talloc_guide.txt

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Using talloc in Samba4
----------------------

Andrew Tridgell
September 2004

The most current version of this document is available at
   http://samba.org/ftp/unpacked/samba4/source/lib/talloc/talloc_guide.txt

If you are used to the "old" talloc from Samba3 before 3.0.20 then please read
this carefully, as talloc has changed a lot. With 3.0.20 (or 3.0.14?) the
Samba4 talloc has been ported back to Samba3, so this guide applies to both.

The new talloc is a hierarchical, reference counted memory pool system
with destructors. Quite a mounthful really, but not too bad once you
get used to it.

Perhaps the biggest change from Samba3 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:

  struct foo *X = talloc(mem_ctx, struct foo);
  X->name = talloc_strdup(X, "foo");

and 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 I suggest you 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.


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.


talloc API
----------

The following is a complete guide to the talloc API. Read it all at
least twice.

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.


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(type *)talloc(const void *context, type);

The talloc() macro is the core of the talloc library. It takes a
memory context and a type, and returns a pointer to a new area of
memory of the given type.

The returned pointer is itself a talloc context, so you can use it as
the context argument to more calls to talloc if you wish.

The returned pointer is a "child" of the supplied context. This means
that if you talloc_free() the context then the new child disappears as
well. Alternatively you can free just the child.

The context argument to talloc() can be NULL, in which case a new top
level context is created. 


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void *talloc_size(const void *context, size_t size);

The function talloc_size() should be used when you don't have a
convenient type to pass to talloc(). Unlike talloc(), it is not type
safe (as it returns a void *), so you are on your own for type checking.

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(typeof(ptr)) talloc_ptrtype(const void *ctx, ptr);

The talloc_ptrtype() macro should be used when you have a pointer and
want to allocate memory to point at with this pointer. When compiling
with gcc >= 3 it is typesafe. Note this is a wrapper of talloc_size()
and talloc_get_name() will return the current location in the source file.
and not the type.

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int talloc_free(void *ptr);

The talloc_free() function frees a piece of talloc memory, and all its
children. You can call talloc_free() on any pointer returned by
talloc().

The return value of talloc_free() indicates success or failure, with 0
returned for success and -1 for failure. The only possible failure
condition is if the pointer had a destructor attached to it and the
destructor returned -1. See talloc_set_destructor() for details on
destructors.

If this pointer has an additional parent when talloc_free() is called
then the memory is not actually released, but instead the most
recently established parent is destroyed. See talloc_reference() for
details on establishing additional parents.

For more control on which parent is removed, see talloc_unlink()

talloc_free() operates recursively on its children.


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int talloc_free_children(void *ptr);

The talloc_free_children() walks along the list of all children of a
talloc context and talloc_free()s only the children, not the context
itself.


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void *talloc_reference(const void *context, const void *ptr);

The talloc_reference() function makes "context" an additional parent
of "ptr".

The return value of talloc_reference() is always the original pointer
"ptr", unless talloc ran out of memory in creating the reference in
which case it will return NULL (each additional reference consumes
around 48 bytes of memory on intel x86 platforms).

If "ptr" is NULL, then the function is a no-op, and simply returns NULL.

After creating a reference you can free it in one of the following
ways:

  - you can talloc_free() any parent of the original pointer. That
    will reduce the number of parents of this pointer by 1, and will
    cause this pointer to be freed if it runs out of parents.

  - you can talloc_free() the pointer itself. That will destroy the
    most recently established parent to the pointer and leave the
    pointer as a child of its current parent.

For more control on which parent to remove, see talloc_unlink()


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int talloc_unlink(const void *context, const void *ptr);

The talloc_unlink() function removes a specific parent from ptr. The
context passed must either be a context used in talloc_reference()
with this pointer, or must be a direct parent of ptr. 

Note that if the parent has already been removed using talloc_free()
then this function will fail and will return -1.  Likewise, if "ptr"
is NULL, then the function will make no modifications and return -1.

Usually you can just use talloc_free() instead of talloc_unlink(), but
sometimes it is useful to have the additional control on which parent
is removed.


