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