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_unreference(const void *context, const void *ptr);
130 The talloc_unreference() function removes a reference added by
131 talloc_reference(). It must be called with exactly the same arguments
132 as talloc_reference().
134 Note that if the reference has already been removed using
135 talloc_free() then this function will fail and will return NULL.
138 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
139 void talloc_set_destructor(const void *ptr, int (*destructor)(void *));
141 The function talloc_set_destructor() sets the "destructor" for the
142 pointer "ptr". A destructor is a function that is called when the
143 memory used by a pointer is about to be released. The destructor
144 receives the pointer as an argument, and should return 0 for success
147 The destructor can do anything it wants to, including freeing other
148 pieces of memory. A common use for destructors is to clean up
149 operating system resources (such as open file descriptors) contained
150 in the structure the destructor is placed on.
152 You can only place one destructor on a pointer. If you need more than
153 one destructor then you can create a zero-length child of the pointer
154 and place an additional destructor on that.
156 To remove a destructor call talloc_set_destructor() with NULL for the
159 If your destructor attempts to talloc_free() the pointer that it is
160 the destructor for then talloc_free() will return -1 and the free will
161 be ignored. This would be a pointless operation anyway, as the
162 destructor is only called when the memory is just about to go away.
165 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
166 void talloc_increase_ref_count(const void *ptr);
168 The talloc_increase_ref_count(ptr) function is exactly equivalent to:
170 talloc_reference(NULL, ptr);
172 You can use either syntax, depending on which you think is clearer in
176 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
177 void talloc_set_name(const void *ptr, const char *fmt, ...);
179 Each talloc pointer has a "name". The name is used principally for
180 debugging purposes, although it is also possible to set and get the
181 name on a pointer in as a way of "marking" pointers in your code.
183 The main use for names on pointer is for "talloc reports". See
184 talloc_report() and talloc_report_full() for details. Also see
185 talloc_enable_leak_report() and talloc_enable_leak_report_full().
188 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
189 void talloc_set_name_const(const void *ptr, const char *name);
191 The function talloc_set_name_const() is just like talloc_set_name(),
192 but it takes a string constant, and is much faster. It is extensively
193 used by the "auto naming" macros, such as talloc_p().
196 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
197 void *talloc_named(const void *context, size_t size, const char *fmt, ...);
199 The talloc_named() function creates a named talloc pointer. It is
202 ptr = talloc(context, size);
203 talloc_set_name(ptr, fmt, ....);
206 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
207 void *talloc_named_const(const void *context, size_t size, const char *name);
209 This is equivalent to:
211 ptr = talloc(context, size);
212 talloc_set_name_const(ptr, name);
215 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
216 const char *talloc_get_name(const void *ptr);
218 This returns the current name for the given talloc pointer. See
219 talloc_set_name() for details.
222 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
223 void *talloc_init(const char *fmt, ...);
225 This function creates a zero length named talloc context as a top
226 level context. It is equivalent to:
228 talloc_named(NULL, 0, fmt, ...);
231 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
232 void *talloc_realloc(const void *context, void *ptr, size_t size);
234 The talloc_realloc() function changes the size of a talloc
235 pointer. It has the following equivalences:
237 talloc_realloc(context, NULL, size) ==> talloc(context, size);
238 talloc_realloc(context, ptr, 0) ==> talloc_free(ptr);
240 The "context" argument is only used if "ptr" is not NULL, otherwise it
243 talloc_realloc() returns the new pointer, or NULL on failure. The call
244 will fail either due to a lack of memory, or because the pointer has
245 an reference (see talloc_reference()).
248 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
249 void *talloc_steal(const void *new_ctx, const void *ptr);
251 The talloc_steal() function changes the parent context of a talloc
252 pointer. It is typically used when the context that the pointer is
253 currently a child of is going to be freed and you wish to keep the
254 memory for a longer time.
256 The talloc_steal() function returns the pointer that you pass it. It
257 does not have any failure modes.
259 NOTE: It is possible to produce loops in the parent/child relationship
260 if you are not careful with talloc_steal(). No guarantees are provided
261 as to your sanity or the safety of your data if you do this.
264 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
265 off_t talloc_total_size(const void *ptr);
267 The talloc_total_size() function returns the total size in bytes used
268 by this pointer and all child pointers. Mostly useful for debugging.
271 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
272 void talloc_report(const void *ptr, FILE *f);
274 The talloc_report() function prints a summary report of all memory
275 used by ptr. One line of report is printed for each immediate child of
276 ptr, showing the total memory and number of blocks used by that child.
278 You can pass NULL for the pointer, in which case a report is printed
279 for the top level memory context.
282 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
283 void talloc_report_full(const void *ptr, FILE *f);
285 This provides a more detailed report than talloc_report(). It will
286 recursively print the ensire tree of memory referenced by the
287 pointer. References in the tree are shown by giving the name of the
288 pointer that is referenced.
290 You can pass NULL for the pointer, in which case a report is printed
291 for the top level memory context.
294 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
295 void talloc_enable_leak_report(void);
297 This enables calling of talloc_report(NULL, stderr) when the program
298 exits. In Samba4 this is enabled by using the --leak-report command
302 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
303 void talloc_enable_leak_report_full(void);
305 This enables calling of talloc_report_full(NULL, stderr) when the
306 program exits. In Samba4 this is enabled by using the
307 --leak-report-full command line option.
310 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
311 void *talloc_zero(const void *ctx, size_t size);
313 The talloc_zero() function is equivalent to:
315 ptr = talloc(ctx, size);
316 if (ptr) memset(ptr, 0, size);
319 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
320 void *talloc_memdup(const void *ctx, const void *p, size_t size);
322 The talloc_memdup() function is equivalent to:
324 ptr = talloc(ctx, size);
325 if (ptr) memcpy(ptr, p, size);
328 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
329 char *talloc_strdup(const void *ctx, const char *p);
331 The talloc_strdup() function is equivalent to:
333 ptr = talloc(ctx, strlen(p)+1);
334 if (ptr) memcpy(ptr, p, strlen(p)+1);
337 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
338 char *talloc_strndup(const void *t, const char *p, size_t n);
340 The talloc_strndup() function is the talloc equivalent of the C
341 library function strndup()
344 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
345 char *talloc_vasprintf(const void *t, const char *fmt, va_list ap);
347 The talloc_vasprintf() function is the talloc equivalent of the C
348 library function vasprintf()
351 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
352 char *talloc_asprintf(const void *t, const char *fmt, ...);
354 The talloc_asprintf() function is the talloc equivalent of the C
355 library function asprintf()
358 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
359 char *talloc_asprintf_append(char *s, const char *fmt, ...);
361 The talloc_asprintf_append() function appends the given formatted
362 string to the given string.
365 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
366 void *talloc_array_p(const void *ctx, type, uint_t count);
368 The talloc_array_p() macro is equivalent to:
370 (type *)talloc(ctx, sizeof(type) * count);
372 except that it provides integer overflow protection for the multiply,
373 returning NULL if the multiply overflows.
376 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
377 void *talloc_realloc_p(const void *ctx, void *ptr, type, uint_t count);
379 The talloc_realloc_p() macro is equivalent to:
381 (type *)talloc_realloc(ctx, ptr, sizeof(type) * count);
383 except that it provides integer overflow protection for the multiply,
384 returning NULL if the multiply overflows.