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void talloc_set_destructor(const void *ptr, int (*destructor)(void *));

The function talloc_set_destructor() sets the "destructor" for the
pointer "ptr". A destructor is a function that is called when the
memory used by a pointer is about to be released. The destructor
receives the pointer as an argument, and should return 0 for success
and -1 for failure.

The destructor can do anything it wants to, including freeing other
pieces of memory. A common use for destructors is to clean up
operating system resources (such as open file descriptors) contained
in the structure the destructor is placed on.

You can only place one destructor on a pointer. If you need more than
one destructor then you can create a zero-length child of the pointer
and place an additional destructor on that.

To remove a destructor call talloc_set_destructor() with NULL for the
destructor.

If your destructor attempts to talloc_free() the pointer that it is
the destructor for then talloc_free() will return -1 and the free will
be ignored. This would be a pointless operation anyway, as the
destructor is only called when the memory is just about to go away.


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int talloc_increase_ref_count(const void *ptr);

The talloc_increase_ref_count(ptr) function is exactly equivalent to:

  talloc_reference(NULL, ptr);

You can use either syntax, depending on which you think is clearer in
your code.

It returns 0 on success and -1 on failure.

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size_t talloc_reference_count(const void *ptr);

Return the number of references to the pointer.

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void talloc_set_name(const void *ptr, const char *fmt, ...);

Each talloc pointer has a "name". The name is used principally for
debugging purposes, although it is also possible to set and get the
name on a pointer in as a way of "marking" pointers in your code.

The main use for names on pointer is for "talloc reports". See
talloc_report() and talloc_report_full() for details. Also see
talloc_enable_leak_report() and talloc_enable_leak_report_full().

The talloc_set_name() function allocates memory as a child of the
pointer. It is logically equivalent to:
  talloc_set_name_const(ptr, talloc_asprintf(ptr, fmt, ...));

Note that multiple calls to talloc_set_name() will allocate more
memory without releasing the name. All of the memory is released when
the ptr is freed using talloc_free().


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void talloc_set_name_const(const void *ptr, const char *name);

The function talloc_set_name_const() is just like talloc_set_name(),
but it takes a string constant, and is much faster. It is extensively
used by the "auto naming" macros, such as talloc_p().

This function does not allocate any memory. It just copies the
supplied pointer into the internal representation of the talloc
ptr. This means you must not pass a name pointer to memory that will
disappear before the ptr is freed with talloc_free().


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void *talloc_named(const void *context, size_t size, const char *fmt, ...);

The talloc_named() function creates a named talloc pointer. It is
equivalent to:

   ptr = talloc_size(context, size);
   talloc_set_name(ptr, fmt, ....);


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void *talloc_named_const(const void *context, size_t size, const char *name);

This is equivalent to:

   ptr = talloc_size(context, size);
   talloc_set_name_const(ptr, name);


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const char *talloc_get_name(const void *ptr);

This returns the current name for the given talloc pointer. See
talloc_set_name() for details.


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void *talloc_init(const char *fmt, ...);

This function creates a zero length named talloc context as a top
level context. It is equivalent to:

  talloc_named(NULL, 0, fmt, ...);


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void *talloc_new(void *ctx);

This is a utility macro that creates a new memory context hanging
off an exiting context, automatically naming it "talloc_new: __location__"
where __location__ is the source line it is called from. It is
particularly useful for creating a new temporary working context.


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(type *)talloc_realloc(const void *context, void *ptr, type, count);

The talloc_realloc() macro changes the size of a talloc
pointer. The "count" argument is the number of elements of type "type"
that you want the resulting pointer to hold. 

talloc_realloc() has the following equivalences:

  talloc_realloc(context, NULL, type, 1) ==> talloc(context, type);
  talloc_realloc(context, NULL, type, N) ==> talloc_array(context, type, N);
  talloc_realloc(context, ptr, type, 0)  ==> talloc_free(ptr);

The "context" argument is only used if "ptr" is NULL, otherwise it is
ignored.

talloc_realloc() returns the new pointer, or NULL on failure. The call
will fail either due to a lack of memory, or because the pointer has
more than one parent (see talloc_reference()).


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void *talloc_realloc_size(const void *context, void *ptr, size_t size);