Jeff King <peff@xxxxxxxx> writes:
> 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
I am not sure if that answers the question asked.
If you have
int frotz(...) {
int fd = open(...);
if (fd < 0)
return error("open failed (%s)", strerror(errno));
return fd;
}
and the caller calls it and cares about the errno from this open,
what does the caller do? Jonathan's worried about a codepath that
may be familiar to us as we recently saw a patch similar to it:
int fd = frotz(...);
if (fd < 0) {
if (errno == ENOENT || errno == EISDIR)
; /* not quite an error */
else
exit(1);
}
If ENOENT/EISDIR is expected and a non-error, it is not useful for
frotz() to give an error message on its own.
I think a more appropriate answer to Jonathan's question is why is
the callee (i.e. frotz()) calling error() in the first place if an
unconditional error message is an issue.
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