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 reference when talloc_free() is
105 called then the memory is not actually released, but instead the most
106 recently established reference is destroyed. See talloc_reference()
107 for details on establishing additional references.
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 returns an additional reference to
118 "ptr", and makes this additional reference a child of "context".
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 After creating a reference you can free it in one of the following
128 - you can talloc_free() a parent of the original pointer. That will
129 destroy the reference and make the pointer a child of the
130 "context" argument from the most recently called
131 talloc_reference() on the pointer.
133 - you can talloc_free() the pointer itself. That will destroy the
134 most recently established reference to the pointer and leave the
135 pointer as a child of its current parent.
137 - you can talloc_free() the context where you placed the
138 reference. That will destroy the reference, and leave the pointer
141 For more control on which parent to remove, see talloc_unlink()
144 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
145 int talloc_unlink(const void *context, const void *ptr);
147 The talloc_unlink() function removes a specific parent from ptr. The
148 context passed must either be a context used in talloc_reference()
149 with this pointer, or must be a direct parent of ptr.
151 Note that if the parent has already been removed using talloc_free()
152 then this function will fail and will return -1.
154 Usually you can just use talloc_free() instead of talloc_unlink(), but
155 sometimes it is useful to have the additional control on which parent
159 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
160 void talloc_set_destructor(const void *ptr, int (*destructor)(void *));
162 The function talloc_set_destructor() sets the "destructor" for the
163 pointer "ptr". A destructor is a function that is called when the
164 memory used by a pointer is about to be released. The destructor
165 receives the pointer as an argument, and should return 0 for success
168 The destructor can do anything it wants to, including freeing other
169 pieces of memory. A common use for destructors is to clean up
170 operating system resources (such as open file descriptors) contained
171 in the structure the destructor is placed on.
173 You can only place one destructor on a pointer. If you need more than
174 one destructor then you can create a zero-length child of the pointer
175 and place an additional destructor on that.
177 To remove a destructor call talloc_set_destructor() with NULL for the
180 If your destructor attempts to talloc_free() the pointer that it is
181 the destructor for then talloc_free() will return -1 and the free will
182 be ignored. This would be a pointless operation anyway, as the
183 destructor is only called when the memory is just about to go away.
186 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
187 void talloc_increase_ref_count(const void *ptr);
189 The talloc_increase_ref_count(ptr) function is exactly equivalent to:
191 talloc_reference(NULL, ptr);
193 You can use either syntax, depending on which you think is clearer in
197 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
198 void talloc_set_name(const void *ptr, const char *fmt, ...);
200 Each talloc pointer has a "name". The name is used principally for
201 debugging purposes, although it is also possible to set and get the
202 name on a pointer in as a way of "marking" pointers in your code.
204 The main use for names on pointer is for "talloc reports". See
205 talloc_report() and talloc_report_full() for details. Also see
206 talloc_enable_leak_report() and talloc_enable_leak_report_full().
209 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
210 void talloc_set_name_const(const void *ptr, const char *name);
212 The function talloc_set_name_const() is just like talloc_set_name(),
213 but it takes a string constant, and is much faster. It is extensively
214 used by the "auto naming" macros, such as talloc_p().
217 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
218 void *talloc_named(const void *context, size_t size, const char *fmt, ...);
220 The talloc_named() function creates a named talloc pointer. It is
223 ptr = talloc(context, size);
224 talloc_set_name(ptr, fmt, ....);
227 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
228 void *talloc_named_const(const void *context, size_t size, const char *name);
230 This is equivalent to:
232 ptr = talloc(context, size);
233 talloc_set_name_const(ptr, name);
236 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
237 const char *talloc_get_name(const void *ptr);
239 This returns the current name for the given talloc pointer. See
240 talloc_set_name() for details.
243 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
244 void *talloc_init(const char *fmt, ...);
246 This function creates a zero length named talloc context as a top
247 level context. It is equivalent to:
249 talloc_named(NULL, 0, fmt, ...);
252 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
253 void *talloc_realloc(const void *context, void *ptr, size_t size);
255 The talloc_realloc() function changes the size of a talloc
256 pointer. It has the following equivalences:
258 talloc_realloc(context, NULL, size) ==> talloc(context, size);
259 talloc_realloc(context, ptr, 0) ==> talloc_free(ptr);
261 The "context" argument is only used if "ptr" is not NULL, otherwise it
264 talloc_realloc() returns the new pointer, or NULL on failure. The call
265 will fail either due to a lack of memory, or because the pointer has
266 an reference (see talloc_reference()).
269 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
270 void *talloc_steal(const void *new_ctx, const void *ptr);
272 The talloc_steal() function changes the parent context of a talloc
273 pointer. It is typically used when the context that the pointer is
274 currently a child of is going to be freed and you wish to keep the
275 memory for a longer time.
277 The talloc_steal() function returns the pointer that you pass it. It
278 does not have any failure modes.
280 NOTE: It is possible to produce loops in the parent/child relationship
281 if you are not careful with talloc_steal(). No guarantees are provided
282 as to your sanity or the safety of your data if you do this.
285 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
286 off_t talloc_total_size(const void *ptr);
288 The talloc_total_size() function returns the total size in bytes used
289 by this pointer and all child pointers. Mostly useful for debugging.
