[Gambas-user] Working with .so library

[Cedron Dawg]

What you need is a pointer to a pointer.

libfptr_handle fptr;
libfptr_create (&fptr);

In C, the first line is a varibale declaration allocating space for a pointer called "fptr".  The second line is the function call where you are passing the address of the pointer (a pointer to the pointer) so the routine can stuff the pointer with a value.  So in Gambas, your call should be something like:

dim a, b as pointer

b = varptr(a)

libfptr_create(b)

Then "a" is the variable you can pass to the other routines.  I haven't tried it, but it should work.

On the other hand, I have found it cleaner when interfacing to external C libraries to write my own shared library as an inbetween.  That way you can hide this kind of detail down in the C code.

You can find an example here for the FFTW library:

https://forum.gambas.one/viewtopic.php?f=4&t=689

Ced


----- Original Message -----
From: "Admin" <admin at allunix.ru>
To: "user" <user at lists.gambas-basic.org>
Sent: Monday, June 24, 2019 9:13:22 AM
Subject: Re: [Gambas-user] Working with .so library

Greetings, people of Gambas!

So, this thread is couple of years old now, and yet I want to thank 
everyone again, who helped me with that issue. Everything worked out 
just fine, I was able to create a working cash register software which 
lived through at least thirty versions, was cloned on github by a few 
people to create similar programs for their businesses, and the emails I 
got from them are very satisfying. It works, and many good people saved 
a lot of money dodging the bullet of mandatory Windows installations to 
support those devices that we use nationwide.

So now there's another challenge. Recently linux driver for those 
devices was updated to a new major version, and it is basically a whole 
new .so library. In previous versions, as I've mentioned here, we needed 
to create an interface with the driver from our program using 
CreateFptrInterface function. This function returned a pointer to a 
driver, and thanks to you kind people I was able to understand that we 
needed such a structure in gambas to work with it:

Extern CreateFptrInterface(ver as integer) as Pointer

Public drv as Pointer

Public Sub Form_Load()
     drv = CreateFptrInterface(12)
End sub

and now we basically have drv as a pointer to a Driver, wich can then be 
passed to any library function, so, for example, Beep(drv) will make a 
device to create a sound (that is if we declare Extern Beep(p as 
pointer) ofcourse).

That structure worked until new Driver was created very differently. The 
new .so library is also documented, but, as always, only for those who 
write their applications in C++, and now it says that we need to 
initialize the Driver like this:

libfptr_handle fptr;
libfptr_create (&fptr);

Well, I tried to apply the same knowledge that I've gained with the 
previous version and it did not help.
libfptr_handle does not seem to be a library function at all, yet it is 
described in a header file (which is also available here: 
http://gazizova.net/pub/install/_Devices/atol-30f/10.2.0/ios/fptr10.framework/Headers/libfptr10.h) 

libfptr_create IS a library function, but I could not figure out how to 
corretly call it.

At first I thought that logic is the same: we call a function that 
creates an interface and it gives us back a pointer to a Driver, so in 
Gambas it should look something like:

Extern libfptr_create() as Pointer

Public drv as Pointer

Public Sub Form_Load()
    drv = libfptr_create()
End sub

but ofcourse that is kind of strange, because function libfptr_create as 
it seems to be still wants some argument. And it sure looks like this 
argument is the pointer. And yes, if I call a function like this, I get 
drv = 0. So, then, should I call this function like this:

Public Sub Form_Load()
    libfptr_create(drv)
End sub

putting (p as Pointer) in Extern description ofcourse.

But no, that does not work either. In that case Gambas just segfaults, 
wich, as I now know, just indicates that parameters passed to an 
external function are of wrong type or just there should be more or less 
of them. Looking inside the log file of a Driver I can also see another 
interesting fact: If I, just out of curiosity, call a function 
libfptr_create and pass some random number to it declaring it as integer 
(just like it was in a previous version of a driver where it required to 
have a driver version as integer passed with CreateFptrInterface) - it 
definitly initializes to a, say, much further point then if I pass a 
pointer to it or nothing at all, and fptr becomes equal FFFFFFF. If I 
pass not an Integer, but a random string, the driver initializes to that 
same point, but ftpr stays equal 0. I mean a structure like fptr = 
libfptr_create(12345) or fptr = libfptr_create("test")
I've tried a lot of different combos but still had not figured it out, 
how do I initialize in Gambas what initializes in C++ like this:

libfptr_handle fptr;
libfptr_create(&fptr);

So, as always, any advice will be much appriciated!

Best regards,
Dmitry.