7 The most current version of this document is available at
8 http://samba.org/ftp/unpacked/samba4/talloc_guide.txt
10 If you are used to talloc from Samba3 then please read this carefully,
11 as talloc has changed a lot.
13 The new talloc is a hierarchical, reference counted memory pool system
14 with destructors. Quite a mounthful really, but not too bad once you
17 Perhaps the biggest change from Samba3 is that there is no distinction
18 between a "talloc context" and a "talloc pointer". Any pointer
19 returned from talloc() is itself a valid talloc context. This means
22 struct foo *X = talloc_p(mem_ctx, struct foo);
23 X->name = talloc_strdup(X, "foo");
25 and the pointer X->name would be a "child" of the talloc context "X"
26 which is itself a child of mem_ctx. So if you do talloc_free(mem_ctx)
27 then it is all destroyed, whereas if you do talloc_free(X) then just X
28 and X->name are destroyed, and if you do talloc_free(X->name) then
29 just the name element of X is destroyed.
31 If you think about this, then what this effectively gives you is an
32 n-ary tree, where you can free any part of the tree with
35 If you find this confusing, then I suggest you run the LOCAL-TALLOC
36 smbtorture test with the --leak-report-full option to watch talloc in
37 action. You may also like to add your own tests to
38 source/torture/local/talloc.c to clarify how some particular situation
45 All the additional features of talloc() over malloc() do come at a
46 price. We have a simple performance test in Samba4 that measures
47 talloc() versus malloc() performance, and it seems that talloc() is
48 about 10% slower than malloc() on my x86 Debian Linux box. For Samba,
49 the great reduction in code complexity that we get by using talloc
50 makes this worthwhile, especially as the total overhead of
51 talloc/malloc in Samba is already quite small.
57 The following is a complete guide to the talloc API. Read it all at
61 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
62 void *talloc(const void *context, size_t size);
64 The talloc() function is the core of the talloc library. It takes a
65 memory context, and returns a pointer to a new area of memory of the
68 The returned pointer is itself a talloc context, so you can use it as
69 the context argument to more calls to talloc if you wish.
71 The returned pointer is a "child" of the supplied context. This means
72 that if you talloc_free() the context then the new child disappears as
73 well. Alternatively you can free just the child.
75 The context argument to talloc() can be NULL, in which case a new top
76 level context is created.
79 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
80 void *talloc_p(const void *context, type);
82 The talloc_p() macro is the equivalent of
84 (type *)talloc(ctx, sizeof(type))
86 You should use it in preference to talloc() whenever possible, as it
87 provides additional type safety. It also automatically calls the
88 talloc_set_name_const() function with the name being a string holding
92 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
93 int talloc_free(void *ptr);
95 The talloc_free() function frees a piece of talloc memory, and all its
96 children. You can call talloc_free() on any pointer returned by
99 The return value of talloc_free() indicates success or failure, with 0
100 returned for success and -1 for failure. The only possible failure
101 condition is if the pointer had a destructor attached to it and the
102 destructor returned -1. See talloc_set_destructor() for details on
105 If this pointer has an additional reference when talloc_free() is
106 called then the memory is not actually released, but instead the
107 reference is destroyed. See talloc_reference() for details on
108 establishing additional references.
110 talloc_free() operates recursively on its children.
113 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
114 void *talloc_reference(const void *context, const void *ptr);
116 The talloc_reference() function returns an additional reference to
117 "ptr", and makes this additional reference a child of "context".
119 The return value of talloc_reference() is always the original pointer
120 "ptr", unless talloc ran out of memory in creating the reference in
121 which case it will return NULL (each additional reference consumes
122 around 48 bytes of memory on intel x86 platforms).
124 After creating a reference you can free it in one of the following
127 - you can talloc_free() a parent of the original pointer. That will
128 destroy the reference and make the pointer a child of "context".
130 - you can talloc_free() the pointer itself. That will destroy the
131 most recently established reference to the pointer and leave the
132 pointer as a child of its current parent.
134 - you can talloc_free() the context where you placed the
135 reference. That will destroy the reference, and leave the pointer
139 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
140 void *talloc_unreference(const void *context, const void *ptr);
142 The talloc_unreference() function removes a reference added by
143 talloc_reference(). It must be called with exactly the same arguments
144 as talloc_reference().
