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

[Demosthenes Koptsis]

On Mon, 2011-01-17 at 11:08 +0100, Benoît Minisini wrote:
> > Here is a last case about pointers i found.
> > 
> > CASE 1
> > -------
> > When a C++ function does not malloc any memory it may return a pointer.
> > In this case we can do
> > 
> > DIM pPointer as Pointer
> > pPointer = Alloc(4096)' alloc 4096 bytes
> > 
> > pPointer = getwd(pBuf)
> > 
> > 'use pPointer with Str@() or other xxx@()
> > 
> > Free(pPointer)
> > pPointer=0
> > 
> > OK!
> > 
> > CASE 2
> > -------
> > When a c++ function itself allocate memory with malloc() what can we do?
> > 
> > Example.
> > --------
> > ' 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 '0xc12938
> > 
> >   pFunc = get_current_dir_name()
> >   Print pFunc '0xc147a0
> > 
> >   'now we cannot Free(pFunc) it is not 0xc12938 anymore
> >   'get_current_dir_name() malloc itself memory and return a new pointer
> > 
> >   sWorkingDirectory = Str@(pFunc)
> >   Print sWorkingDirectory
> > 
> > ' Free(pFunc)
> > 'if i unrem Free i get a crash, because i try to free the 0xc147a0
> >   pFunc = 0
> > 
> > End
> > 
> > What can we do in such cases when a function itself allocates memory?
> > Should we free it? Can we?
> > 
> > May be we can use pointers without Alloc()?
> > 
> > Example, no Alloc(), no Free, just DIM pPointer
> > ---------
> > ' 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 = get_current_dir_name()
> >   Print pFunc '0xc147a0
> > 
> >   sWorkingDirectory = Str@(pFunc)
> >   Print sWorkingDirectory
> > 
> >   pFunc = 0
> > 
> > End
> > 
> 
> What must be done with with a pointer returned by a C / C++ function depends 
> on the function semantic.
> 
> You must read and *understand* the function documentation to know.

ok...

> 
> Some functions return a pointer that you must free yourself with Free().
> 
> Some functions return a pointer that you must not free.
> 
> Some functions return a pointer that you must free by calling a dedicated 
> function, and not Free().

so in case of get_current_dir_name(),

i should use the free function of c++ ?

that is i understand form manual.
"get_current_dir_name() will malloc(3) an array big enough to hold the
absolute pathname of the current working directory. If the environment
variable PWD is set, and its value is correct, then that value will be
returned. The caller should free(3) the returned buffer.'


> ... Everything is possible.
> 
> Regards,
> 

-- 
Regards,
Demosthenes Koptsis.