291 Passing NULL is allowed, but it will only give a meaningful result if
292 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
296 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
297 off_t talloc_total_blocks(const void *ptr);
299 The talloc_total_blocks() function returns the total memory block
300 count used by this pointer and all child pointers. Mostly useful for
303 Passing NULL is allowed, but it will only give a meaningful result if
304 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
308 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
309 void talloc_report(const void *ptr, FILE *f);
311 The talloc_report() function prints a summary report of all memory
312 used by ptr. One line of report is printed for each immediate child of
313 ptr, showing the total memory and number of blocks used by that child.
315 You can pass NULL for the pointer, in which case a report is printed
316 for the top level memory context, but only if
317 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
321 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
322 void talloc_report_full(const void *ptr, FILE *f);
324 This provides a more detailed report than talloc_report(). It will
325 recursively print the ensire tree of memory referenced by the
326 pointer. References in the tree are shown by giving the name of the
327 pointer that is referenced.
329 You can pass NULL for the pointer, in which case a report is printed
330 for the top level memory context, but only if
331 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
335 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
336 void talloc_enable_leak_report(void);
338 This enables calling of talloc_report(NULL, stderr) when the program
339 exits. In Samba4 this is enabled by using the --leak-report command
342 For it to be useful, this function must be called before any other
343 talloc function as it establishes a "null context" that acts as the
344 top of the tree. If you don't call this function first then passing
345 NULL to talloc_report() or talloc_report_full() won't give you the
348 Here is a typical talloc report:
350 talloc report on 'null_context' (total 267 bytes in 15 blocks)
351 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
352 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
353 iconv(UTF8,CP850) contains 42 bytes in 2 blocks
354 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
355 iconv(CP850,UTF8) contains 42 bytes in 2 blocks
356 iconv(UTF8,UTF-16LE) contains 45 bytes in 2 blocks
357 iconv(UTF-16LE,UTF8) contains 45 bytes in 2 blocks
360 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
361 void talloc_enable_leak_report_full(void);
363 This enables calling of talloc_report_full(NULL, stderr) when the
364 program exits. In Samba4 this is enabled by using the
365 --leak-report-full command line option.
367 For it to be useful, this function must be called before any other
368 talloc function as it establishes a "null context" that acts as the
369 top of the tree. If you don't call this function first then passing
370 NULL to talloc_report() or talloc_report_full() won't give you the
373 Here is a typical full report:
375 full talloc report on 'root' (total 18 bytes in 8 blocks)
376 p1 contains 18 bytes in 7 blocks (ref 0)
377 r1 contains 13 bytes in 2 blocks (ref 0)
379 p2 contains 1 bytes in 1 blocks (ref 1)
380 x3 contains 1 bytes in 1 blocks (ref 0)
381 x2 contains 1 bytes in 1 blocks (ref 0)
382 x1 contains 1 bytes in 1 blocks (ref 0)
385 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
386 void *talloc_zero(const void *ctx, size_t size);
388 The talloc_zero() function is equivalent to:
390 ptr = talloc(ctx, size);
391 if (ptr) memset(ptr, 0, size);
394 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
395 void *talloc_memdup(const void *ctx, const void *p, size_t size);
397 The talloc_memdup() function is equivalent to:
399 ptr = talloc(ctx, size);
400 if (ptr) memcpy(ptr, p, size);
403 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
404 char *talloc_strdup(const void *ctx, const char *p);
406 The talloc_strdup() function is equivalent to:
408 ptr = talloc(ctx, strlen(p)+1);
409 if (ptr) memcpy(ptr, p, strlen(p)+1);
412 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
413 char *talloc_strndup(const void *t, const char *p, size_t n);
415 The talloc_strndup() function is the talloc equivalent of the C
416 library function strndup()
419 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
420 char *talloc_vasprintf(const void *t, const char *fmt, va_list ap);
422 The talloc_vasprintf() function is the talloc equivalent of the C
423 library function vasprintf()
426 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
427 char *talloc_asprintf(const void *t, const char *fmt, ...);
429 The talloc_asprintf() function is the talloc equivalent of the C
430 library function asprintf()
433 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
434 char *talloc_asprintf_append(char *s, const char *fmt, ...);
436 The talloc_asprintf_append() function appends the given formatted
437 string to the given string.
440 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
441 void *talloc_array_p(const void *ctx, type, uint_t count);
443 The talloc_array_p() macro is equivalent to:
445 (type *)talloc(ctx, sizeof(type) * count);
447 except that it provides integer overflow protection for the multiply,
448 returning NULL if the multiply overflows.
451 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
452 void *talloc_realloc_p(const void *ctx, void *ptr, type, uint_t count);
454 The talloc_realloc_p() macro is equivalent to:
456 (type *)talloc_realloc(ctx, ptr, sizeof(type) * count);
458 except that it provides integer overflow protection for the multiply,
459 returning NULL if the multiply overflows.
462 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
463 void *talloc_realloc_fn(const void *ctx, void *ptr, size_t size);
465 This is a non-macro version of talloc_realloc(), which is useful
466 as libraries sometimes want a ralloc function pointer. A realloc()
467 implementation encapsulates the functionality of malloc(), free() and
468 realloc() in one call, which is why it is useful to be able to pass
469 around a single function pointer.