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The main change is to get rid of talloc_parent_chunk() from all commonly used code paths, so talloc_free() is now O(1) again. It was originally O(1), but the last round of changes broke that. Also some documentation updates
490 lines
18 KiB
Plaintext
490 lines
18 KiB
Plaintext
Using talloc in Samba4
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----------------------
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Andrew Tridgell
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September 2004
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The most current version of this document is available at
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http://samba.org/ftp/unpacked/samba4/talloc_guide.txt
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If you are used to talloc from Samba3 then please read this carefully,
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as talloc has changed a lot.
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The new talloc is a hierarchical, reference counted memory pool system
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with destructors. Quite a mounthful really, but not too bad once you
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get used to it.
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Perhaps the biggest change from Samba3 is that there is no distinction
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between a "talloc context" and a "talloc pointer". Any pointer
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returned from talloc() is itself a valid talloc context. This means
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you can do this:
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struct foo *X = talloc_p(mem_ctx, struct foo);
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X->name = talloc_strdup(X, "foo");
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and the pointer X->name would be a "child" of the talloc context "X"
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which is itself a child of mem_ctx. So if you do talloc_free(mem_ctx)
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then it is all destroyed, whereas if you do talloc_free(X) then just X
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and X->name are destroyed, and if you do talloc_free(X->name) then
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just the name element of X is destroyed.
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If you think about this, then what this effectively gives you is an
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n-ary tree, where you can free any part of the tree with
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talloc_free().
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If you find this confusing, then I suggest you run the LOCAL-TALLOC
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smbtorture test to watch talloc in action. You may also like to add
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your own tests to source/torture/local/talloc.c to clarify how some
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particular situation is handled.
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Performance
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-----------
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All the additional features of talloc() over malloc() do come at a
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price. We have a simple performance test in Samba4 that measures
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talloc() versus malloc() performance, and it seems that talloc() is
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about 10% slower than malloc() on my x86 Debian Linux box. For Samba,
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the great reduction in code complexity that we get by using talloc
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makes this worthwhile, especially as the total overhead of
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talloc/malloc in Samba is already quite small.
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talloc API
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----------
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The following is a complete guide to the talloc API. Read it all at
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least twice.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc(const void *context, size_t size);
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The talloc() function is the core of the talloc library. It takes a
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memory context, and returns a pointer to a new area of memory of the
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given size.
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The returned pointer is itself a talloc context, so you can use it as
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the context argument to more calls to talloc if you wish.
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The returned pointer is a "child" of the supplied context. This means
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that if you talloc_free() the context then the new child disappears as
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well. Alternatively you can free just the child.
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The context argument to talloc() can be NULL, in which case a new top
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level context is created.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_p(const void *context, type);
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The talloc_p() macro is the equivalent of
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(type *)talloc(ctx, sizeof(type))
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You should use it in preference to talloc() whenever possible, as it
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provides additional type safety. It also automatically calls the
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talloc_set_name_const() function with the name being a string holding
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the name of the type.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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int talloc_free(void *ptr);
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The talloc_free() function frees a piece of talloc memory, and all its
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children. You can call talloc_free() on any pointer returned by
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talloc().
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The return value of talloc_free() indicates success or failure, with 0
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returned for success and -1 for failure. The only possible failure
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condition is if the pointer had a destructor attached to it and the
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destructor returned -1. See talloc_set_destructor() for details on
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destructors.
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If this pointer has an additional parent when talloc_free() is called
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then the memory is not actually released, but instead the most
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recently established parent is destroyed. See talloc_reference() for
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details on establishing additional parents.
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For more control on which parent is removed, see talloc_unlink()
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talloc_free() operates recursively on its children.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_reference(const void *context, const void *ptr);
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The talloc_reference() function makes "context" an additional parent
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of "ptr".
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The return value of talloc_reference() is always the original pointer
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"ptr", unless talloc ran out of memory in creating the reference in
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which case it will return NULL (each additional reference consumes
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around 48 bytes of memory on intel x86 platforms).
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After creating a reference you can free it in one of the following
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ways:
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- you can talloc_free() any parent of the original pointer. That
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will reduce the number of parents of this pointer by 1, and will
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cause this pointer to be freed if it runs out of parents.
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- you can talloc_free() the pointer itself. That will destroy the
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most recently established parent to the pointer and leave the
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pointer as a child of its current parent.
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For more control on which parent to remove, see talloc_unlink()
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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int talloc_unlink(const void *context, const void *ptr);
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The talloc_unlink() function removes a specific parent from ptr. The
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context passed must either be a context used in talloc_reference()
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with this pointer, or must be a direct parent of ptr.
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Note that if the parent has already been removed using talloc_free()
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then this function will fail and will return -1.
