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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?

    Cheers,

    Andre Ben Hamou
    Imperial College London

    --

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