p1.c

#include <errno.h>
#include <fcntl.h>
#include "apue.h"

void sigint(int signo) {}

int main(void) {
  pid_t pid1, pid2, pid3;
  int fd;

  setbuf(stdout, NULL);
  signal_intr(SIGINT, sigint);

  if ((fd = open("lockfile", O_RDWR | O_CREAT, 0666)) < 0) {
    err_sys("can't open/create lockfile");
  }

  if ((pid1 = fork()) < 0) {
    err_sys("fork failed");
  } else if (pid1 == 0) {
    if (lock_reg(fd, F_SETLK, F_RDLCK, 0, SEEK_SET, 0) < 0) {
      err_sys("child 1: can't read-lock file");
    }
    printf("child 1: obtained read lock on file\n");
    pause();
    printf("child 1: exit after pause\n");
    exit(0);
  } else {
    sleep(2);
  }

  if ((pid2 = fork()) < 0) {
    err_sys("fork failed");
  } else if (pid2 == 0) {
    if (lock_reg(fd, F_SETLK, F_RDLCK, 0, SEEK_SET, 0) < 0) {
      err_sys("child 2: can't read-lock file");
    }
    printf("child 2: obtained read lock on file\n");
    pause();
    printf("child 2: exit after pause\n");
    exit(0);
  } else {
    sleep(2);
  }

  if ((pid3 = fork()) < 0) {
    err_sys("fork failed");
  } else if (pid3 == 0) {
    if (lock_reg(fd, F_SETLK, F_RDLCK, 0, SEEK_SET, 0) < 0) {
      printf("child about to block in write-lock%s\n", strerror(errno));
    }
    printf("child 3 about to block in write-lock...\n");
    if (lock_reg(fd, F_SETLKW, F_WRLCK, 0, SEEK_SET, 0) < 0) {
      err_sys("child 3: can't write-lock file");
    }
    printf("child 3 returned and got write lock????\n");
    pause();
    printf("child 3: exit after pause\n");
    exit(0);
  } else {
    sleep(2);
  }
  
  if (lock_reg(fd, F_SETLK, F_RDLCK, 0, SEEK_SET, 0) < 0) {
    printf("parent: can't set read lock: %s\n", strerror(errno));
  } else {
    printf("parent: obtained additional read lock while write lock is pending\n");
  }
  printf("killing child 1...\n");
  kill(pid1, SIGINT);
  printf("killing child 2...\n");
  kill(pid2, SIGINT);
  printf("killing child 3...\n");
  kill(pid3, SIGINT);
  exit(0);
}