7 The most current version of this document is available at
8 http://samba.org/ftp/unpacked/samba4/source/lib/talloc/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(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 testsuite to
36 watch talloc in action. You may also like to add your own tests to
37 testsuite.c to clarify how some particular situation is handled.
43 All the additional features of talloc() over malloc() do come at a
44 price. We have a simple performance test in Samba4 that measures
45 talloc() versus malloc() performance, and it seems that talloc() is
46 about 10% slower than malloc() on my x86 Debian Linux box. For Samba,
47 the great reduction in code complexity that we get by using talloc
48 makes this worthwhile, especially as the total overhead of
49 talloc/malloc in Samba is already quite small.
55 The following is a complete guide to the talloc API. Read it all at
59 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
60 (type *)talloc(const void *context, type);
62 The talloc() macro is the core of the talloc library. It takes a
63 memory context and a type, and returns a pointer to a new area of
64 memory of the given type.
66 The returned pointer is itself a talloc context, so you can use it as
67 the context argument to more calls to talloc if you wish.
69 The returned pointer is a "child" of the supplied context. This means
70 that if you talloc_free() the context then the new child disappears as
71 well. Alternatively you can free just the child.
73 The context argument to talloc() can be NULL, in which case a new top
74 level context is created.
77 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
78 void *talloc_size(const void *context, size_t size);
80 The function talloc_size() should be used when you don't have a
81 convenient type to pass to talloc(). Unlike talloc(), it is not type
82 safe (as it returns a void *), so you are on your own for type checking.
85 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
86 int talloc_free(void *ptr);
88 The talloc_free() function frees a piece of talloc memory, and all its
89 children. You can call talloc_free() on any pointer returned by
92 The return value of talloc_free() indicates success or failure, with 0
93 returned for success and -1 for failure. The only possible failure
94 condition is if the pointer had a destructor attached to it and the
95 destructor returned -1. See talloc_set_destructor() for details on
98 If this pointer has an additional parent when talloc_free() is called
99 then the memory is not actually released, but instead the most
100 recently established parent is destroyed. See talloc_reference() for
101 details on establishing additional parents.
103 For more control on which parent is removed, see talloc_unlink()
105 talloc_free() operates recursively on its children.
108 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
109 void *talloc_reference(const void *context, const void *ptr);
111 The talloc_reference() function makes "context" an additional parent
114 The return value of talloc_reference() is always the original pointer
115 "ptr", unless talloc ran out of memory in creating the reference in
116 which case it will return NULL (each additional reference consumes
117 around 48 bytes of memory on intel x86 platforms).
119 If "ptr" is NULL, then the function is a no-op, and simply returns NULL.
121 After creating a reference you can free it in one of the following
124 - you can talloc_free() any parent of the original pointer. That
125 will reduce the number of parents of this pointer by 1, and will
126 cause this pointer to be freed if it runs out of parents.
128 - you can talloc_free() the pointer itself. That will destroy the
129 most recently established parent to the pointer and leave the
130 pointer as a child of its current parent.
132 For more control on which parent to remove, see talloc_unlink()
135 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
136 int talloc_unlink(const void *context, const void *ptr);
138 The talloc_unlink() function removes a specific parent from ptr. The
139 context passed must either be a context used in talloc_reference()
140 with this pointer, or must be a direct parent of ptr.
142 Note that if the parent has already been removed using talloc_free()
143 then this function will fail and will return -1. Likewise, if "ptr"
144 is NULL, then the function will make no modifications and return -1.
146 Usually you can just use talloc_free() instead of talloc_unlink(), but
147 sometimes it is useful to have the additional control on which parent
151 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
152 void talloc_set_destructor(const void *ptr, int (*destructor)(void *));
154 The function talloc_set_destructor() sets the "destructor" for the
155 pointer "ptr". A destructor is a function that is called when the
156 memory used by a pointer is about to be released. The destructor
157 receives the pointer as an argument, and should return 0 for success
160 The destructor can do anything it wants to, including freeing other
161 pieces of memory. A common use for destructors is to clean up
162 operating system resources (such as open file descriptors) contained
163 in the structure the destructor is placed on.
