Subject: bug#18592: FFI should have portable access to ‘errno’

From: Mark H Weaver <[email-masked]>
Date: Sat, 24 Jan 2015 03:08:23 -0500

[email-masked] (Ludovic Courtès) writes:

> Chaos Eternal <[email-masked]> skribis:
>
>> Proposals to solve this bug:
>>
>> Proposal 1.
>>
>> Adding a keyword argument to pointer->procedure, if set to true, the
>> generated wrapper will check 'errno' immediately after ffi_call and
>> return the errno as second value.
>>
>> the proposed pointer->procedure maybe like this:
>> pointer->procedure return_type func_ptr arg_types #:return-errno
>>
>> Proposal 2.
>>
>> let pointer->procedure check return_type, if it is a list:
>> (func_return_type, 'errno)
>> then return multiple values, as errno be second value.
>
> That’s my favorite because it’s both pragmatic and extensible (we can
> also add support for h_errno, etc.)

We can't do this, because although it's not documented in our manual, a
list passed as a foreign type already has a meaning: it means a struct.
See 'parse_ffi_type' in foreign.c.  I mentioned this fact on guile-user:

  https://lists.gnu.org/archive/html/guile-user/2012-05/msg00037.html

Of the proposals given here, I would support #1, but maybe there's a
better option.

     Thanks,
       Mark