[Gambas-user] Working with .so library

[Admin]

15.06.2017 16:19, Tobias Boege пишет:
>> All your help was very important for me, I now have completed my cash
>> register software to the point where it does everything my company needs. I
>> must say Gambas is a great language, it's very easy to learn from scratch,
>> I'm surprised how obvious everything is. But there is a lot of work for me
>> left to do mostly in terms of managing wrong human actions. My software
>> works good if the employee doesn't do any mistakes, but that's unrealistic,
>> so there's a lot of things I want to control and check. And that's where I'm
>> stuck.
>>
>> This library (which still calls itself a driver) theoretically is able to
>> return a lot of values that I need, but I can't understand basic rules of
>> how do we take output from a C-lib in Gambas.
>>
>>  From http://gambaswiki.org/wiki/howto/extern I understood that I need to
>> locate a space in memory and pass a pointer to a library so that it can
>> write data into that place in ram, which I would then read and set free.
>>
>> So I have to declare a pointer, then Alloc(8) it, then pass it to my library
>> and then read from it like it is a stream. Does this principle still work in
>> current version of Gambas?
>>
> If you do Alloc(8), then you get 8 bytes of memory. You most likely *don't*
> want to read that like a stream, but use Integer@() or similar functions.
>
>> What I don't understand is how I construct the code in my particular case.
>>
>> To make an interface to the library I declare external pointer like this:
>>
>>      Extern CreateFptrInterface(ver As Integer) As Pointer
>>
>> Then I declare some pointers that I'll use with help of the interface I
>> created:
>>
>>      Extern put_DeviceEnable(p as Pointer, mode as Integer)
>>
>>      Extern GetStatus(p as Pointer, StatRequest as String)
>>
>> Then I declare the pointer which will be that interface:
>>
>>      Public kkmDrv as Pointer
>>
>> So then in sub I can do
>>
>> kkmDrv = CreateFptrInterface(12) ' this establishes the interface
>>
>> put_DeviceEnabled(kkmDrv, 1) ' this transfers the comand to the library
>> through the interface.
>>
>> And it works great.
>>
>> But then If I want to get some data from the library, as I understand, I
>> have to declare another pointer, allocate ram for it and pass my request.
>>
>> I don't understand how should I pass that pointer to GetStatus() while also
>> passing my interface pointer to it, let alone reading data back. Totally
>> confused.
>>
> This entirely depends on how the C functions in your library are declared.
> I don't know about your specific library but commonly the occurence of an
> error is indicated by an integer return code, e.g. this might be the
> signature of one of the functions in your library:
>
>    int myfunction(void *interface, int argument)
>
> If the documentation says that the return value (int) of this function
> indicates an error, then you just need to get that return value back into
> your Gambas program, which you accomplish by declaring the function in
> Gambas as
>
>    Extern myfunction(interface As Pointer, argument As Integer) As Integer
>
> (notice the trailing "As Integer"). Then you can use "myfunction" in your
> Gambas code like any other function and get and interpret its return value.
>
> So, if this convention for error reporting is used, it is much simpler to
> get information about errors, without using Alloc() and co. Your library
> may use a different convention which actually involves pointers, but
> I wouldn't know.
>
> Regards,
> Tobi
>
I should've said it in the beginning. Ofcourse any function returns 
integer value of 0 as success or -1 as error, but that only indicates 
that function was successfully executed or not. So GetStatus() will 
always return 0 because it shurely ran, nothing can go wrong here. But 
that's not the result I want. GetStatus() actually gives back a string 
with the status I asked for. Not that I fully understand how it does 
that. I already gave links to the libfptr.so library itself 
(http://allunix.ru/back/atol.tar.gz) and it's header files 
(http://allunix.ru/back/atol-header.tar.gz) so that it's clearer, what 
I'm talking about, unfortunately I am absolute zero in C to figure 
things out myself.

For example I can see that to get serial number of the device driven by 
that library i can use a function described like this:

get_SerialNumber(void *ptr, wchar_t *bfr, int bfrSize);

As far as I can tell what it does is it gets data needed and puts it 
into some buffer. The result of executing this function through 
put_SerialNumber(kkmDrv) will always be returned to me as 0.

So to see what's in that buffer, I have to then invoke GetStatus(kkmDrv) 
describe in .h file like GetStatus(void *ptr); and the integer result of 
this operation will also always be 0, which means that GetStatus itself 
ran successfully, but I don't care about that, I want to see what it 
actually told me, not that if it told me it successfully or not. So 
that's the main confusion. If all this is too complicated and lamely 
explained then nevermind, I expect it to be so and I'm sorry, that's the 
best I can do. I'm just really confused that I recieve two answers, one 
boolean telling if function successfully invoked and one string, 
carrying the actual data I want.


Best Regards,

Dmitry.