165 You can only place one destructor on a pointer. If you need more than
166 one destructor then you can create a zero-length child of the pointer
167 and place an additional destructor on that.
169 To remove a destructor call talloc_set_destructor() with NULL for the
172 If your destructor attempts to talloc_free() the pointer that it is
173 the destructor for then talloc_free() will return -1 and the free will
174 be ignored. This would be a pointless operation anyway, as the
175 destructor is only called when the memory is just about to go away.
178 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
179 void talloc_increase_ref_count(const void *ptr);
181 The talloc_increase_ref_count(ptr) function is exactly equivalent to:
183 talloc_reference(NULL, ptr);
185 You can use either syntax, depending on which you think is clearer in
189 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
190 void talloc_set_name(const void *ptr, const char *fmt, ...);
192 Each talloc pointer has a "name". The name is used principally for
193 debugging purposes, although it is also possible to set and get the
194 name on a pointer in as a way of "marking" pointers in your code.
196 The main use for names on pointer is for "talloc reports". See
197 talloc_report() and talloc_report_full() for details. Also see
198 talloc_enable_leak_report() and talloc_enable_leak_report_full().
200 The talloc_set_name() function allocates memory as a child of the
201 pointer. It is logically equivalent to:
202 talloc_set_name_const(ptr, talloc_asprintf(ptr, fmt, ...));
204 Note that multiple calls to talloc_set_name() will allocate more
205 memory without releasing the name. All of the memory is released when
206 the ptr is freed using talloc_free().
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().
216 This function does not allocate any memory. It just copies the
217 supplied pointer into the internal representation of the talloc
218 ptr. This means you must not pass a name pointer to memory that will
219 disappear before the ptr is freed with talloc_free().
222 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
223 void *talloc_named(const void *context, size_t size, const char *fmt, ...);
225 The talloc_named() function creates a named talloc pointer. It is
228 ptr = talloc_size(context, size);
229 talloc_set_name(ptr, fmt, ....);
232 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
233 void *talloc_named_const(const void *context, size_t size, const char *name);
235 This is equivalent to:
237 ptr = talloc_size(context, size);
238 talloc_set_name_const(ptr, name);
241 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
242 const char *talloc_get_name(const void *ptr);
244 This returns the current name for the given talloc pointer. See
245 talloc_set_name() for details.
248 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
249 void *talloc_init(const char *fmt, ...);
251 This function creates a zero length named talloc context as a top
252 level context. It is equivalent to:
254 talloc_named(NULL, 0, fmt, ...);
257 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
258 void *talloc_new(void *ctx);
260 This is a utility macro that creates a new memory context hanging
261 off an exiting context, automatically naming it "talloc_new: __location__"
262 where __location__ is the source line it is called from. It is
263 particularly useful for creating a new temporary working context.
266 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
267 (type *)talloc_realloc(const void *context, void *ptr, type, count);
269 The talloc_realloc() macro changes the size of a talloc
270 pointer. The "count" argument is the number of elements of type "type"
271 that you want the resulting pointer to hold.
273 talloc_realloc() has the following equivalences:
275 talloc_realloc(context, NULL, type, 1) ==> talloc(context, type);
276 talloc_realloc(context, NULL, type, N) ==> talloc_array(context, type, N);
277 talloc_realloc(context, ptr, type, 0) ==> talloc_free(ptr);
279 The "context" argument is only used if "ptr" is not NULL, otherwise it
282 talloc_realloc() returns the new pointer, or NULL on failure. The call
283 will fail either due to a lack of memory, or because the pointer has
284 more than one parent (see talloc_reference()).
287 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
288 void *talloc_realloc_size(const void *context, void *ptr, size_t size);
290 the talloc_realloc_size() function is useful when the type is not
291 known so the typesafe talloc_realloc() cannot be used.
