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 The following is a complete guide to the talloc API. Read it all at
49 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
50 void *talloc(const void *context, size_t size);
52 The talloc() function is the core of the talloc library. It takes a
53 memory context, and returns a pointer to a new area of memory of the
56 The returned pointer is itself a talloc context, so you can use it as
57 the context argument to more calls to talloc if you wish.
59 The returned pointer is a "child" of the supplied context. This means
60 that if you talloc_free() the context then the new child disappears as
61 well. Alternatively you can free just the child.
63 The context argument to talloc() can be NULL, in which case a new top
64 level context is created.
67 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
68 void *talloc_p(const void *context, type);
70 The talloc_p() macro is the equivalent of
72 (type *)talloc(ctx, sizeof(type))
74 You should use it in preference to talloc() whenever possible, as it
75 provides additional type safety. It also automatically calls the
76 talloc_set_name_const() function with the name being a string holding
80 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
81 int talloc_free(void *ptr);
83 The talloc_free() function frees a piece of talloc memory, and all its
84 children. You can call talloc_free() on any pointer returned by
87 The return value of talloc_free() indicates success or failure, with 0
88 returned for success and -1 for failure. The only possible failure
89 condition is if the pointer had a destructor attached to it and the
90 destructor returned -1. See talloc_set_destructor() for details on
93 If this pointer has an additional reference when talloc_free() is
94 called then the memory is not actually released, but instead the
95 reference is destroyed and the memory becomes a child of the
96 referrer. See talloc_reference() for details on establishing
97 additional references.
99 talloc_free() operates recursively on its children.
102 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
103 void *talloc_reference(const void *context, const void *ptr);
105 The talloc_reference() function returns an additional reference to
106 "ptr", and makes this additional reference a child of "context".
108 The return value of talloc_reference() is always the original pointer
109 "ptr", unless talloc ran out of memory in creating the reference in
110 which case it will return NULL (each additional reference consumes
111 around 48 bytes of memory on intel x86 platforms).
113 After creating a reference you can free it in one of the following
116 - you can talloc_free() a parent of the original pointer. That will
117 destroy the reference and make the pointer a child of "context".
119 - you can talloc_free() the pointer itself. That will destroy the
120 reference and make the pointer a child of "context".
122 - you can talloc_free() the context where you placed the
123 reference. That will destroy the reference, and leave the pointer
127 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
128 void talloc_set_destructor(const void *ptr, int (*destructor)(void *));
130 The function talloc_set_destructor() sets the "destructor" for the
131 pointer "ptr". A destructor is a function that is called when the
132 memory used by a pointer is about to be released. The destructor
133 receives the pointer as an argument, and should return 0 for success
136 The destructor can do anything it wants to, including freeing other
137 pieces of memory. A common use for destructors is to clean up
138 operating system resources (such as open file descriptors) contained
139 in the structure the destructor is placed on.
141 You can only place one destructor on a pointer. If you need more than
142 one destructor then you can create a zero-length child of the pointer
143 and place an additional destructor on that.
145 To remove a destructor call talloc_set_destructor() with NULL for the
148 If your destructor attempts to talloc_free() the pointer that it is
149 the destructor for then talloc_free() will return -1 and the free will
150 be ignored. This would be a pointless operation anyway, as the
151 destructor is only called when the memory is just about to go away.
154 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
155 void talloc_increase_ref_count(const void *ptr);
157 The talloc_increase_ref_count(ptr) function is exactly equivalent to:
159 talloc_reference(NULL, ptr);
161 You can use either syntax, depending on which you think is clearer in
165 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
166 void talloc_set_name(const void *ptr, const char *fmt, ...);
168 Each talloc pointer has a "name". The name is used principally for
169 debugging purposes, although it is also possible to set and get the
170 name on a pointer in as a way of "marking" pointers in your code.
172 The main use for names on pointer is for "talloc reports". See
173 talloc_report() and talloc_report_full() for details. Also see
174 talloc_enable_leak_report() and talloc_enable_leak_report_full().
177 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
178 void talloc_set_name_const(const void *ptr, const char *name);
180 The function talloc_set_name_const() is just like talloc_set_name(),
181 but it takes a string constant, and is much faster. It is extensively
182 used by the "auto naming" macros, such as talloc_p().
185 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
186 void *talloc_named(const void *context, size_t size, const char *fmt, ...);
188 The talloc_named() function creates a named talloc pointer. It is
191 ptr = talloc(context, size);
192 talloc_set_name(ptr, fmt, ....);
195 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
196 void *talloc_named_const(const void *context, size_t size, const char *name);
198 This is equivalent to:
200 ptr = talloc(context, size);
201 talloc_set_name_const(ptr, name);
204 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
205 const char *talloc_get_name(const void *ptr);
207 This returns the current name for the given talloc pointer. See
208 talloc_set_name() for details.
