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 to watch talloc in action. You may also like to add
37 your own tests to source/torture/local/talloc.c to clarify how some
38 particular situation is handled.
44 All the additional features of talloc() over malloc() do come at a
45 price. We have a simple performance test in Samba4 that measures
46 talloc() versus malloc() performance, and it seems that talloc() is
47 about 10% slower than malloc() on my x86 Debian Linux box. For Samba,
48 the great reduction in code complexity that we get by using talloc
49 makes this worthwhile, especially as the total overhead of
50 talloc/malloc in Samba is already quite small.
56 The following is a complete guide to the talloc API. Read it all at
60 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
61 void *talloc(const void *context, size_t size);
63 The talloc() function is the core of the talloc library. It takes a
64 memory context, and returns a pointer to a new area of memory of the
67 The returned pointer is itself a talloc context, so you can use it as
68 the context argument to more calls to talloc if you wish.
70 The returned pointer is a "child" of the supplied context. This means
71 that if you talloc_free() the context then the new child disappears as
72 well. Alternatively you can free just the child.
74 The context argument to talloc() can be NULL, in which case a new top
75 level context is created.
78 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
79 void *talloc_p(const void *context, type);
81 The talloc_p() macro is the equivalent of
83 (type *)talloc(ctx, sizeof(type))
85 You should use it in preference to talloc() whenever possible, as it
86 provides additional type safety. It also automatically calls the
87 talloc_set_name_const() function with the name being a string holding
91 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
92 int talloc_free(void *ptr);
94 The talloc_free() function frees a piece of talloc memory, and all its
95 children. You can call talloc_free() on any pointer returned by
98 The return value of talloc_free() indicates success or failure, with 0
99 returned for success and -1 for failure. The only possible failure
100 condition is if the pointer had a destructor attached to it and the
101 destructor returned -1. See talloc_set_destructor() for details on
104 If this pointer has an additional parent when talloc_free() is called
105 then the memory is not actually released, but instead the most
106 recently established parent is destroyed. See talloc_reference() for
107 details on establishing additional parents.
109 For more control on which parent is removed, see talloc_unlink()
111 talloc_free() operates recursively on its children.
114 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
115 void *talloc_reference(const void *context, const void *ptr);
117 The talloc_reference() function makes "context" an additional parent
120 The return value of talloc_reference() is always the original pointer
121 "ptr", unless talloc ran out of memory in creating the reference in
122 which case it will return NULL (each additional reference consumes
123 around 48 bytes of memory on intel x86 platforms).
125 If "ptr" is NULL, then the function is a no-op, and simply returns NULL.
127 After creating a reference you can free it in one of the following
130 - you can talloc_free() any parent of the original pointer. That
131 will reduce the number of parents of this pointer by 1, and will
132 cause this pointer to be freed if it runs out of parents.
134 - you can talloc_free() the pointer itself. That will destroy the
135 most recently established parent to the pointer and leave the
136 pointer as a child of its current parent.
138 For more control on which parent to remove, see talloc_unlink()
141 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
142 int talloc_unlink(const void *context, const void *ptr);
144 The talloc_unlink() function removes a specific parent from ptr. The
145 context passed must either be a context used in talloc_reference()
146 with this pointer, or must be a direct parent of ptr.
148 Note that if the parent has already been removed using talloc_free()
149 then this function will fail and will return -1. Likewise, if "ptr"
150 is NULL, then the function will make no modifications and return -1.
152 Usually you can just use talloc_free() instead of talloc_unlink(), but
153 sometimes it is useful to have the additional control on which parent
157 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
158 void talloc_set_destructor(const void *ptr, int (*destructor)(void *));
160 The function talloc_set_destructor() sets the "destructor" for the
161 pointer "ptr". A destructor is a function that is called when the
162 memory used by a pointer is about to be released. The destructor
163 receives the pointer as an argument, and should return 0 for success
166 The destructor can do anything it wants to, including freeing other
167 pieces of memory. A common use for destructors is to clean up
168 operating system resources (such as open file descriptors) contained
169 in the structure the destructor is placed on.
171 You can only place one destructor on a pointer. If you need more than
172 one destructor then you can create a zero-length child of the pointer
173 and place an additional destructor on that.
175 To remove a destructor call talloc_set_destructor() with NULL for the
178 If your destructor attempts to talloc_free() the pointer that it is
179 the destructor for then talloc_free() will return -1 and the free will
180 be ignored. This would be a pointless operation anyway, as the
181 destructor is only called when the memory is just about to go away.
184 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
185 void talloc_increase_ref_count(const void *ptr);
187 The talloc_increase_ref_count(ptr) function is exactly equivalent to:
189 talloc_reference(NULL, ptr);
191 You can use either syntax, depending on which you think is clearer in
195 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
196 void talloc_set_name(const void *ptr, const char *fmt, ...);
198 Each talloc pointer has a "name". The name is used principally for
199 debugging purposes, although it is also possible to set and get the
200 name on a pointer in as a way of "marking" pointers in your code.
