Re: Free-ing memory: not springing leaks?

[Ludovic Brenta <ludovic.brenta@insalien.org>]

chris <spamoff.danx@ntlworld.com> writes:

> Matthew Heaney wrote:
> > If you know it was malloc'd, then yes, just write a binding to free and
> > call that.
> 
> And if you don't, then it's hit and miss? ... it figures.   I wonder
> how C programmers deal with this :(

This is supposedly documented by your library.  Some libraries return
"statically allocated" memory which you must not free.  This is for
example the case of readdir(3).

In the particular case of dlerror(3), I've just had a look at the
sources because the documentation is very evasive about this.  As it
turns out, dlerror() runs in one of two modes, thread-safe or not.

In thread-safe mode, it allocates a dynamic block of memory for each
thread.  This block contains a pointer to the string returned.  The
function allocates and frees this pointer, and _you should not free
the memory yourself_, because you cannot set the pointer to NULL
(remember that C is pass-by-value only, so what you receive from
dlerror() is a copy of the pointer).

If for some reason, dlerror() cannot use thread-safe mode, it falls
back to a thread-unsafe mode in which there is a single, statically
allocated, block of memory containing the pointer to the error string.
As before, dlerror() allocates and frees the memory for the string.

If you wand to forcefully free the memory, your best bet is to call
dlerror() twice, like this (I have not tried to compile this, mind
you):

Dynamic_Loader_Error : Exception;
procedure Dl_Error is
   function dlerror returns Interfaces.C.Strings.chars_ptr;
   pragma Import (C, dlerror, "dlerror");
   Error_Message : Interfaces.C.Strings.chars_ptr := dlerror;
   use type Interfaces.C.Strings.chars_ptr;
begin
   if Error_Message /= Interfaces.C.Strings.Null_Ptr then
      declare
         S : String := Interfaces.C.Strings.Value (Error_Message);
      begin
         Error_Message := dlerror; -- ask it to free the memory
         Ada.Exceptions.Raise_Exception (Dynamic_Loader_Error'Identity, S);
      end if;
   end if;
end Dl_Error;

-- 
Ludovic Brenta.