问题

I use FIFO in my server. Due to some limitations, I have to open the FIFO in write-only firstly, then I use fork to create another process to open FIFO in read-only. But I find that, it will be blocked when I open the FIFO in write-only firstly, thus the server can not continue to run to fork.

由于某些限制，我必须首先以只写模式打开 FIFO，然后使用 fork 创建另一个进程以只读模式打开 FIFO。但我发现当我首先以只写模式打开 FIFO 时，它会被阻塞，因此服务器无法继续运行到 fork。

For some reason I can not open FIFO in read-only firstly, Is there a way to open FIFO in write-only without blocked? I have tried open("/tmp/ngfifo", O_WRONLY | O_NONBLOCK) but will return error.

由于某些原因，我不能首先以只读模式打开 FIFO，有没有办法以只写模式打开 FIFO 而不被阻塞？我已经尝试过 open("/tmp/ngfifo", O_WRONLY | O_NONBLOCK)，但会返回错误。

The small example is like this :

示例代码如下：

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>

int main()
{
    int pipe_fd = open("/tmp/ngfifo", O_WRONLY);  // It will be blocked here, is there a way to open without block.
    if (pipe_fd < 0) {
        fprintf(stderr, "Opened fifo failed %d\n", errno);
        return -1;
    }
    printf("wait..\n");

    if (fork() > 0) {
        printf("begin to write \n");
    } else {
        int read_fd = open("/tmp/ngfifo", O_RDONLY);
        // ...
        printf("begin to read \n");
    }
    return 0;
}

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>

int main()
{
    int pipe_fd = open("/tmp/ngfifo", O_WRONLY);  // 会在这里被阻塞，是否有办法在不阻塞的情况下打开。
    if (pipe_fd < 0) {
        fprintf(stderr, "打开FIFO失败 %d\n", errno);
        return -1;
    }
    printf("等待..\n");

    if (fork() > 0) {
        printf("开始写入\n");
    } else {
        int read_fd = open("/tmp/ngfifo", O_RDONLY);
        // ...
        printf("开始读取\n");
    }
    return 0;
}

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>

int main()
{
    int pipe_fd = open("/tmp/ngfifo", O_WRONLY);  // It will be blocked here, is there a way to open without block.
    if(pipe_fd < 0) {
        fprintf(stderr, "Opened fifo failed %d\n", errno);
        return -1;
    }
    printf("wait..\n");

    if (fork() > 0) {
        printf("begin to write \n");
    } else {
        int read_fd = open("/tmp/ngfifo", O_RDONLY);
        // ...
        printf("begin to read \n");
    }
    return 0;
}

答案1

得分: 3

通常情况下，打开 FIFO 会阻塞，直到另一端也被打开。

解决方法很简单：在分叉（fork）子进程之后再打开文件。

答案2

得分: 1

从Linux FIFO手册页面：

在Linux下，以读写方式打开FIFO将在阻塞模式和非阻塞模式下均成功。POSIX未定义此行为。这可用于在没有可用读取器的情况下打开FIFO进行写入。

因此，在Linux上，写入进程可以调用open("/tmp/ngfifo", O_RDWR | O_NONBLOCK)以非阻塞方式打开管道的写入端，因为它也用于读取。然而，这在其他操作系统上不具备可移植性。