202 The main use for names on pointer is for "talloc reports". See
203 talloc_report() and talloc_report_full() for details. Also see
204 talloc_enable_leak_report() and talloc_enable_leak_report_full().
206 The talloc_set_name() function allocates memory as a child of the
207 pointer. It is logically equivalent to:
208 talloc_set_name_const(ptr, talloc_asprintf(ptr, fmt, ...));
210 Note that multiple calls to talloc_set_name() will allocate more
211 memory without releasing the name. All of the memory is released when
212 the ptr is freed using talloc_free().
215 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
216 void talloc_set_name_const(const void *ptr, const char *name);
218 The function talloc_set_name_const() is just like talloc_set_name(),
219 but it takes a string constant, and is much faster. It is extensively
220 used by the "auto naming" macros, such as talloc_p().
222 This function does not allocate any memory. It just copies the
223 supplied pointer into the internal representation of the talloc
224 ptr. This means you must not pass a name pointer to memory that will
225 disappear before the ptr is freed with talloc_free().
228 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
229 void *talloc_named(const void *context, size_t size, const char *fmt, ...);
231 The talloc_named() function creates a named talloc pointer. It is
234 ptr = talloc(context, size);
235 talloc_set_name(ptr, fmt, ....);
238 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
239 void *talloc_named_const(const void *context, size_t size, const char *name);
241 This is equivalent to:
243 ptr = talloc(context, size);
244 talloc_set_name_const(ptr, name);
247 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
248 const char *talloc_get_name(const void *ptr);
250 This returns the current name for the given talloc pointer. See
251 talloc_set_name() for details.
254 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
255 void *talloc_init(const char *fmt, ...);
257 This function creates a zero length named talloc context as a top
258 level context. It is equivalent to:
260 talloc_named(NULL, 0, fmt, ...);
263 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
264 void *talloc_new(void *ctx);
266 This is a utility macro that creates a new memory context hanging
267 off an exiting context, automatically naming it "talloc_new: __location__"
268 where __location__ is the source line it is called from. It is
269 particularly useful for creating a new temporary working context.
272 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
273 void *talloc_realloc(const void *context, void *ptr, size_t size);
275 The talloc_realloc() function changes the size of a talloc
276 pointer. It has the following equivalences:
278 talloc_realloc(context, NULL, size) ==> talloc(context, size);
279 talloc_realloc(context, ptr, 0) ==> talloc_free(ptr);
281 The "context" argument is only used if "ptr" is not NULL, otherwise it
284 talloc_realloc() returns the new pointer, or NULL on failure. The call
285 will fail either due to a lack of memory, or because the pointer has
286 more than one parent (see talloc_reference()).
289 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
290 void *talloc_steal(const void *new_ctx, const void *ptr);
292 The talloc_steal() function changes the parent context of a talloc
293 pointer. It is typically used when the context that the pointer is
294 currently a child of is going to be freed and you wish to keep the
295 memory for a longer time.
297 The talloc_steal() function returns the pointer that you pass it. It
298 does not have any failure modes.
300 NOTE: It is possible to produce loops in the parent/child relationship
301 if you are not careful with talloc_steal(). No guarantees are provided
302 as to your sanity or the safety of your data if you do this.
305 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
306 off_t talloc_total_size(const void *ptr);
308 The talloc_total_size() function returns the total size in bytes used
309 by this pointer and all child pointers. Mostly useful for debugging.
311 Passing NULL is allowed, but it will only give a meaningful result if
312 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
316 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
317 off_t talloc_total_blocks(const void *ptr);
319 The talloc_total_blocks() function returns the total memory block
320 count used by this pointer and all child pointers. Mostly useful for
323 Passing NULL is allowed, but it will only give a meaningful result if
324 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
328 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
329 void talloc_report(const void *ptr, FILE *f);
331 The talloc_report() function prints a summary report of all memory
332 used by ptr. One line of report is printed for each immediate child of
333 ptr, showing the total memory and number of blocks used by that child.
335 You can pass NULL for the pointer, in which case a report is printed
336 for the top level memory context, but only if
337 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
341 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
342 void talloc_report_full(const void *ptr, FILE *f);
344 This provides a more detailed report than talloc_report(). It will
345 recursively print the ensire tree of memory referenced by the
346 pointer. References in the tree are shown by giving the name of the
347 pointer that is referenced.
