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Re: PATCH: bug fix: infinite loop in sysread
- X-seq: zsh-workers 45384
- From: Roman Perepelitsa <roman.perepelitsa@xxxxxxxxx>
- To: Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>
- Subject: Re: PATCH: bug fix: infinite loop in sysread
- Date: Wed, 5 Feb 2020 22:04:32 +0100
- Cc: Zsh hackers list <zsh-workers@xxxxxxx>
- In-reply-to: <CAH+w=7aTpOsSMJOoiRnj6mpREfPc8AHJE+Os0FKjuNug23nxMQ@mail.gmail.com>
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On Wed, Feb 5, 2020 at 9:55 PM Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx> wrote:
> On Wed, Feb 5, 2020 at 6:20 AM Roman Perepelitsa
> <roman.perepelitsa@xxxxxxxxx> wrote:
> > Here select() keeps returning 0, indicating timeout. This is not an
> > error, so errno doesn't get set. If it was EINTR prior to the call,
> > it stays EINTR, and the loop keeps spinning.
> > The fix is to replace `< 1` with `< 0` in the loop condition.
> Wouldn't a better fix be to set errno = 0 before the call?
That would be incorrect. select() returns -1 on error and in general
functions must set errno on error and may set it on success. When
select() returns 0, errno can be anything.
Before sending this patch I've also checked if there are similar bugs
in other places. There are none. The result of select() is always
tested with `< 0` everywhere in Zsh expect this one place.
The function I'm modifying has a very similar branch that calls poll()
instead of select. That one compares with `< 0`.
From `man select`:
On success, select() and pselect() return the number of file
descriptors contained in the three returned descriptor sets
(that is, the total number of bits that are set in readfds,
writefds, exceptfds) which may be zero if the timeout
expires before anything interesting happens. On error, -1
is returned, and errno is set to indicate the error; the
file descriptor sets are unmodified, and timeout becomes
From `man errno`:
The value in errno is significant only when the return value
of the call indicated an error (i.e., -1 from most system
calls; -1 or NULL from most library functions); a function
that succeeds is allowed to change errno. The value of
errno is never set to zero by any system call or library
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