146 Note that if the reference has already been removed using
147 talloc_free() then this function will fail and will return NULL.
149 Usually you can just use talloc_free() instead of
150 talloc_unreference(), but sometimes it is useful to have the
151 additional control on who becomes the parent of the pointer given by
152 talloc_unreference().
155 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
156 void talloc_set_destructor(const void *ptr, int (*destructor)(void *));
158 The function talloc_set_destructor() sets the "destructor" for the
159 pointer "ptr". A destructor is a function that is called when the
160 memory used by a pointer is about to be released. The destructor
161 receives the pointer as an argument, and should return 0 for success
164 The destructor can do anything it wants to, including freeing other
165 pieces of memory. A common use for destructors is to clean up
166 operating system resources (such as open file descriptors) contained
167 in the structure the destructor is placed on.
169 You can only place one destructor on a pointer. If you need more than
170 one destructor then you can create a zero-length child of the pointer
171 and place an additional destructor on that.
173 To remove a destructor call talloc_set_destructor() with NULL for the
176 If your destructor attempts to talloc_free() the pointer that it is
177 the destructor for then talloc_free() will return -1 and the free will
178 be ignored. This would be a pointless operation anyway, as the
179 destructor is only called when the memory is just about to go away.
182 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
183 void talloc_increase_ref_count(const void *ptr);
185 The talloc_increase_ref_count(ptr) function is exactly equivalent to:
187 talloc_reference(NULL, ptr);
189 You can use either syntax, depending on which you think is clearer in
193 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
194 void talloc_set_name(const void *ptr, const char *fmt, ...);
196 Each talloc pointer has a "name". The name is used principally for
197 debugging purposes, although it is also possible to set and get the
198 name on a pointer in as a way of "marking" pointers in your code.
200 The main use for names on pointer is for "talloc reports". See
201 talloc_report() and talloc_report_full() for details. Also see
202 talloc_enable_leak_report() and talloc_enable_leak_report_full().
205 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
206 void talloc_set_name_const(const void *ptr, const char *name);
208 The function talloc_set_name_const() is just like talloc_set_name(),
209 but it takes a string constant, and is much faster. It is extensively
210 used by the "auto naming" macros, such as talloc_p().
213 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
214 void *talloc_named(const void *context, size_t size, const char *fmt, ...);
216 The talloc_named() function creates a named talloc pointer. It is
219 ptr = talloc(context, size);
220 talloc_set_name(ptr, fmt, ....);
223 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
224 void *talloc_named_const(const void *context, size_t size, const char *name);
226 This is equivalent to:
228 ptr = talloc(context, size);
229 talloc_set_name_const(ptr, name);
232 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
233 const char *talloc_get_name(const void *ptr);
235 This returns the current name for the given talloc pointer. See
236 talloc_set_name() for details.
239 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
240 void *talloc_init(const char *fmt, ...);
242 This function creates a zero length named talloc context as a top
243 level context. It is equivalent to:
245 talloc_named(NULL, 0, fmt, ...);
248 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
249 void *talloc_realloc(const void *context, void *ptr, size_t size);
251 The talloc_realloc() function changes the size of a talloc
252 pointer. It has the following equivalences:
254 talloc_realloc(context, NULL, size) ==> talloc(context, size);
255 talloc_realloc(context, ptr, 0) ==> talloc_free(ptr);
257 The "context" argument is only used if "ptr" is not NULL, otherwise it
260 talloc_realloc() returns the new pointer, or NULL on failure. The call
261 will fail either due to a lack of memory, or because the pointer has
262 an reference (see talloc_reference()).
265 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
266 void *talloc_steal(const void *new_ctx, const void *ptr);
268 The talloc_steal() function changes the parent context of a talloc
269 pointer. It is typically used when the context that the pointer is
270 currently a child of is going to be freed and you wish to keep the
271 memory for a longer time.
273 The talloc_steal() function returns the pointer that you pass it. It
274 does not have any failure modes.
276 NOTE: It is possible to produce loops in the parent/child relationship
277 if you are not careful with talloc_steal(). No guarantees are provided
278 as to your sanity or the safety of your data if you do this.
