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Date: Fri, 24 Feb 2023 09:42:00 -0500
From: Tamir Duberstein <>
To: NRK <>
Cc: Rich Felker <>,
Subject: Re: undefined behavior in fread.c

We could take the lock and still avoid UB with an early return.

On Fri, Feb 24, 2023, 09:17 NRK <> wrote:

> On Fri, Feb 24, 2023 at 09:07:41AM -0500, Rich Felker wrote:
> > stdio functions are required (by POSIX) to behave as if they take a
> > mutex on the FILE. If fread with a length of zero makes forward
> > progress when another thread holds the lock, this is non-conforming.
> OK, I see. Wasn't aware of that.
> On a sidenote, if I'm reading the right function (libio/iofread.c) then
> glibc is taking the lock _after_ they do the size check.
>           if (bytes_requested == 0)
>             return 0;
>           _IO_acquire_lock (fp);
>         /* ... */
> - NRK

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