#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
#define LOCKTRIES       10
#define STRINGIFY_(x)	#x
#define STRINGIFY(x)    STRINGIFY_(x)
#define LOCKTRIESSTR    STRINGIFY(LOCKTRIES)
#define LOCKSLEEPSECS   1

void parent_try_lock(int mfd, int successcount, int status)
{
  int		i;
  struct flock	lck;

  for (i=0; 1 /* always true */; ++i)
    {
      lck.l_type   = F_WRLCK;
      lck.l_whence = SEEK_SET;
      lck.l_start  = (off_t)0;
      lck.l_len    = (off_t)0;
      if (fcntl(mfd, F_SETLK, &lck) == -1)
	{
	  if (errno == EAGAIN)
	    {
	      if ( i == LOCKTRIES )
		{
		  fprintf(stderr,
			  "unexpected status from child %08X\n"
			  "successful locking attempts: %d\n",
			  status, successcount);
		  exit(1);
		}
	      else
		{
		  sleep(LOCKSLEEPSECS);
		}
	    }
	  else
	    perror("[parent] fcntl(F_SETLK)"), exit(1);
	}
      else
	{
	  lck.l_type   = F_UNLCK;
	  lck.l_whence = SEEK_SET;
	  lck.l_start  = (off_t)0;
	  lck.l_len    = (off_t)0;
	  if (fcntl(mfd, F_SETLK, &lck) == -1)
	    perror("[parent] fcntl(F_SETLK) UNLCK"), exit(1);
	  if (status)
	    fprintf(stderr, "[parent] transient error, going on...\n");
	  break;
	}
    }
}

int
main()
{
  int	successcount = 0;
  int	mfd;
  /**/

  mfd = open(FNAME, O_RDWR|O_CREAT, 0666);
  if (mfd == -1)
    perror("open()"), exit(1);
  parent_try_lock(mfd, successcount, 0);

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

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

	  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));
	  while(1)
	    write(fd, buf, sizeof(buf));
	}

      usleep(rand()%1000);
      kill(childpid, DEATHSIG);
      if (waitpid(childpid, &status, 0) != childpid)
	perror("waitpid"), exit(1);
      if ( ! (WIFSIGNALED(status) && WTERMSIG(status) == DEATHSIG))
	parent_try_lock(mfd, successcount, status);
      ++successcount;
    }
}