281 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
282 off_t talloc_total_size(const void *ptr);
284 The talloc_total_size() function returns the total size in bytes used
285 by this pointer and all child pointers. Mostly useful for debugging.
287 Passing NULL is allowed, but it will only give a meaningful result if
288 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
292 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
293 off_t talloc_total_blocks(const void *ptr);
295 The talloc_total_blocks() function returns the total memory block
296 count used by this pointer and all child pointers. Mostly useful for
299 Passing NULL is allowed, but it will only give a meaningful result if
300 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
304 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
305 void talloc_report(const void *ptr, FILE *f);
307 The talloc_report() function prints a summary report of all memory
308 used by ptr. One line of report is printed for each immediate child of
309 ptr, showing the total memory and number of blocks used by that child.
311 You can pass NULL for the pointer, in which case a report is printed
312 for the top level memory context, but only if
313 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
317 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
318 void talloc_report_full(const void *ptr, FILE *f);
320 This provides a more detailed report than talloc_report(). It will
321 recursively print the ensire tree of memory referenced by the
322 pointer. References in the tree are shown by giving the name of the
323 pointer that is referenced.
325 You can pass NULL for the pointer, in which case a report is printed
326 for the top level memory context, but only if
327 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
331 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
332 void talloc_enable_leak_report(void);
334 This enables calling of talloc_report(NULL, stderr) when the program
335 exits. In Samba4 this is enabled by using the --leak-report command
338 For it to be useful, this function must be called before any other
339 talloc function as it establishes a "null context" that acts as the
340 top of the tree. If you don't call this function first then passing
341 NULL to talloc_report() or talloc_report_full() won't give you the
345 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
346 void talloc_enable_leak_report_full(void);
348 This enables calling of talloc_report_full(NULL, stderr) when the
349 program exits. In Samba4 this is enabled by using the
350 --leak-report-full command line option.
352 For it to be useful, this function must be called before any other
353 talloc function as it establishes a "null context" that acts as the
354 top of the tree. If you don't call this function first then passing
355 NULL to talloc_report() or talloc_report_full() won't give you the
359 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
360 void *talloc_zero(const void *ctx, size_t size);
362 The talloc_zero() function is equivalent to:
364 ptr = talloc(ctx, size);
365 if (ptr) memset(ptr, 0, size);
368 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
369 void *talloc_memdup(const void *ctx, const void *p, size_t size);
371 The talloc_memdup() function is equivalent to:
373 ptr = talloc(ctx, size);
374 if (ptr) memcpy(ptr, p, size);
377 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
378 char *talloc_strdup(const void *ctx, const char *p);
380 The talloc_strdup() function is equivalent to:
382 ptr = talloc(ctx, strlen(p)+1);
383 if (ptr) memcpy(ptr, p, strlen(p)+1);
386 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
387 char *talloc_strndup(const void *t, const char *p, size_t n);
389 The talloc_strndup() function is the talloc equivalent of the C
390 library function strndup()
393 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
394 char *talloc_vasprintf(const void *t, const char *fmt, va_list ap);
396 The talloc_vasprintf() function is the talloc equivalent of the C
397 library function vasprintf()
400 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
401 char *talloc_asprintf(const void *t, const char *fmt, ...);
403 The talloc_asprintf() function is the talloc equivalent of the C
404 library function asprintf()
407 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
408 char *talloc_asprintf_append(char *s, const char *fmt, ...);
410 The talloc_asprintf_append() function appends the given formatted
411 string to the given string.
414 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
415 void *talloc_array_p(const void *ctx, type, uint_t count);
417 The talloc_array_p() macro is equivalent to:
419 (type *)talloc(ctx, sizeof(type) * count);
421 except that it provides integer overflow protection for the multiply,
422 returning NULL if the multiply overflows.
425 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
426 void *talloc_realloc_p(const void *ctx, void *ptr, type, uint_t count);
428 The talloc_realloc_p() macro is equivalent to:
430 (type *)talloc_realloc(ctx, ptr, sizeof(type) * count);
432 except that it provides integer overflow protection for the multiply,
433 returning NULL if the multiply overflows.