successcount;
    }
}