Re: [Xen-devel] Suggestion: Improve hypercall Interface to get real return value

[Mats Petersson <mats.petersson@xxxxxxxxxx>]

On 05/12/12 11:05, George Dunlap wrote:
> On Wed, Dec 5, 2012 at 6:21 AM, Yanzhang Li <liyz@xxxxxxxxxx 
> <mailto:liyz@xxxxxxxxxx>> wrote:
>
>
>     Do you think this would be a good modification?
>     Also, I am curious why the original design didn't do that. Is it a
>     bug or is it designed that way intentionally?
>     Any suggestions and comments will be highly appreciated.
>
>
> The reason we just return -1 is because that is the standard practice 
> for Unix system libraries: to return -1 but set the error value in 
> "errno".  I couldn't tell you why Unix does this, but there's an 
> advantage to following standard interfaces, because it reduces the 
> surprise factor, and reduces the amount of information programmers 
> need to keep in their head.
I think returning -1 instead of "the error" allows for simpler code when 
you do something like this:
int func()
{
     FILE *f = fopen(...);
     if(!f) return -1;

     while(...)
     {
        if (fread(f, ...) < 0)
        {
           fclose(f);
           return -1;
         }
         ...
        if (...)
           if (fwrite(f, ...) < 0)
           {
              fclose(f);
              return -1;
           }
     }
     fclose(f);
     return 0;
}

Now, we don't need extra code to "remember the errno from the failing 
function". [And I'm assuming here that fclose isn't "interfering" with 
the errno - if you REALLY need to know for sure what the errno was at 
fread or fwrite, you still need to "remember errno".
(Note that some functions do not return -1 for failure in the above 
code, but for example NULL, and some function would not be able to 
return -errno, as that may well be a "valid" return value - so keeping 
the interface as alike as possible is a good idea)
--
Mats
>  -George

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