Re: Multithread select() bug

From: Andre Ben Hamou
Date: Mon May 10 2004 - 17:15:17 EST

Eric Dumazet wrote:
Your program is racy and have undefined behavior.

A thread should not close a handle 'used by another thread blocked in a sytemcall'

The race is : if a thread does a close(fd), then the fd value may be reused by another thread during an open()/socket()/dup()... syscall, and the first thread could issue the select() syscall (or read()/write()/...) on the bad file.

Apologies, but I don't follow this.

It was my understanding that the (potentially) many threads of a single process all share a canonical file descriptor table. Hence as long as the various calls you mention are issued in a guaranteed order, maintaining state as you go (which is what the 1 second sleep in the test code was a very quick and dirty way to almost do), I don't see how a race condition arises.

If I were to replace the sleep (1) with, say a global semaphore or something similar, would your explanation still hold?


Andre Ben Hamou
Imperial College London


...and, on the seventh day, God switched off his Mac.
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