294 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
295 void *talloc_steal(const void *new_ctx, const void *ptr);
297 The talloc_steal() function changes the parent context of a talloc
298 pointer. It is typically used when the context that the pointer is
299 currently a child of is going to be freed and you wish to keep the
300 memory for a longer time.
302 The talloc_steal() function returns the pointer that you pass it. It
303 does not have any failure modes.
305 NOTE: It is possible to produce loops in the parent/child relationship
306 if you are not careful with talloc_steal(). No guarantees are provided
307 as to your sanity or the safety of your data if you do this.
309 talloc_steal (new_ctx, NULL) will return NULL with no sideeffects.
311 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
312 off_t talloc_total_size(const void *ptr);
314 The talloc_total_size() function returns the total size in bytes used
315 by this pointer and all child pointers. Mostly useful for debugging.
317 Passing NULL is allowed, but it will only give a meaningful result if
318 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
322 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
323 off_t talloc_total_blocks(const void *ptr);
325 The talloc_total_blocks() function returns the total memory block
326 count used by this pointer and all child pointers. Mostly useful for
329 Passing NULL is allowed, but it will only give a meaningful result if
330 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
334 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
335 void talloc_report(const void *ptr, FILE *f);
337 The talloc_report() function prints a summary report of all memory
338 used by ptr. One line of report is printed for each immediate child of
339 ptr, showing the total memory and number of blocks used by that child.
341 You can pass NULL for the pointer, in which case a report is printed
342 for the top level memory context, but only if
343 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
347 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
348 void talloc_report_full(const void *ptr, FILE *f);
350 This provides a more detailed report than talloc_report(). It will
351 recursively print the ensire tree of memory referenced by the
352 pointer. References in the tree are shown by giving the name of the
353 pointer that is referenced.
355 You can pass NULL for the pointer, in which case a report is printed
356 for the top level memory context, but only if
357 talloc_enable_leak_report() or talloc_enable_leak_report_full() has
361 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
362 void talloc_enable_leak_report(void);
364 This enables calling of talloc_report(NULL, stderr) when the program
365 exits. In Samba4 this is enabled by using the --leak-report command
368 For it to be useful, this function must be called before any other
369 talloc function as it establishes a "null context" that acts as the
370 top of the tree. If you don't call this function first then passing
371 NULL to talloc_report() or talloc_report_full() won't give you the
374 Here is a typical talloc report:
376 talloc report on 'null_context' (total 267 bytes in 15 blocks)
377 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
378 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
379 iconv(UTF8,CP850) contains 42 bytes in 2 blocks
380 libcli/auth/spnego_parse.c:55 contains 31 bytes in 2 blocks
381 iconv(CP850,UTF8) contains 42 bytes in 2 blocks
382 iconv(UTF8,UTF-16LE) contains 45 bytes in 2 blocks
383 iconv(UTF-16LE,UTF8) contains 45 bytes in 2 blocks
386 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
387 void talloc_enable_leak_report_full(void);
389 This enables calling of talloc_report_full(NULL, stderr) when the
390 program exits. In Samba4 this is enabled by using the
391 --leak-report-full command line option.
