Non-blocking I/O

Adding nonblock_read() and nonblock_write() would save a considerable amount of time and effort in typical select/poll libraries such as my io library. It would eliminate bugs in quite a few programs that don't go to the same amount of effort.

Here are the alternatives to nonblock_read() and nonblock_write():

• O_NONBLOCK (aka O_NDELAY) at the beginning of the program. The problem with O_NONBLOCK is that it's too heavy: it hits an entire ofile, not just an fd. For example, suppose I set O_NONBLOCK on a pty on stdout, and another program has the same stdout. That other program then has O_NONBLOCK set too. Many programs can't handle O_NONBLOCK and will break horribly in this situation.
• O_NONBLOCK immediately before the operation, and turn it off immediately after. This is clumsy, and fails horribly if more than one program is active at once.
• An imaginary per-fd (rather than per-ofile) version of O_NONBLOCK. This would work, but it's clumsier than nonblock_read() and nonblock_write(), and there are no benefits to the indirection.
• MSG_DONTWAIT. This isn't portable, and it works only for sockets.
• Checking readability a moment before the operation. This is popular, but can fail horribly if more than one program is active at once. For example, after select() says a pipe is writable, a program that tries writing to the pipe might nevertheless block, because the pipe was filled by another program after select() returned.
• Interval timers that interrupt blocking calls every 10 milliseconds. This works (aside from the 10-millisecond delay), at least in programs that don't use SIGALRM for anything else, but it's clumsy.

Here's what I'm doing now:

1. Call read() or write() without further ado if the descriptor was originally provided in O_NONBLOCK mode or refers to a regular file. (Of course, it's possible that somebody subsequently turned O_NONBLOCK off; every program relying on O_NONBLOCK is taking this risk.)
2. Use poll(), if it's available and functioning, to check whether the descriptor is ready.
3. Set an interval timer.
4. Call read() or write().
5. Turn the interval timer off.