linux 异步I/O 信号

    if (ioctl(ngx_processes[s].channel[0], FIOASYNC, &on) == -1) {
            ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_errno,
                          "ioctl(FIOASYNC) failed while spawning \"%s\"", name);
            ngx_close_channel(ngx_processes[s].channel, cycle->log);
            return NGX_INVALID_PID;
        }

        if (fcntl(ngx_processes[s].channel[0], F_SETOWN, ngx_pid) == -1) {
            ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_errno,
                          "fcntl(F_SETOWN) failed while spawning \"%s\"", name);
            ngx_close_channel(ngx_processes[s].channel, cycle->log);
            return NGX_INVALID_PID;
        }
            

分析：

• 设置channel[0]的信号驱动异步I/O标志 ，FIOASYNC：该状态标志决定是否收取针对socket的异步I/O信号（SIGIO）
• 其与O_ASYNC文件状态标志等效，可通过fcntl的F_SETFL命令设置or清除
• F_SETOWN：用于指定接收SIGIO和SIGURG信号的socket属主（进程ID或进程组ID） 这里意思是指定Master进程接收SIGIO和SIGURG信号 ， 
• SIGIO信号必须是在socket设置为信号驱动异步I/O才能产生，即上一步操作 SIGURG信号是在新的带外数据到达socket时产生的 

FIOASYNC toggles the O_ASYNC flag (which is usually set in open(2) or fcntl(2)) for a file descriptor, which will ask the kernel to send SIGIO or SIGPOLL to the process when the file descriptor is ready for IO.

O_ASYNC is not used often:

• it is extremely difficult to properly handle IO in signal handlers; they are best left as tiny as possible
• because signals interrupt the control flow of the program, they 'cost more' to run than standard system calls, such as select(2) or poll(2)
• signals provide less information than other calls: they only report one fd ready vs many fds that might be ready.

The O_NONBLOCK doesn't provide any notification to the user process that a fd is ready for read(2) or write(2) -- instead, it changes the behavior of read(2) and write(2) and similar calls to return immediately if the file descriptor isn't ready for reading or writing. O_NONBLOCK is typically used in conjunction with select(2) or poll(2) or similar calls to guarantee that the main loop of a client or server won't block on one specific peer, and thus starve all its peers.

ioctl和FIOASYNC等价于fcntl和O_ASYNC。

ioctl和FIONBIO等价于fcntl和O_NONBLOCK。

FIOASYNC设置O_ASYNC标记，该标记决定fd可以IO时进程是否会收到SIGIO和SIGPOLL信号。

FIONBIO设置O_NONBLOCK标记，该标记会改变read，write和同类函数的行为，使得在fd还不能IO时立即返回而不是hang住。

后者经常跟select，poll等函数一起使用，使得主程序不会因为个别socket而影响其他。

一般来说使用select和poll结合非阻塞的文件指针可以对应大部分情况，但是某些时候 需要使用异步的文件指针。比如：如果一个函数处理数据，但是处理时间很长，在其处理的时候 我们需要运行这个函数的进程及时响应网络事件或者内核信号，这时就需要将其置为异步

对于socket来说，如果需要设置异步的话需要三个步骤

1. 必须注册一个响应SIGIO的信号回调函数
2. 通过fcntl设置F_SETOWN,使得socket属于某个进程
3. 通过fcntl设置O——ASYNC将该socket设置为异步

https://stackoverflow.com/questions/6260114/whats-the-difference-between-async-and-nonblocking-in-unix-socket

https://stackoverflow.com/questions/7440571/whats-the-difference-between-fionbio-and-fioasync-for-socket