Re: [PATCH] trace2: refactor to avoid gcc warning under -O3

Jeff King [off-list ref] writes:

On Tue, May 11, 2021 at 04:34:49PM +0200, Johannes Schindelin wrote:

quoted

On Fri, 7 May 2021, Junio C Hamano wrote:

quoted

		/* GCC thinks socket()/connect() might fail to set errno */
		return errno ? errno : EIO;

If a compiler thinks errno may *not* be set, can 'errno' be reliably
used to decide if it can be returned as-is or a fallback value EIO
should be used, without triggering the same (incorrect) working in
the first place?

Oh, I guess I mistook the problem for something else, then.

If the problem is that `errno` is not set in case of failure, the
resolution is easy (and even recommended in the manual page of `errno`):
simply set it to 0 before the syscall (or in the function that relies on
`errno == 0` means success).

I don't think that is the problem. According to the standard, errno is
always set to a non-zero value after a syscall failure.

The problem is only that the compiler does not incorporate that special
knowledge of the variable, so it generates a warning even though we can
reason that the situation it describes is impossible.

Yes, that is what I tried to say (i.e. if the compiler does not know
errno has a defined value at certain places in our code and
complain, then "return errno ? errno : EIO" would get the same
warning because bases its outcome on the value of errno the compiler
thinks is possibly undefined at that point), but apparently I failed
to convey that clearly enough.

Thanks.