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Usually you can just use talloc_free() instead of talloc_unlink(), but
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sometimes it is useful to have the additional control on which parent
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is removed.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void talloc_set_destructor(const void *ptr, int (*destructor)(void *));
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The function talloc_set_destructor() sets the "destructor" for the
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pointer "ptr". A destructor is a function that is called when the
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memory used by a pointer is about to be released. The destructor
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receives the pointer as an argument, and should return 0 for success
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and -1 for failure.
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The destructor can do anything it wants to, including freeing other
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pieces of memory. A common use for destructors is to clean up
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operating system resources (such as open file descriptors) contained
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in the structure the destructor is placed on.
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You can only place one destructor on a pointer. If you need more than
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one destructor then you can create a zero-length child of the pointer
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and place an additional destructor on that.
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To remove a destructor call talloc_set_destructor() with NULL for the
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destructor.
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If your destructor attempts to talloc_free() the pointer that it is
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the destructor for then talloc_free() will return -1 and the free will
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be ignored. This would be a pointless operation anyway, as the
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destructor is only called when the memory is just about to go away.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void talloc_increase_ref_count(const void *ptr);
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The talloc_increase_ref_count(ptr) function is exactly equivalent to:
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talloc_reference(NULL, ptr);
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You can use either syntax, depending on which you think is clearer in
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your code.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void talloc_set_name(const void *ptr, const char *fmt, ...);
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Each talloc pointer has a "name". The name is used principally for
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debugging purposes, although it is also possible to set and get the
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name on a pointer in as a way of "marking" pointers in your code.
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The main use for names on pointer is for "talloc reports". See
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talloc_report() and talloc_report_full() for details. Also see
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talloc_enable_leak_report() and talloc_enable_leak_report_full().
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The talloc_set_name() function allocates memory as a child of the
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pointer. It is logically equivalent to:
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talloc_set_name_const(ptr, talloc_asprintf(ptr, fmt, ...));
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Note that multiple calls to talloc_set_name() will allocate more
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memory without releasing the name. All of the memory is released when
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the ptr is freed using talloc_free().
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void talloc_set_name_const(const void *ptr, const char *name);
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The function talloc_set_name_const() is just like talloc_set_name(),
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but it takes a string constant, and is much faster. It is extensively
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used by the "auto naming" macros, such as talloc_p().
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This function does not allocate any memory. It just copies the
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supplied pointer into the internal representation of the talloc
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ptr. This means you must not pass a name pointer to memory that will
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disappear before the ptr is freed with talloc_free().
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_named(const void *context, size_t size, const char *fmt, ...);
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The talloc_named() function creates a named talloc pointer. It is
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equivalent to:
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ptr = talloc(context, size);
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talloc_set_name(ptr, fmt, ....);
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_named_const(const void *context, size_t size, const char *name);
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This is equivalent to:
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ptr = talloc(context, size);
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talloc_set_name_const(ptr, name);
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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const char *talloc_get_name(const void *ptr);
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This returns the current name for the given talloc pointer. See
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talloc_set_name() for details.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_init(const char *fmt, ...);
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This function creates a zero length named talloc context as a top
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level context. It is equivalent to:
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talloc_named(NULL, 0, fmt, ...);
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_realloc(const void *context, void *ptr, size_t size);
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The talloc_realloc() function changes the size of a talloc
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pointer. It has the following equivalences:
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talloc_realloc(context, NULL, size) ==> talloc(context, size);
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talloc_realloc(context, ptr, 0) ==> talloc_free(ptr);
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The "context" argument is only used if "ptr" is not NULL, otherwise it
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is ignored.
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talloc_realloc() returns the new pointer, or NULL on failure. The call
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will fail either due to a lack of memory, or because the pointer has
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more than one parent (see talloc_reference()).
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_steal(const void *new_ctx, const void *ptr);
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The talloc_steal() function changes the parent context of a talloc
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pointer. It is typically used when the context that the pointer is
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currently a child of is going to be freed and you wish to keep the
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memory for a longer time.
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The talloc_steal() function returns the pointer that you pass it. It
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does not have any failure modes.
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NOTE: It is possible to produce loops in the parent/child relationship
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if you are not careful with talloc_steal(). No guarantees are provided
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as to your sanity or the safety of your data if you do this.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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off_t talloc_total_size(const void *ptr);
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The talloc_total_size() function returns the total size in bytes used
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by this pointer and all child pointers. Mostly useful for debugging.
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Passing NULL is allowed, but it will only give a meaningful result if
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talloc_enable_leak_report() or talloc_enable_leak_report_full() has
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been called.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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off_t talloc_total_blocks(const void *ptr);
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The talloc_total_blocks() function returns the total memory block
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count used by this pointer and all child pointers. Mostly useful for
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debugging.
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Passing NULL is allowed, but it will only give a meaningful result if
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talloc_enable_leak_report() or talloc_enable_leak_report_full() has
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been called.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void talloc_report(const void *ptr, FILE *f);
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The talloc_report() function prints a summary report of all memory
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used by ptr. One line of report is printed for each immediate child of
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ptr, showing the total memory and number of blocks used by that child.
