What's the difference between async and nonblocking in unix socket?

c network-programming nonblocking asyncsocket
cpuer picture cpuer · Jun 7, 2011 · Viewed 7.3k times · Source

I'm seeing such code in nginx:

if(fcntl(ngx_processes[s].channel[0], F_SETFL, fcntl(s, F_GETFL) | O_NONBLOCK) == -1) {
...
if (ioctl(ngx_processes[s].channel[0], FIOASYNC, &on) == -1) {
...

Anyone can tell me what's the difference between fcntl(s, F_SETFL, fcntl(s, F_GETFL) | O_NONBLOCK) and ioctl(s, FIOASYNC, &on) ,aren't async and nonblocking the same thing??

Answer

sarnold picture sarnold · Jun 7, 2011

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.

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