Should I free the pointer returned by native method manually?

[Red IO]

I know that the original question is answered, but how would you free
memory in case the libraries documentation says to use free. Would you just
create a method handle to libc free and call it with the pointer or is
there an api to free native memory?
Great regards
RedIODev

On Mon, Jan 22, 2024, 19:36 Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> Hi tison,
> in this case, the function is returning a pointer. What happens to that
> pointer is 100% library-dependent. In this case, doing `man getpwnam`
> gives us some clues:
>
> > The return value may point to a static area, and may be overwritten  by
> >        subsequent  calls  to  getpwent(3), getpwnam(), or getpwuid().
> > (Do not
> >        pass the returned pointer to free(3).)
>
> Basically, the returned pointer points to an area that is managed by the
> library, so you don't have to free.
>
> This means the code you have seems correct to me.
>
> Other libraries will behave differently, for instance they might have a
> pair of functions such as "createFoo" and "destroyFoo", the former
> allocating region of memory, and the latter destroying such region. In
> such cases it might be helpful to associate the return memory segment to
> an existing arena *and* also attach a cleanup function (via
> MemorySegment::reinterpret), so that, when the arena is closed,
> "destroyFoo" is called on the pointer.
>
> Hope this helps
>
> Maurizio
>
>
> On 22/01/2024 18:28, tison wrote:
> > Here is the code snippet:
> >
> >      public static void main(String[] args) throws Throwable {
> >          final Linker linker = Linker.nativeLinker();
> >          final SymbolLookup libc = linker.defaultLookup();
> >          final MethodHandle handle =
> > linker.downcallHandle(libc.find("getpwnam").orElseThrow(),
> > FunctionDescriptor.of(ValueLayout.ADDRESS, ValueLayout.ADDRESS));
> >          try (Arena arena = Arena.ofConfined()) {
> >              final MemorySegment passwd = (MemorySegment)
> > handle.invoke(arena.allocateUtf8String("tison"));
> >              System.out.println("passwd=" + passwd);
> >              System.out.println("pw_name=" +
> > passwd.reinterpret(Long.MAX_VALUE).get(ValueLayout.ADDRESS,
> > 48).reinterpret(Long.MAX_VALUE).getUtf8String(0));
> >          }
> >      }
> >
> > I wonder if I should explicitly free passwd that is returned from a
> > native method. If so, how? If not, how the Arena tracks it and free on
> > closed?
> >
> > Best,
> > tison.
>
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