Importing a Unix function call

Importing a Unix function call

[Steve Crowe]

Someone has bound to have done this before... hopefully.

Using Ada to import a function is easy, but if the function parameter
returns
a value, how is the Ada error overcome.  As I want the return value as
well
as the out parameter.  Is there away round this using a pragma?

GNAT AdA compiler running under Unix, the function being imported is a
Unix 'pipe'.

Anyone have any ideas on this?


--
MZ�

Re: Importing a Unix function call

[Larry Kilgallen]

In article <36683FA3.2512DECF@ndirect.co.uk>, Steve Crowe <stevecrowe@ndirect.co.uk> writes:

> Using Ada to import a function is easy, but if the function parameter
> returns
> a value, how is the Ada error overcome.  As I want the return value as
> well
> as the out parameter.  Is there away round this using a pragma?

Use IMPORT_VALUED_PROCEDURE in Gnat or DEC Ada.

Your purpose is exactly why it exists.

Larry Kilgallen

Re: Importing a Unix function call

[David Botton]

Do you have a piece of sample code to illustrate this?

David Botton

Larry Kilgallen wrote in message <1998Dec4.154225.1@eisner>...
>In article <36683FA3.2512DECF@ndirect.co.uk>, Steve Crowe
<stevecrowe@ndirect.co.uk> writes:
>
>> Using Ada to import a function is easy, but if the function parameter
>> returns
>> a value, how is the Ada error overcome.  As I want the return value as
>> well
>> as the out parameter.  Is there away round this using a pragma?
>
>Use IMPORT_VALUED_PROCEDURE in Gnat or DEC Ada.
>
>Your purpose is exactly why it exists.
>
>Larry Kilgallen

Re: Importing a Unix function call

[Larry Kilgallen]

In article <749jfj$1i8q$1@news.gate.net>, "David Botton" <dbotton@hotmail.com> writes:
> Do you have a piece of sample code to illustrate this?

I don't have anything that can be made public, but the DEC Ada manuals
are pretty clear.

Larry Kilgallen

Re: Importing a Unix function call

[dennison]

In article <36683FA3.2512DECF@ndirect.co.uk>,
  Steve Crowe <stevecrowe@ndirect.co.uk> wrote:
> Someone has bound to have done this before... hopefully.
>
> Using Ada to import a function is easy, but if the function parameter
> returns
> a value, how is the Ada error overcome.  As I want the return value as
> well
> as the out parameter.  Is there away round this using a pragma?
>
> GNAT AdA compiler running under Unix, the function being imported is a
> Unix 'pipe'.

If you are using gnat, you can use pragma Import_Valued_Procedure.

The portable way around this problem is to pass the parameters the same way C
does: all "in"s, with some being pointer objects. C doesn't have "out"
parameters either, you know.

--
T.E.D.

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Re: Importing a Unix function call

[Matthew Heaney]

Steve Crowe <stevecrowe@ndirect.co.uk> writes:

> GNAT AdA compiler running under Unix, the function being imported is a
> Unix 'pipe'.

From my man page:

       int pipe(int filedes[2]);

       pipe  creates  a  pair  of file descriptors, pointing to a
       pipe inode, and places them in the  array  pointed  to  by
       filedes.   filedes[0]  is  for  reading, filedes[1] is for
       writing.


C doesn't have out parameters: only in parameters that point to the
return data.

How about something like:

with Interfaces.C;  use Interfaces;
package UNIX is

  type File_Descriptor is new C.int;

  type File_Descriptor_Array is
     array (Positive range 1 .. 2) of File_Descriptor;

  pragma Convention (C, File_Descriptor_Array);

  type File_Descriptor_Array_Access is
     access all File_Descriptor_Array;

  pragma Convention (C, File_Descriptor_Array_Access);

  function Pipe
    (Files : in File_Descriptor_Array_Access)
     return C.Int;

  pragma Import (C, Pipe);

end UNIX;


You'd use it like:

declare
  Files  : alised File_Descriptor_Array;
  Status : constant C.Int := Pipe (Files'Access);
begin
...


Is that what you had in mind?

  

Re: Importing a Unix function call

[David Botton]

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Since in Ada non-scalar types are passed by reference (ie as pointers) to C
functions you could get rid of the Access type and treat it just like C:

For our case see RM95 B.3 (70) :

� An Ada parameter of an array type with component type T, of any mode, is
passed as a t* argument to a C function, where t is the C type corresponding
to the Ada type T.

function Pipe
    (Files : in File_Descriptor_Array)
return C.Int;
pragma Import (C, Pipe);


declare
  Files  : File_Descriptor_Array;
  Status : constant C.Int := Pipe (Files);
begin


It drives me crazy that many bindings are not written with these rules in
mind making them so much easier to use.


David Botton


Matthew Heaney wrote in message ...
>Steve Crowe <stevecrowe@ndirect.co.uk> writes:
>
>From my man page:
>
>       int pipe(int filedes[2]);
>
>C doesn't have out parameters: only in parameters that point to the
>return data.
>
>How about something like:
>
>with Interfaces.C;  use Interfaces;
>package UNIX is
>
>  type File_Descriptor is new C.int;
>
>  type File_Descriptor_Array is
>     array (Positive range 1 .. 2) of File_Descriptor;
>
>  pragma Convention (C, File_Descriptor_Array);
>

Re: Importing a Unix function call

[Matthew Heaney]

"David Botton" <dbotton@hotmail.com> writes:

> Since in Ada non-scalar types are passed by reference (ie as pointers) to C
> functions you could get rid of the Access type and treat it just like C:

The file descriptor array is an out parameter.  The solution you propose
makes the array an in parameter.

I think this is another example of the fact that value-returning
subprograms (which we spell F-U-N-C-T-I-O-N in Ada) should be allowed to
have in out parameters.  

Clearly, what we want is the function

  function Pipe (Files : out File_Descriptor_Array) return Int;

but we can't express this thought using the language.

Re: Importing a Unix function call

[dennison]

In article <m3ogph5b2p.fsf@mheaney.ni.net>,
  Matthew Heaney <matthew_heaney@acm.org> wrote:
> "David Botton" <dbotton@hotmail.com> writes:
>
> > Since in Ada non-scalar types are passed by reference (ie as pointers) to C
> > functions you could get rid of the Access type and treat it just like C:
>
> Clearly, what we want is the function
>
>   function Pipe (Files : out File_Descriptor_Array) return Int;
>
> but we can't express this thought using the language.

Yes. But neither can C, so for our purposes here its a good enough match.

--
T.E.D.

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