[PATCH] new macro: talloc_ctxdup

Stephen Gallagher sgallagh at redhat.com
Tue May 1 05:40:33 MDT 2012

On Tue, 2012-05-01 at 21:26 +1000, Andrew Bartlett wrote:
> On Tue, 2012-05-01 at 07:12 -0400, Stephen Gallagher wrote:
> > This patch came out of a discussion that Pavel and I were having about
> > copying memory out of a talloc pool into a new context. This seemed an
> > expedient way to do it. Obviously as Andrew points out, you need to also
> > be aware of whether the children of this context are also allocated from
> > the pool.
> > 
> > Perhaps it would be better to design a talloc_steal_ext() that could be
> > configured to recursively move child memory out of a pool, where
> > appropriate.
> I'm a little confused, as talloc_steal() already moves the context and
> any child memory to the new parent.  What would your talloc_steal_ext()
> do?

Actually, that's a false assumption. In the following construct:

TALLOC_CTX *tmp_ctx = talloc_pool(NULL, 65536);
struct user *user = talloc(tmp_ctx, struct user);
user->username = talloc_strdup(user, "user1");

Ok, so now we have 64k of memory allocated to a pool. We've taken some
portion of it for use with user and its username.

new_user = talloc_steal(mem_ctx, user);
This reassigns the parent to the mem_ctx, but the memory is still
allocated in the pool.

talloc_free(tmp_ctx); /* This doesn't do what you think it does */

The memory has its PARENTAGE as mem_ctx now, but the pool itself will
not actually be freed until such time as the new_user is freed (and by
extension new_user->username). So in the meantime, we're holding on to
the entire 64k pool memory.

The idea behind talloc_steal_ext() would be that under the hood it would
iterate through and determine if any part of the memory was in a
talloc_pool. If so, it would perform a talloc_ctxdup() to reallocate it
on the new context hierarchy (which may or may not be a NEW
talloc_pool). The end result would be that the original talloc_pool
could then be freed immediately, instead of hanging on to potentially
large amounts of memory that would look like a memory leak without being
detectable as one by valgrind.
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