UNIX for Dummies Questions & Answers

#!/bin/sh

if ! mkfifo /tmp/myappname
then
        echo "Locked" >&2
        exit 1
fi

trap "rm -f /tmp/myappname" EXIT

	case IOWRITE:
		flags = O_WRONLY | O_CREAT | O_TRUNC;
		/* The stat() is here to allow redirections to
		 * things like /dev/null without error.
		 */
		if (Flag(FNOCLOBBER) && !(iop->flag & IOCLOB) &&
		    (stat(cp, &statb) < 0 || S_ISREG(statb.st_mode)))
			flags |= O_EXCL;
		break;

  /* If the file was not present (r != 0), make sure we open it
     exclusively so that if it is created before we open it, our open
     will fail.  Make sure that we do not truncate an existing file.
     Note that we don't turn on O_EXCL unless the stat failed -- if
     the file was not a regular file, we leave O_EXCL off. */
  flags &= ~O_TRUNC;
  if (r != 0)
    {
      fd = open (filename, flags|O_EXCL, mode);
      return ((fd < 0 && errno == EEXIST) ? NOCLOBBER_REDIRECT : fd);
    }