Returning errno as a second value

[James Bielman]

On 08/07/2013 10:44 AM, Luís Oliveira wrote:
>> It would look like this: (foreign-funcall ("strlen" :errno t) :string
>> "foo" :int), or (defcfun (strlen "strlen" :errno t) :int (s :string)).
>>
>> Calling (strlen) would return two values, the return value of the
>> foreign call, and errno.
>>
>> In some Lisps, the only way to get errno is to make an additional
>> foreign call. Then perhaps that call could be made by CFFI and returns
>> as the second value.
> That makes sense. Except for the second value bit because CFFI types can
> yield more than one value. Alternatives that pop up:
>
>    1. Return it as the first value. (Probably requires some
>       multiple-value-list mangling which might not be acceptable
>       performance-wise?)
>
>    2. Have the foreign function accept an extra errno argument that takes
>       some structure than can be modified with the errno value.
>
> Not quite happy with either. Any other ideas?
>
Perhaps instead of returning "errno" as an additional value, we could 
modify an internal global thread-local variable with the value of errno 
when the function is flagged with ":errno t", and export (cffi:errno) to 
return the thread's saved errno value?  That's how the underlying 
machinery works at the C level after all.

Of course, then CFFI-SYS needs a portable mechanism for thread-locals...

If I was binding a library that was user-hostile enough to use "errno" 
as its error reporting mechanism, I'd be tempted to wrap its function 
calls to pass the error code out with an explicit output parameter.  I 
wonder if there's any way to automate that.

James


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