why( warning: left-hand operand of comma expression has no effect)

Sam Bobroff sbobroff at shoretel.com
Tue Nov 20 03:54:50 CET 2012


On 20/11/12 12:32, Marc Lehmann wrote:
> On Tue, Nov 20, 2012 at 01:16:04AM +0000, Sam Bobroff <sbobroff at shoretel.com> wrote:
>> Maybe those hack strings should removed from the assert calls and be
>> moved into comments in the code?
> Calling this a "hack" just shows your immaturity in the C language - not
> everything that is new to you is badly done.
Sorry if you felt that I'd disrespected your code, that wasn't my
intention (and FWIW, I'm not new to C or assert()).
>
> This use of assert is part of the C language that is well supported and in
> very common use for a long time now, and does what it is suppsoed to do
> very well and reliably. It's the only way to reliably achieve the effect
> libev needs.
Well... it's not an obvious, straight forward usage: it seems to me that
if the "normal usage" of assert was to print a string message along with
the test, assert() would take two arguments or even a format string.
>
> Putting error messages into a comment would completely defeat the purpose
> of having runtime error messages in the first place, namely telling whats
> wrong at *runtime*, when the comment is long gone.
I don't agree with you here: I think assert is for reporting "bugs in
the code" rather than some problem a user could understand, which is why
it always prints out the file and line number at which it occurred: this
information is mostly only useful to a programmer who has access to the
source code. If they have access to the source and follow the assert
message to where it occurred, they would immediately see a code comment.
That was what I was suggesting.

What I do disagree with, a little, is that assert() is the right way to
generate run-time error messages. Assert is specific, in that it crashes
your code with a core dump and prints out the file and line number: it
"seems" to me to be for that kind of serious problem. If, as a
programmer, you have more information than that and could generate a
better, user friendly, message then I think you should do so via
fprintf() or similar, and follow up with an exit() or abort() only if
appropriate.
> If you are worried about runtime error messages, you can compile with
> -DNDEBUG to get rid of them (and any warnings that might have been
> enabled).
>
I'm not worried, since I'm not the original poster ;-) I was just trying
to explain your initial answer a bit, as I thought the situation was
more complicated (and therefore interesting).

Again, sorry if I've offended you! I think libev is a great project :-)

Sam.

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