349 You can pass NULL for the pointer, in which case a report is printed
350 for the top level memory context, but only if
351 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
355 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
356 void talloc_enable_leak_report(void);
358 This enables calling of talloc_report(NULL, stderr) when the program
359 exits. In Samba4 this is enabled by using the --leak-report command
362 For it to be useful, this function must be called before any other
363 talloc function as it establishes a "null context" that acts as the
364 top of the tree. If you don't call this function first then passing
365 NULL to talloc_report() or talloc_report_full() won't give you the
368 Here is a typical talloc report:
370 talloc report on 'null_context' (total 267 bytes in 15 blocks)
371 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
372 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
373 iconv(UTF8,CP850) contains 42 bytes in 2 blocks
374 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
375 iconv(CP850,UTF8) contains 42 bytes in 2 blocks
376 iconv(UTF8,UTF-16LE) contains 45 bytes in 2 blocks
377 iconv(UTF-16LE,UTF8) contains 45 bytes in 2 blocks
380 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
381 void talloc_enable_leak_report_full(void);
383 This enables calling of talloc_report_full(NULL, stderr) when the
384 program exits. In Samba4 this is enabled by using the
385 --leak-report-full command line option.
387 For it to be useful, this function must be called before any other
388 talloc function as it establishes a "null context" that acts as the
389 top of the tree. If you don't call this function first then passing
390 NULL to talloc_report() or talloc_report_full() won't give you the
393 Here is a typical full report:
395 full talloc report on 'root' (total 18 bytes in 8 blocks)
396 p1 contains 18 bytes in 7 blocks (ref 0)
397 r1 contains 13 bytes in 2 blocks (ref 0)
399 p2 contains 1 bytes in 1 blocks (ref 1)
400 x3 contains 1 bytes in 1 blocks (ref 0)
401 x2 contains 1 bytes in 1 blocks (ref 0)
402 x1 contains 1 bytes in 1 blocks (ref 0)
405 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
406 void *talloc_zero(const void *ctx, size_t size);
408 The talloc_zero() function is equivalent to:
410 ptr = talloc(ctx, size);
411 if (ptr) memset(ptr, 0, size);
414 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
415 void *talloc_memdup(const void *ctx, const void *p, size_t size);
417 The talloc_memdup() function is equivalent to:
419 ptr = talloc(ctx, size);
420 if (ptr) memcpy(ptr, p, size);
423 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
424 char *talloc_strdup(const void *ctx, const char *p);
426 The talloc_strdup() function is equivalent to:
428 ptr = talloc(ctx, strlen(p)+1);
429 if (ptr) memcpy(ptr, p, strlen(p)+1);
431 This functions sets the name of the new pointer to the passed
432 string. This is equivalent to:
433 talloc_set_name_const(ptr, ptr)
435 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
436 char *talloc_strndup(const void *t, const char *p, size_t n);
438 The talloc_strndup() function is the talloc equivalent of the C
439 library function strndup()
441 This functions sets the name of the new pointer to the passed
442 string. This is equivalent to:
443 talloc_set_name_const(ptr, ptr)
446 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
447 char *talloc_vasprintf(const void *t, const char *fmt, va_list ap);
449 The talloc_vasprintf() function is the talloc equivalent of the C
450 library function vasprintf()
453 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
454 char *talloc_asprintf(const void *t, const char *fmt, ...);
456 The talloc_asprintf() function is the talloc equivalent of the C
457 library function asprintf()
459 This functions sets the name of the new pointer to the passed
460 string. This is equivalent to:
461 talloc_set_name_const(ptr, ptr)
464 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
465 char *talloc_asprintf_append(char *s, const char *fmt, ...);
467 The talloc_asprintf_append() function appends the given formatted
468 string to the given string.
471 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
472 void *talloc_array_p(const void *ctx, type, uint_t count);
474 The talloc_array_p() macro is equivalent to:
476 (type *)talloc(ctx, sizeof(type) * count);
478 except that it provides integer overflow protection for the multiply,
479 returning NULL if the multiply overflows.
482 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
483 void *talloc_realloc_p(const void *ctx, void *ptr, type, uint_t count);
485 The talloc_realloc_p() macro is equivalent to:
487 (type *)talloc_realloc(ctx, ptr, sizeof(type) * count);
489 except that it provides integer overflow protection for the multiply,
490 returning NULL if the multiply overflows.
493 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
494 void *talloc_realloc_fn(const void *ctx, void *ptr, size_t size);
496 This is a non-macro version of talloc_realloc(), which is useful
497 as libraries sometimes want a ralloc function pointer. A realloc()
498 implementation encapsulates the functionality of malloc(), free() and
499 realloc() in one call, which is why it is useful to be able to pass
500 around a single function pointer.
502 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
503 void *talloc_autofree_context(void);
505 This is a handy utility function that returns a talloc context
506 which will be automatically freed on program exit. This can be used
507 to reduce the noise in memory leak reports.