Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> writes:
> 02.04.2020 11:55, Markus Armbruster wrote:
>> Peter Maydell <peter.mayd...@linaro.org> writes:
>>
>>> On Thu, 2 Apr 2020 at 07:11, Vladimir Sementsov-Ogievskiy
>>> <vsement...@virtuozzo.com> wrote:
>>>> Somehow, in general, especially with long function names and long
>>>> parameter lists I prefer
>>>>
>>>> ret = func(..);
>>>> if (ret < 0) {
>>>> return ret;
>>>> }
>>>
>>> Personally I prefer the other approach -- this one has an extra line
>>> in the source and
>>> needs an extra local variable.
>>
>> Me too, except when func(...) is so long that
>>
>> if (func(...) < 0) {
>>
>> becomes illegible like
>>
>> if (func(... yadda, yadda,
>> yadda, ...) < 0) {
>>
>> Then an extra variable can improve things.
>>
>>>> Are you sure that adding a lot of boolean functions is a good idea? I
>>>> somehow feel better with more usual int functions with -errno on failure.
>>>>
>>>> Bool is a good return value for functions which are boolean by nature:
>>>> checks, is something correspond to some criteria. But for reporting an
>>>> error I'd prefer -errno.
>>>
>>> When would we want to return an errno? I thought the whole point of the
>>> Error* was that that was where information about the error was returned.
>>> If all your callsites are just going to do "if (ret < 0) { ... } then having
>>> the functions pick an errno value to return is just extra work.
>>
>> 0/-1 vs. true/false is a matter of convention. Lacking convention, it's
>> a matter of taste. >
>> 0/-1 vs. 0/-errno depends on the function and its callers. When -errno
>> enables callers to distinguish failures in a sane and simple way, use
>> it. When -errno feels "natural", I'd say feel free to use it even when
>> all existing callers only check < 0.
>>
>> When you return non-null/null or true/false on success/error, neglecting
>> to document that in a function contract can perhaps be tolerated; you're
>> using the return type the common, obvious way. But when you return 0/-1
>> or 0/-errno, please spell it out. I've seen too many "Operation not
>> permitted" that were actually -1 mistaken for -EPERM. Also too many
>> functions that return -1 for some failures and -errno for others.
>>
>
> I just want to add one note:
>
> OK, you like the pattern
>
> if (func()) {
> <handle error>
> }
>
> , I can live with it.
>
> I believe, we have a lot of such patterns already in code.
>
> Now, we are going to add a lot of functions, returning true on success and
> false on failure, so add a lot of patterns
>
> if (!func()) {
> <handle error>
> }
We already have this pattern all over the place with functions returning
non-null pointers on success, null pointer on failure.
> ---
>
> After it, looking at something like
>
> if (!func()) {} / if (func()) {}
>
> I'll have to always jump to function definition, to check is it int or bool
> function, to understand what exactly is meant and is there a mistake in the
> code..
> So, I'm afraid that such conversion will not help reviewing/understanding the
> code. I'd prefer to avoid using two opposite conventions in on project.
C sucks :)
Conventions help mitigate. Here's one: when fun() returns
non-negative/negative on success/error, always use
fun(...) < 0
or
fun(...) >= 0
to check. I dislike the latter.
When returns 0/negative, always use
fun(...) < 0
Avoid
fun(...) >= 0
because that suggests it could return a positive value, which is wrong.
Avoid
fun(...)
because that requires the reader to know the return type.
When its returns non-null/null or true/false on success/failure, always
use
!fun(...)
Avoid
fun(...)
because that requires the reader to know the return type.
Note that we have a usable check for failure in all cases. Goes well
with the habit to handle errors like this whenever practical
if (failed) {
handle failure
bail out
}
handle success
> I can also imagine combining different function types (int/bool) in if
> conditions o_O, what will save us from it?
Can you give an example?
> And don't forget about bool functions, which just check something, and false
> is not an error, but just negative answer on some question.
For what it's worth, GLib specifically advises against such function
contracts:
By convention, if you return a boolean value indicating success then
TRUE means success and FALSE means failure. Avoid creating
functions which have a boolean return value and a GError parameter,
but where the boolean does something other than signal whether the
GError is set. Among other problems, it requires C callers to
allocate a temporary error. Instead, provide a "gboolean *" out
parameter. There are functions in GLib itself such as
g_key_file_has_key() that are deprecated because of this.