393 For it to be useful, this function must be called before any other
394 talloc function as it establishes a "null context" that acts as the
395 top of the tree. If you don't call this function first then passing
396 NULL to talloc_report() or talloc_report_full() won't give you the
399 Here is a typical full report:
401 full talloc report on 'root' (total 18 bytes in 8 blocks)
402 p1 contains 18 bytes in 7 blocks (ref 0)
403 r1 contains 13 bytes in 2 blocks (ref 0)
405 p2 contains 1 bytes in 1 blocks (ref 1)
406 x3 contains 1 bytes in 1 blocks (ref 0)
407 x2 contains 1 bytes in 1 blocks (ref 0)
408 x1 contains 1 bytes in 1 blocks (ref 0)
411 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
412 void talloc_enable_null_tracking(void);
414 This enables tracking of the NULL memory context without enabling leak
415 reporting on exit. Useful for when you want to do your own leak
416 reporting call via talloc_report_null_full();
419 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
420 (type *)talloc_zero(const void *ctx, type);
422 The talloc_zero() macro is equivalent to:
424 ptr = talloc(ctx, type);
425 if (ptr) memset(ptr, 0, sizeof(type));
428 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
429 void *talloc_zero_size(const void *ctx, size_t size)
431 The talloc_zero_size() function is useful when you don't have a known type
434 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
435 void *talloc_memdup(const void *ctx, const void *p, size_t size);
437 The talloc_memdup() function is equivalent to:
439 ptr = talloc_size(ctx, size);
440 if (ptr) memcpy(ptr, p, size);
443 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
444 char *talloc_strdup(const void *ctx, const char *p);
446 The talloc_strdup() function is equivalent to:
448 ptr = talloc_size(ctx, strlen(p)+1);
449 if (ptr) memcpy(ptr, p, strlen(p)+1);
451 This functions sets the name of the new pointer to the passed
452 string. This is equivalent to:
453 talloc_set_name_const(ptr, ptr)
455 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
456 char *talloc_strndup(const void *t, const char *p, size_t n);
458 The talloc_strndup() function is the talloc equivalent of the C
459 library function strndup()
461 This functions sets the name of the new pointer to the passed
462 string. This is equivalent to:
463 talloc_set_name_const(ptr, ptr)
466 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
467 char *talloc_vasprintf(const void *t, const char *fmt, va_list ap);
469 The talloc_vasprintf() function is the talloc equivalent of the C
470 library function vasprintf()
473 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
474 char *talloc_asprintf(const void *t, const char *fmt, ...);
476 The talloc_asprintf() function is the talloc equivalent of the C
477 library function asprintf()
479 This functions sets the name of the new pointer to the passed
480 string. This is equivalent to:
481 talloc_set_name_const(ptr, ptr)
484 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
485 char *talloc_asprintf_append(char *s, const char *fmt, ...);
487 The talloc_asprintf_append() function appends the given formatted
488 string to the given string.
491 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
492 (type *)talloc_array(const void *ctx, type, uint_t count);
494 The talloc_array() macro is equivalent to:
496 (type *)talloc_size(ctx, sizeof(type) * count);
498 except that it provides integer overflow protection for the multiply,
499 returning NULL if the multiply overflows.
502 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
503 void *talloc_array_size(const void *ctx, size_t size, uint_t count);
505 The talloc_array_size() function is useful when the type is not
506 known. It operates in the same way as talloc_array(), but takes a size
510 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
511 void *talloc_realloc_fn(const void *ctx, void *ptr, size_t size);
513 This is a non-macro version of talloc_realloc(), which is useful
514 as libraries sometimes want a ralloc function pointer. A realloc()
515 implementation encapsulates the functionality of malloc(), free() and
516 realloc() in one call, which is why it is useful to be able to pass
517 around a single function pointer.
520 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
521 void *talloc_autofree_context(void);
523 This is a handy utility function that returns a talloc context
524 which will be automatically freed on program exit. This can be used
525 to reduce the noise in memory leak reports.
528 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
529 void *talloc_check_name(const void *ptr, const char *name);
531 This function checks if a pointer has the specified name. If it does
532 then the pointer is returned. It it doesn't then NULL is returned.
535 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
536 (type *)talloc_get_type(const void *ptr, type);
538 This macro allows you to do type checking on talloc pointers. It is
539 particularly useful for void* private pointers. It is equivalent to
542 (type *)talloc_check_name(ptr, #type)
545 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
546 talloc_set_type(const void *ptr, type);
548 This macro allows you to force the name of a pointer to be a
549 particular type. This can be used in conjunction with
550 talloc_get_type() to do type checking on void* pointers.
552 It is equivalent to this:
553 talloc_set_name_const(ptr, #type)