读取和写入管道问题

Question

Here is the program:

  int fd[2];
  char buf1 [31];
  int i;
  char buf2;

  pipe(fd);

  if (fork() == 0) { // child
    close(fd[1]); // close writing pipe
    for(i = 0; i< 20; i++) { 
        read(fd[0], buf1, 30);
        printf("%s\n", buf1);

    }
    close(fd[0]);
  }

  else { // parent
    close(fd[0]); // close reading end
    buf2 = 'a';
    for (i = 0; i < 20; i++) {
        write(fd[1], &buf2, sizeof(buf2));
    }
    close(fd[1]);
  }
  wait(NULL);

Since read blocks if there is nothing in there, we will first write to the pipe.

My question is regarding the write function. The for loop runs 20 times, and each time I am writing an a to the pipe. After 20 iterations, the pipe on write end will contains aaaaaaaaaaaaaaaaaaaa

In the child, I read all 20 a's into buf1 , and I print the result.

Now is the pipe empty, since we have wrote, and read what we wrote?

Since there is also a for loop here, we read again, but this time, I think the pipe is empty, so theres nothing to read. So I think the final output after 20 iterations of reading is just aaaaaaaaaaaaaaaaaaaa

But actually it's this:

aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa

Why? Is the pipe not empty after we read?

Answer 1

You are correct that if the child calls read before the parent has written anything, the call will block; however, after the parent closes the file descriptor, this is no longer the case and read will return 0 for EOF.

The read call doesn't clear the buffer. The OS successfully reads zero characters, and therefore changes the first zero characters of the buffer. That is, it doesn't change buf1 at all. Thus, once all the data has been read into the buffer, the printf call will always do the same thing.

Note that you aren't guaranteed to get this output. It is perfectly possible that the child will be run after some but not all of the write calls from the parent, in which case it will not read all 20 a s at once and print shorter strings. That's why above I said that the printf call will always do the same thing once all the data has been read . To handle the case where not all the data has been written, you should check the return value of read . If it is less than 20 you didn't read the full string into the buffer (and should call read again with the destination pointer advanced so you don't overwrite your data); if it is zero then you've reached end-of-file (the write end is closed); if it is negative there was an error and you should check errno to handle it.