From: Andrei
Subject: Strange behavior of "with-alien" in CMUCL
Date: Sat, 18 Sep 2004 00:03:42 +0000
Message-ID: <2e04fd43.0409171524.2858cbc7@posting.google.com>

Hi,
I came across a strange problem that I can't explain.
if I execute a code like:
 (with-alien ((RetVal (* integer)))
   (unix-ioctl fd VIDIOC_G_INPUT RetVal)
   (format t "Output1 is ~a~%" (deref RetVal)))
 (with-alien ((RetVal1 (* integer)))
   (unix-ioctl fd VIDIOC_G_INPUT RetVal1)
   (format t "Output1 is ~a~%" (deref RetVal1)))))

Lisp doesn't allocate RetVal1 and the second call fails as the input
pointer is NULL. On the other hand if I do this:


 (with-alien ((RetVal (* integer)) (RetVal2 (* integer)))
   (unix-ioctl fd VIDIOC_G_INPUT RetVal)
   (unix-ioctl fd VIDIOC_G_INPUT RetVal2)
   (format t "Output1 is ~a~%" (deref RetVal)))
 (with-alien ((RetVal1 (* integer)))
   (format t "RetVal1 is ~a~%" RetVal1)
   (unix-ioctl fd VIDIOC_G_INPUT RetVal1)
   (format t "Output1 is ~a~%" (deref RetVal1)))))

There is no problem. Everything works.
Could anyone shed some light on it, please?

Thanks,
Andrew

• Re: Strange behavior of "with-alien" in CMUCL Alexey Dejneka
  • Re: Strange behavior of "with-alien" in CMUCL Andrei Stebkov

From: Alexey Dejneka
Subject: Re: Strange behavior of "with-alien" in CMUCL
Date: Sat, 18 Sep 2004 04:13:54 +0000
Message-ID: <m3mzzonri5.fsf@comail.ru>

Hello,

·········@yahoo.com (Andrei) writes:

> I came across a strange problem that I can't explain.
> if I execute a code like:
>  (with-alien ((RetVal (* integer)))
>    (unix-ioctl fd VIDIOC_G_INPUT RetVal)
>    (format t "Output1 is ~a~%" (deref RetVal)))
>  (with-alien ((RetVal1 (* integer)))
>    (unix-ioctl fd VIDIOC_G_INPUT RetVal1)
>    (format t "Output1 is ~a~%" (deref RetVal1)))))
> 
> Lisp doesn't allocate RetVal1 and the second call fails as the input
> pointer is NULL.

This is strange. You make a non-initialized pointer, pass it to `ioctl',
and take the value it points to. What is the C equivalent of your
code? Did you mean

 (with-alien ((RetVal integer))
   (unix-ioctl fd VIDIOC_G_INPUT (addr RetVal))
   (format t "Output1 is ~a~%" RetVal))

-- 
Regards,
Alexey Dejneka

"Alas, the spheres of truth are less transparent than those of
illusion." -- L.E.J. Brouwer

From: Andrei Stebkov
Subject: Re: Strange behavior of "with-alien" in CMUCL
Date: Sat, 18 Sep 2004 12:06:05 +0000
Message-ID: <414c24b8@news.nnrp.ca>

Alexey Dejneka wrote:

> Hello,
> 
> ·········@yahoo.com (Andrei) writes:
> 
>> I came across a strange problem that I can't explain.
>> if I execute a code like:
>>  (with-alien ((RetVal (* integer)))
>>    (unix-ioctl fd VIDIOC_G_INPUT RetVal)
>>    (format t "Output1 is ~a~%" (deref RetVal)))
>>  (with-alien ((RetVal1 (* integer)))
>>    (unix-ioctl fd VIDIOC_G_INPUT RetVal1)
>>    (format t "Output1 is ~a~%" (deref RetVal1)))))
>> 
>> Lisp doesn't allocate RetVal1 and the second call fails as the input
>> pointer is NULL.
> 
> This is strange. You make a non-initialized pointer, pass it to `ioctl',
> and take the value it points to. What is the C equivalent of your
> code? Did you mean
> 
>  (with-alien ((RetVal integer))
>    (unix-ioctl fd VIDIOC_G_INPUT (addr RetVal))
>    (format t "Output1 is ~a~%" RetVal))
> 
Hi, Alexey

I thought that (with-alien ((RetVal (* integer))) construct was supposed to
allocate memory for the pointer (so you can alien-free it after).
I'll try your code that looks much more up to the task than mine.

Thanks,
Andrew