[Gambas-user] Gambas3 Pointers example - signal #6

[John Spikowski]

On Tue, 2011-01-18 at 10:08 +0200, Demosthenes Koptsis wrote:
> > 
> > Isn't pFunc a Gambas variable (memory location) which is maintained by
> > the Basic? Why would you free it externally? I would think Gambas would
> > complain about that.
> 
> Yes pFunc is a pointer. A pointer is a variable that holds a memory
> address.
> 
> At line 
> Dim pFunc As Pointer
> i declare the variable.
> 
> But i do not use Alloc() to alocate a memory address with Gambas3.
> 
> i do this because the external function get_current_dir_name() allocates
> a memory address internal.
> 
> This is described in manual
> http://manpages.ubuntu.com/manpages/dapper/en/man3/get_current_dir_name.3.html
> 
> "get_current_dir_name, which is only prototyped if _GNU_SOURCE is
> defined, will malloc(3) an array"
> 
> to free a pointer in such a case i have to use free() function of C.
> 
> Next is the same example but with an error.
> The error is that i allocate memory with Gambas3 and try to Free() the
> memory later. But when i call get_current_dir_name() this function set
> again a new pointer and the address from Gambas3 Alloc() is not
> accesible any more. So when i try to Free() the pointer in fact i try to
> free an unlocated memory!
> 
> ' Gambas module file
> 'char *get_current_dir_name(void);
> Extern get_current_dir_name() As Pointer In "libc:6"
> 
> Public Sub Main()
> 
>   Dim pFunc As Pointer
>   Dim sWorkingDirectory As String
> 
> 'get_current_dir_name
>   pFunc = Alloc(4096)
>   Print pFunc '(Pointer 0x185b918)
>   
>   pFunc = get_current_dir_name()
>   Print pFunc '(Pointer 0x185d780)
>   
>   'notice that address was changed
>   ' from now on the address 0x185b918
>   'is unreachable
>   'and the new address 0x185d780
>   'is not allocated by Gambas3 !
>   'so we cannot free both of them
>   'with Free() function of Gambas3!
>    
>   sWorkingDirectory = Str@(pFunc)
>   Print sWorkingDirectory
> 
>   Free(pFunc)
>   pFunc = 0
> 
> End

I was thinking that you were freeing pFunc (using it's VARPTR) rather
than freeing the memory associated to the pointer being returned. I have
to agree with Benoît that working with external memory (variables) and
structures are a 'at your own risk' task in Gambas. (rules vary by what
your trying to access)