通过多个过程从stdin读取 [英] Reading from stdin by multiple processes

问题描述

我正在编写一个程序，其中父进程使用fork()创建N子进程(将N作为参数提供)，以便每个子进程都直接由这个父进程派生.

I am writing a program in which the parent process uses fork() to create N child processes (N is provided as an argument), so that every child is directly forked by this one parent.

每个子进程都需要从stdin中读取一行，并打印到屏幕上. 编译并执行程序后，将通过以下文本文件提供文本:

Every child process needs to read a line from stdin, and print on to the screen. After compiling and executing the program, the text is provided through a text file like this:

./prog1 3 < fileWithText.txt

我的问题是，我希望看到每个孩子都在为阅读输入而打架"，但是我实际上看到的是，总是只有一个子进程负责处理输入.这是我用于子进程的代码:

My problem is, that I am expecting to see every child "fight" for reading the input, however what I actually see is that there is always only one child process taking care of handling the input. This is the code I use for the child process:

void do_child_reader(int j) {
  int pid;
  int counter = 0;
  char* buffer;
  char* readCheck;
  int readerRunning = TRUE;

  pid = getpid();
  buffer = (char*)malloc(BUFFER_SIZE * sizeof(char));

  while (readerRunning == TRUE) {
    readCheck = fgets(buffer, BUFFER_SIZE, stdin);
    if (readCheck == NULL) {
      readerRunning = FALSE;
    }
    else {
      fprintf(stdout, "(READER %d pid-%d) %s", j, pid, buffer);
      counter++;
    }
  }

  fprintf(stderr, "(READER %d pid-%d) processed %d messages, going to exit\n", j, pid, counter);
  free(buffer);
  exit(counter);
}

这是我在运行N = 3时得到的输出:

Here's the output I get when running for N=3:

(READER 0 pid-72655) I am a message - line 1
(READER 0 pid-72655) I am a message - line 2
(READER 0 pid-72655) I am a message - line 3
(READER 0 pid-72655) processed 3 messages, going to exit
(READER 1 pid-72657) processed 0 messages, going to exit
(READER 2 pid-72659) processed 0 messages, going to exit

父进程正在等待子进程完成后退出.

The parent process is waiting for the children to finish before exiting.

我正在尝试找出导致此行为的原因，以及它是通过fgets()的方式还是以我尝试读取字符串的方式.

I am trying to figure out what would cause this behaviour, and whether it is in the way fgets() works or perhaps in the way I am trying to read the strings.

谢谢！

推荐答案

如果文件太小，一个进程可以读取一个BUFSIZ字节(来自<stdio.h>)字节，那么其他进程就什么也没剩下读书.增大输入文件的大小(是BUFSIZ大小的倍数)，并确保客户机在缓冲区已满后读取速度足够慢，并且您会看到它们都在工作.

If the file is so small that one unit of BUFSIZ (from <stdio.h>) bytes can be read by one process, the other processes have nothing left to read. Make the input file bigger — multiple times the size of BUFSIZ — and make sure the clients read slowly enough after getting a buffer full, and you will see them all working on it.

这是为您提供的I/O缓冲.您可以尝试弄乱setvbuf()来设置输入的行缓冲.我不确定是否可以.

This is I/O buffering for you. You could try messing with setvbuf() to set line buffering on the input; I'm not sure it would work.

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