211 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
212 void *talloc_init(const char *fmt, ...);
214 This function creates a zero length named talloc context as a top
215 level context. It is equivalent to:
217 talloc_named(NULL, 0, fmt, ...);
220 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
221 void *talloc_realloc(const void *context, void *ptr, size_t size);
223 The talloc_realloc() function changes the size of a talloc
224 pointer. It has the following equivalences:
226 talloc_realloc(context, NULL, size) ==> talloc(context, size);
227 talloc_realloc(context, ptr, 0) ==> talloc_free(ptr);
229 The "context" argument is only used if "ptr" is not NULL, otherwise it
232 talloc_realloc() returns the new pointer, or NULL on failure. The call
233 will fail either due to a lack of memory, or because the pointer has
234 an reference (see talloc_reference()).
237 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
238 void *talloc_steal(const void *new_ctx, const void *ptr);
240 The talloc_steal() function changes the parent context of a talloc
241 pointer. It is typically used when the context that the pointer is
242 currently a child of is going to be freed and you wish to keep the
243 memory for a longer time.
245 The talloc_steal() function returns the pointer that you pass it. It
246 does not have any failure modes.
248 NOTE: It is possible to produce loops in the parent/child relationship
249 if you are not careful with talloc_steal(). No guarantees are provided
250 as to your sanity or the safety of your data if you do this.
253 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
254 off_t talloc_total_size(const void *ptr);
256 The talloc_total_size() function returns the total size in bytes used
257 by this pointer and all child pointers. Mostly useful for debugging.
260 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
261 void talloc_report(const void *ptr, FILE *f);
263 The talloc_report() function prints a summary report of all memory
264 used by ptr. One line of report is printed for each immediate child of
265 ptr, showing the total memory and number of blocks used by that child.
267 You can pass NULL for the pointer, in which case a report is printed
268 for the top level memory context.
271 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
272 void talloc_report_full(const void *ptr, FILE *f);
274 This provides a more detailed report than talloc_report(). It will
275 recursively print the ensire tree of memory referenced by the
276 pointer. References in the tree are shown by giving the name of the
277 pointer that is referenced.
279 You can pass NULL for the pointer, in which case a report is printed
280 for the top level memory context.
283 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
284 void talloc_enable_leak_report(void);
286 This enables calling of talloc_report(NULL, stderr) when the program
287 exits. In Samba4 this is enabled by using the --leak-report command
291 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
292 void talloc_enable_leak_report_full(void);
294 This enables calling of talloc_report_full(NULL, stderr) when the
295 program exits. In Samba4 this is enabled by using the
296 --leak-report-full command line option.
299 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
300 void *talloc_zero(const void *ctx, size_t size);
302 The talloc_zero() function is equivalent to:
304 ptr = talloc(ctx, size);
305 if (ptr) memset(ptr, 0, size);
308 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
309 void *talloc_memdup(const void *ctx, const void *p, size_t size);
311 The talloc_memdup() function is equivalent to:
313 ptr = talloc(ctx, size);
314 if (ptr) memcpy(ptr, p, size);
317 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
318 char *talloc_strdup(const void *ctx, const char *p);
320 The talloc_strdup() function is equivalent to:
322 ptr = talloc(ctx, strlen(p)+1);
323 if (ptr) memcpy(ptr, p, strlen(p)+1);
326 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
327 char *talloc_strndup(const void *t, const char *p, size_t n);
329 The talloc_strndup() function is the talloc equivalent of the C
330 library function strndup()
333 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
334 char *talloc_vasprintf(const void *t, const char *fmt, va_list ap);
336 The talloc_vasprintf() function is the talloc equivalent of the C
337 library function vasprintf()
340 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
341 char *talloc_asprintf(const void *t, const char *fmt, ...);
343 The talloc_asprintf() function is the talloc equivalent of the C
344 library function asprintf()
347 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
348 char *talloc_asprintf_append(char *s, const char *fmt, ...);
350 The talloc_asprintf_append() function appends the given formatted
351 string to the given string.
354 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
355 void *talloc_array_p(const void *ctx, type, uint_t count);
357 The talloc_array_p() macro is equivalent to:
359 (type *)talloc(ctx, sizeof(type) * count);
361 except that it provides integer overflow protection for the multiply,
362 returning NULL if the multiply overflows.
365 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
366 void *talloc_realloc_p(const void *ctx, void *ptr, type, uint_t count);
368 The talloc_realloc_p() macro is equivalent to:
370 (type *)talloc_realloc(ctx, ptr, sizeof(type) * count);
372 except that it provides integer overflow protection for the multiply,
373 returning NULL if the multiply overflows.