ftp client 发送了 STOR 命令后，命令端口就阻塞了，我关闭了数据端口了？

问题

代码

开了一个子进程来发送文件，发送完毕之后就 close 了 client_data_socket （数据 socket ）

// 非阻塞
set_flag(client_data_socket, O_NONBLOCK);
pid_t pid;
if ((pid = fork()) < 0) {
    printf("fork error");
    continue;
} else if (pid == 0) {
    FILE *fp;
    if ((fp = fopen(filename, "rb")) == NULL)
    {
        close(client_data_socket);
        printf("open file failed\n");
        exit(1);
    }
    size_t char_size = sizeof(char);
    char data_buffer[FILE_READ_BUFFER_SIZE];
    int numread;
    for (;;)
    {
        bzero(data_buffer, FILE_READ_BUFFER_SIZE);
        numread = fread(data_buffer, char_size, FILE_READ_BUFFER_SIZE, fp);
        if (numread < 0)
        {
            printf("read file failed\n");
            break;
        } 
        else if (numread > 0)
        {
            int length = send(client_data_socket, data_buffer, numread, 0);
            if (length == 0)
            {
                break;
            }
            else if (length < 0)
            {
                if (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR)
                {
                    continue;
                }
                printf("[PUT] command send data failed\n");
                exit(1);
            }
        }
        if (numread == FILE_READ_BUFFER_SIZE) continue;
        else {
            break;
        }
    }
    close(client_data_socket);
    fclose(fp);
    exit(0);
} else {
    int status = 0;
    waitpid(pid, &status, 0);
    if (status == 0)
        printf("send file %s complete.\n", filename);
    else 
        printf("send file %s failed.\n", filename);
}

然后，发送别的命令在 recv 的时候，命令端 socket 就阻塞了
Github