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Date: Tue, 18 Feb 2020 22:39:52 -0500
From: Rich Felker <>
Subject: Re: race condition in sem_wait

On Wed, Feb 19, 2020 at 01:46:34AM +0100, Sebastian Gottschall wrote:
> Hello
> i discovered recently a race condition while playing with threads
> and sem_wait/sem_post
> sem_wait may fail with errno set EAGAIN which is not valid since
> only sem_trywait is able to set that errno code.
> this was causing a bug with a later select() and accept() which
> failed since accept does not work if errno is set to EAGAIN.
> from my point of view the bug is in sem_timedwait.c
>         if (!sem_trywait(sem)) return 0;
>         int spins = 100;
>         while (spins-- && sem->__val[0] <= 0 && !sem->__val[1]) a_spin();
>         while (sem_trywait(sem)) {
> the fist sem_trywait will fail with -1 and sets EAGAIN. but the
> second sem_trywait will not fail and does return 0. the problem now
> is that errno is still present and not reset.
> this may cause if sem_post is called from a second thread on the
> same semaphore.
> of course the same bug affects sem_timedwait itself.
> so i assume sem_wait is not thread safe which is bad and is not
> follow the posix specification
> or am i wrong here?

errno is only meaningful on failure; unless specified otherwise (a few
functions are special because you can't [easily] distinguish success
from failure for them without examining errno), any standard function
may have changed the value of errno when it returns with success. The
only thing it's not allowed to do is clear it (set it to 0).


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