Re: NativeCall interface for a char ** argument

[Fernando Santagata]

On Wed, May 25, 2016 at 8:50 PM, Brandon Allbery 
wrote:

>
> On Wed, May 25, 2016 at 2:41 PM, Fernando Santagata <
> nando.santag...@gmail.com> wrote:
>
>>
>> gbooleannotify_get_server_info (char **ret_name,
>> char **ret_vendor,
>> char **ret_version,
>> char **ret_spec_version);
>>
>> I think that the four arguments return the information about the system.
>> The problem here is that I'm not an expert on NativeCall, which I'm just
>> beginning to love, and I don't know how to map those char **.
>>
>> What I concocted on the Perl6 side so far is this:
>>
>> sub notify_get_server_info(Str $name is rw, Str $vendor is rw, Str
>> $version is rw, Str $spec_version is rw) returns Bool is native('notify',
>> v4) is export {};
>>
>> The calling program goes like this:
>>
>> my Str ($name, $vendor, $version, $spec_version);
>> if notify_get_server_info($name, $vendor, $version, $spec_version) {
>>   say $name, $vendor, $version, $spec_version;
>> }else{
>>   say 'No server info';
>> }
>>
>> and it prints
>>
>> (Str)(Str)(Str)(Str)
>>
>> So, I get that the mapping is wrong.
>>
>
> In this case, you need to pass a Pointer[Str] which will be filled in by
> the C function. Just using a Str directly passes a (char *), not a (char
> **), and you ended up with a pointer where rakudo expected characters.
>

I rewrote the interface like this:

sub notify_get_server_info(Pointer[Str] $name, Pointer[Str] $vendor,
Pointer[Str] $version, Pointer[Str] $spec_version) returns Bool is
native('notify', v4) is export {};

skipping the "rw" role. I can't declare the variables as Str, because I
would get a mismatch error, but I got this result:

(Any)(Any)(Any)(Any)

Declaring those variables as Pointer[Str] I get:

(NativeCall::Types::Pointer[Str])(NativeCall::Types::Pointer[Str])(NativeCall::Types::Pointer[Str])(NativeCall::Types::Pointer[Str])

Should I try to define a class with the repr('CPointer') role to
encapsulate that variable type, or am I off tracks?

-- 
Fernando Santagata

Re: NativeCall interface for a char ** argument

[Brandon Allbery]

On Wed, May 25, 2016 at 2:41 PM, Fernando Santagata <
nando.santag...@gmail.com> wrote:

>
> gbooleannotify_get_server_info (char **ret_name,
> char **ret_vendor,
> char **ret_version,
> char **ret_spec_version);
>
> I think that the four arguments return the information about the system.
> The problem here is that I'm not an expert on NativeCall, which I'm just
> beginning to love, and I don't know how to map those char **.
>
> What I concocted on the Perl6 side so far is this:
>
> sub notify_get_server_info(Str $name is rw, Str $vendor is rw, Str
> $version is rw, Str $spec_version is rw) returns Bool is native('notify',
> v4) is export {};
>
> The calling program goes like this:
>
> my Str ($name, $vendor, $version, $spec_version);
> if notify_get_server_info($name, $vendor, $version, $spec_version) {
>   say $name, $vendor, $version, $spec_version;
> }else{
>   say 'No server info';
> }
>
> and it prints
>
> (Str)(Str)(Str)(Str)
>
> So, I get that the mapping is wrong.
>

In this case, you need to pass a Pointer[Str] which will be filled in by
the C function. Just using a Str directly passes a (char *), not a (char
**), and you ended up with a pointer where rakudo expected characters.

-- 
brandon s allbery kf8nh   sine nomine associates
allber...@gmail.com  ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net

Re: NativeCall interface for a char ** argument

[Fernando Santagata]

On Wed, May 25, 2016 at 6:20 PM, Brandon Allbery 
wrote:

> On Wed, May 25, 2016 at 11:50 AM, Fernando Santagata <
> nando.santag...@gmail.com> wrote:
>
>> When I write a C program I'm able to call that function and I receive the
>> strings, so I guess my problem is just a mapping one.
>
>
> It can also mean a preallocated array of strings, though; C is sloppy
> about the difference between pointer-to and array-of. In NativeCall, the
> former is Pointer[Str] and the latter is CArray[Str]. From your
> description, you want the former.
>

In this case I'm working on libnotify. The interface to all the core
functionality works fine and I can show notifications on screen.

Now I'm working on this function:

gbooleannotify_get_server_info (char **ret_name,
char **ret_vendor,
char **ret_version,
char **ret_spec_version);

I think that the four arguments return the information about the system.
The problem here is that I'm not an expert on NativeCall, which I'm just
beginning to love, and I don't know how to map those char **.

What I concocted on the Perl6 side so far is this:

sub notify_get_server_info(Str $name is rw, Str $vendor is rw, Str $version
is rw, Str $spec_version is rw) returns Bool is native('notify', v4) is
export {};

The calling program goes like this:

my Str ($name, $vendor, $version, $spec_version);
if notify_get_server_info($name, $vendor, $version, $spec_version) {
  say $name, $vendor, $version, $spec_version;
}else{
  say 'No server info';
}

and it prints

(Str)(Str)(Str)(Str)

So, I get that the mapping is wrong.

-- 
Fernando Santagata

Re: NativeCall interface for a char ** argument

[Brandon Allbery]

On Wed, May 25, 2016 at 11:50 AM, Fernando Santagata <
nando.santag...@gmail.com> wrote:

> When I write a C program I'm able to call that function and I receive the
> strings, so I guess my problem is just a mapping one.


It can also mean a preallocated array of strings, though; C is sloppy about
the difference between pointer-to and array-of. In NativeCall, the former
is Pointer[Str] and the latter is CArray[Str]. From your description, you
want the former.

Does the C API provide a way to free the result, or is it some static
horror that will cause problems in a threaded environment?

-- 
brandon s allbery kf8nh   sine nomine associates
allber...@gmail.com  ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net

Re: NativeCall interface for a char ** argument

[Fernando Santagata]

In that case it's a pointer to a pointer: since in C a function can't
multiple values, when one wants to return two strings, one needs to use
several char ** arguments in the function call.
The caller passes a reference to a pointer and the function returns the
address of a malloc-ed memory area with a return string in it.

When I write a C program I'm able to call that function and I receive the
strings, so I guess my problem is just a mapping one.

On Wed, May 25, 2016 at 5:40 PM, Bennett Todd 
wrote:

> A C char** is an array (of unknown length) of pounters to C strings ( each
> of unknown length).
>
> I don't know NativeCall, can't tell you how it should be declared, but I
> hope that highlights the issue.
>



-- 
Fernando Santagata