c - 子进程如何在读取第一条消息后不离开while循环？

Question

我最近在尝试解决 Linux C 中我自己的管道问题时遇到了这个例子，它确实回答了我的问题，但给了我另一个问题，为什么子进程在第一条消息之后不离开 while 循环？如果它已经读取完输入消息，那么它不是在父进程有机会在 sleep(5) 之后输入第二条消息之前就离开吗？

    #include <stdio.h>
    #include <unistd.h>
    #include <sys/ioctl.h>

    int main()
    {
        int pid = 0;

        // create pipe pair
        int fd[2];
        pipe(fd);

        pid = fork();
        if (pid == 0)
        {
            // child side
            char *buff = NULL;
            char byte = 0;
            int count = 0;

            // close write side. don't need it.
            close(fd[1]);

            // read at least one byte from the pipe.
            while (read(fd[0], &byte, 1) == 1)
            {
                if (ioctl(fd[0], FIONREAD, &count) != -1)
                {
                    fprintf(stdout,"Child: count = %d\n",count);

                    // allocate space for the byte we just read + the rest
                    //  of whatever is on the pipe.
                    buff = malloc(count+1);
                    buff[0] = byte;
                    if (read(fd[0], buff+1, count) == count)
                        fprintf(stdout,"Child: received \"%s\"\n", buff);
                    free(buff);
                }
                else
                {   // could not read in-size
                    perror("Failed to read input size.");
                }
            }

            // close our side
            close(fd[0]);
            fprintf(stdout,"Child: Shutting down.\n");
        }
        else
        {   // close read size. don't need it.
            const char msg1[] = "Message From Parent";
            const char msg2[] = "Another Message From Parent";
            close(fd[0]);
            fprintf(stdout, "Parent: sending \"%s\"\n", msg1);
            write(fd[1], msg1, sizeof(msg1));
            sleep(5); // simulate process wait
            fprintf(stdout, "Parent: sending \"%s\"\n", msg2);
            write(fd[1], msg2, sizeof(msg2));
            close(fd[1]);
            fprintf(stdout,"Parent: Shutting down.\n");
        }
        return 0;
    }

Answer 1

如果没有可用数据，

read 将阻塞直到数据到达(除非管道设为非阻塞)。