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SubjectRe: Multithread select() bug
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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