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You can pass NULL for the pointer, in which case a report is printed
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for the top level memory context, but only if
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talloc_enable_leak_report() or talloc_enable_leak_report_full() has
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been called.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void talloc_report_full(const void *ptr, FILE *f);
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This provides a more detailed report than talloc_report(). It will
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recursively print the ensire tree of memory referenced by the
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pointer. References in the tree are shown by giving the name of the
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pointer that is referenced.
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You can pass NULL for the pointer, in which case a report is printed
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for the top level memory context, but only if
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talloc_enable_leak_report() or talloc_enable_leak_report_full() has
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been called.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void talloc_enable_leak_report(void);
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This enables calling of talloc_report(NULL, stderr) when the program
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exits. In Samba4 this is enabled by using the --leak-report command
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line option.
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For it to be useful, this function must be called before any other
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talloc function as it establishes a "null context" that acts as the
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top of the tree. If you don't call this function first then passing
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NULL to talloc_report() or talloc_report_full() won't give you the
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full tree printout.
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Here is a typical talloc report:
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talloc report on 'null_context' (total 267 bytes in 15 blocks)
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libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
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libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
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iconv(UTF8,CP850) contains 42 bytes in 2 blocks
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libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
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iconv(CP850,UTF8) contains 42 bytes in 2 blocks
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iconv(UTF8,UTF-16LE) contains 45 bytes in 2 blocks
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iconv(UTF-16LE,UTF8) contains 45 bytes in 2 blocks
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void talloc_enable_leak_report_full(void);
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This enables calling of talloc_report_full(NULL, stderr) when the
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program exits. In Samba4 this is enabled by using the
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--leak-report-full command line option.
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For it to be useful, this function must be called before any other
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talloc function as it establishes a "null context" that acts as the
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top of the tree. If you don't call this function first then passing
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NULL to talloc_report() or talloc_report_full() won't give you the
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full tree printout.
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Here is a typical full report:
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full talloc report on 'root' (total 18 bytes in 8 blocks)
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p1 contains 18 bytes in 7 blocks (ref 0)
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r1 contains 13 bytes in 2 blocks (ref 0)
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reference to: p2
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p2 contains 1 bytes in 1 blocks (ref 1)
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x3 contains 1 bytes in 1 blocks (ref 0)
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x2 contains 1 bytes in 1 blocks (ref 0)
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x1 contains 1 bytes in 1 blocks (ref 0)
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_zero(const void *ctx, size_t size);
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The talloc_zero() function is equivalent to:
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ptr = talloc(ctx, size);
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if (ptr) memset(ptr, 0, size);
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_memdup(const void *ctx, const void *p, size_t size);
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The talloc_memdup() function is equivalent to:
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ptr = talloc(ctx, size);
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if (ptr) memcpy(ptr, p, size);
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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char *talloc_strdup(const void *ctx, const char *p);
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The talloc_strdup() function is equivalent to:
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ptr = talloc(ctx, strlen(p)+1);
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if (ptr) memcpy(ptr, p, strlen(p)+1);
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This functions sets the name of the new pointer to the passed
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string. This is equivalent to:
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talloc_set_name_const(ptr, ptr)
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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char *talloc_strndup(const void *t, const char *p, size_t n);
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The talloc_strndup() function is the talloc equivalent of the C
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library function strndup()
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This functions sets the name of the new pointer to the passed
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string. This is equivalent to:
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talloc_set_name_const(ptr, ptr)
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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char *talloc_vasprintf(const void *t, const char *fmt, va_list ap);
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The talloc_vasprintf() function is the talloc equivalent of the C
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library function vasprintf()
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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char *talloc_asprintf(const void *t, const char *fmt, ...);
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The talloc_asprintf() function is the talloc equivalent of the C
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library function asprintf()
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This functions sets the name of the new pointer to the passed
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string. This is equivalent to:
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talloc_set_name_const(ptr, ptr)
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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char *talloc_asprintf_append(char *s, const char *fmt, ...);
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The talloc_asprintf_append() function appends the given formatted
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string to the given string.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_array_p(const void *ctx, type, uint_t count);
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The talloc_array_p() macro is equivalent to:
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(type *)talloc(ctx, sizeof(type) * count);
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except that it provides integer overflow protection for the multiply,
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returning NULL if the multiply overflows.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_realloc_p(const void *ctx, void *ptr, type, uint_t count);
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The talloc_realloc_p() macro is equivalent to:
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(type *)talloc_realloc(ctx, ptr, sizeof(type) * count);
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except that it provides integer overflow protection for the multiply,
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returning NULL if the multiply overflows.
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=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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void *talloc_realloc_fn(const void *ctx, void *ptr, size_t size);
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This is a non-macro version of talloc_realloc(), which is useful
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as libraries sometimes want a ralloc function pointer. A realloc()
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implementation encapsulates the functionality of malloc(), free() and
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realloc() in one call, which is why it is useful to be able to pass
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around a single function pointer.
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