14.3.2.c

#include <errno.h>
#include <fcntl.h>
#include <sys/wait.h>
#include "apue.h"

int main(int argc, char* argv[]) {
  int fd;
  pid_t pid;
  char buf[5];
  struct stat statbuf;

  if (argc != 2) {
    fprintf(stderr, "usage: %s filename\n", argv[0]);
    exit(1);
  }
  if ((fd = open(argv[1], O_RDWR | O_CREAT | O_TRUNC, FILE_MODE)) < 0) {
    err_sys("open error");
  }
  if (write(fd, "abcdef", 6) != 6) {
    err_sys("write error");
  }

  if (fstat(fd, &statbuf) < 0) {
    err_sys("fstat error");
  }
  if (fchmod(fd, (statbuf.st_mode & ~S_IXGRP) | S_ISGID) < 0) {
    err_sys("fchmod error");
  }

  TELL_WAIT();

  if ((pid = fork()) < 0) {
    err_sys("fork error");
  } else if (pid > 0) {
    if (write_lock(fd, 0, SEEK_SET, 0) < 0) {
      err_sys("write_lock error");
    }

    TELL_CHILD(pid);

    if (waitpid(pid, NULL, 0) < 0) {
      err_sys("write_lock error");
    }
  } else {
    WAIT_PARENT();

    set_fl(fd, O_NONBLOCK);

    if (read_lock(fd, 0, SEEK_SET, 0) != -1) {
      err_sys("child: read_lock succeeded");
    }
    printf("read_lock of already-locked region erturns %d\n", errno);

    if (lseek(fd, 0, SEEK_SET) == -1) {
      err_sys("lseek error");
    }
    if (read(fd, buf, 2) < 0) {
      err_ret("read failed (mandatory locking works)");
    } else {
      printf("read OK (no mandatory locking), buf = %2.2s\n", buf);
    }
  }
  exit(0);
}