Move the talloc details to the mainpage.

Signed-off-by: Andrew Tridgell <tridge@samba.org>

1 files changed, 69 insertions, 0 deletions

diff --git a/lib/talloc/doc/mainpage.dox b/lib/talloc/doc/mainpage.dox
index 9629949124..3204e8a5c2 100644
--- a/lib/talloc/doc/mainpage.dox
+++ b/lib/talloc/doc/mainpage.dox
@@ -33,4 +33,73 @@
  *
  * rsync -Pavz samba.org::ftp/unpacked/standalone_projects/lib/talloc .
  *
+ * @section talloc_preample Preamble
+ *
+ * talloc is a hierarchical, reference counted memory pool system with
+ * destructors.
+ *
+ * Perhaps the biggest difference from other memory pool systems is that there
+ * is no distinction between a "talloc context" and a "talloc pointer". Any
+ * pointer returned from talloc() is itself a valid talloc context. This means
+ * you can do this:
+ *
+ * @code
+ *      struct foo *X = talloc(mem_ctx, struct foo);
+ *      X->name = talloc_strdup(X, "foo");
+ * @endcode
+ *
+ * The pointer X->name would be a "child" of the talloc context "X" which is
+ * itself a child of mem_ctx. So if you do talloc_free(mem_ctx) then it is all
+ * destroyed, whereas if you do talloc_free(X) then just X and X->name are
+ * destroyed, and if you do talloc_free(X->name) then just the name element of
+ * X is destroyed.
+ *
+ * If you think about this, then what this effectively gives you is an n-ary
+ * tree, where you can free any part of the tree with talloc_free().
+ *
+ * If you find this confusing, then run the testsuite to watch talloc in
+ * action. You may also like to add your own tests to testsuite.c to clarify
+ * how some particular situation is handled.
+ *
+ * @section talloc_performance Performance
+ *
+ * All the additional features of talloc() over malloc() do come at a price. We
+ * have a simple performance test in Samba4 that measures talloc() versus
+ * malloc() performance, and it seems that talloc() is about 4% slower than
+ * malloc() on my x86 Debian Linux box. For Samba, the great reduction in code
+ * complexity that we get by using talloc makes this worthwhile, especially as
+ * the total overhead of talloc/malloc in Samba is already quite small.
+ *
+ * @section talloc_named Named blocks
+ *
+ * Every talloc chunk has a name that can be used as a dynamic type-checking
+ * system. If for some reason like a callback function you had to cast a
+ * "struct foo *" to a "void *" variable, later you can safely reassign the
+ * "void *" pointer to a "struct foo *" by using the talloc_get_type() or
+ * talloc_get_type_abort() macros.
+ *
+ * @code
+ *      struct foo *X = talloc_get_type_abort(ptr, struct foo);
+ * @endcode
+ *
+ * This will abort if "ptr" does not contain a pointer that has been created
+ * with talloc(mem_ctx, struct foo).
+ *
+ * @section talloc_threading Multi-threading
+ *
+ * talloc itself does not deal with threads. It is thread-safe (assuming the
+ * underlying "malloc" is), as long as each thread uses different memory
+ * contexts.
+ *
+ * If two threads uses the same context then they need to synchronize in order
+ * to be safe. In particular:
+ *
+ *   - when using talloc_enable_leak_report(), giving directly NULL as a parent
+ *     context implicitly refers to a hidden "null context" global variable, so
+ *     this should not be used in a multi-threaded environment without proper
+ *     synchronization.
+ *   - the context returned by talloc_autofree_context() is also global so
+ *     shouldn't be used by several threads simultaneously without
+ *     synchronization.
+ *
  */