From: Jeff King <peff@peff.net>
Date: Wed, 19 Nov 2014 01:47:23 GMT
Subject: Re: [PATCH 1/4] error: save and restore errno
Message-ID: <20141119014722.GB2337@peff.net>
In-Reply-To: <20141119014344.GP6527@google.com>

On Tue, Nov 18, 2014 at 05:43:44PM -0800, Jonathan Nieder wrote:

> Jeff King wrote:
> 
> > It's common to use error() to return from a function, like:
> >
> > 	if (open(...) < 0)
> > 		return error("open failed");
> >
> > Unfortunately this may clobber the errno from the open()
> > call. So we often end up with code like this:
> >
> >         if (open(...) < 0) {
> > 		int saved_errno = errno;
> > 		error("open failed");
> > 		errno = saved_errno;
> > 		return -1;
> > 	}
> >
> > which is less nice.
> 
> What the above doesn't explain is why the caller cares about errno.
> Are they going to print another message with strerror(errno)?  Or are
> they going to consider some errors non-errors (like ENOENT when trying
> to unlink a file), in which case why is printing a message to stderr
> okay?

I guess the unsaid bit is:

  Unfortunately this may clobber the errno from the open() call. Even
  though error() sees the correct errno, the caller to which we are
  returning may see a bogus errno value.

-Peff