1libtalloc_bestpractices(3) talloc libtalloc_bestpractices(3)
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6 libtalloc_bestpractices - Chapter 7: Best practises The following
7 sections contain several best practices and good manners that were
8 found by the Samba and SSSD developers over the years.
9 These will help you to write code which is better, easier to debug and
11 with as few (hopefully none) memory leaks as possible.
12
14 The talloc is a hierarchy memory allocator. The hierarchy nature is
15 what makes the programming more error proof. It makes the memory easier
16 to manage and to free. Therefore, the first thing we should have on our
17 mind is: always project your data structures into the talloc context
18 hierarchy.
19 That means if we have a structure, we should always use it as a parent
21 context for its elements. This way we will not encounter any troubles
22 when freeing the structure or when changing its parent. The same rule
23 applies for arrays.
24 For example, the structure user from section Hierarchy of talloc
26 context should be created with the context hierarchy illustrated on the
27 next image.
28
30 It is a good practice to create a temporary talloc context at the
31 function beginning and free the context just before the return
32 statement. All the data must be allocated on this context or on its
33 children. This ensures that no memory leaks are created as long as we
34 do not forget to free the temporary context.
35 This pattern applies to both situations - when a function does not
37 return any dynamically allocated value and when it does. However, it
38 needs a little extension for the latter case.
39 Functions that do not return any dynamically allocated
41 value
42 If the function does not return any value created on the heap, we will
44 just obey the aforementioned pattern.
45 int bar()
47 {
48 int ret;
49 TALLOC_CTX *tmp_ctx = talloc_new(NULL);
50 if (tmp_ctx == NULL) {
51 ret = ENOMEM;
52 goto done;
53 }
54 /* allocate data on tmp_ctx or on its descendants */
55 ret = EOK;
56 done:
57 talloc_free(tmp_ctx);
58 return ret;
59 }
60 Functions returning dynamically allocated values
62 If our function returns any dynamically allocated data, its first
63 parameter should always be the destination talloc context. This context
64 serves as a parent for the output values. But again, we will create the
65 output values as the descendants of the temporary context. If
66 everything goes well, we will change the parent of the output values
67 from the temporary to the destination talloc context.
68 This pattern ensures that if an error occurs (e.g. I/O error or
70 insufficient amount of the memory), all allocated data is freed and no
71 garbage appears on the destination context.
72 int struct_foo_init(TALLOC_CTX *mem_ctx, struct foo **_foo)
74 {
75 int ret;
76 struct foo *foo = NULL;
77 TALLOC_CTX *tmp_ctx = talloc_new(NULL);
78 if (tmp_ctx == NULL) {
79 ret = ENOMEM;
80 goto done;
81 }
82 foo = talloc_zero(tmp_ctx, struct foo);
83 /* ... */
84 *_foo = talloc_steal(mem_ctx, foo);
85 ret = EOK;
86 done:
87 talloc_free(tmp_ctx);
88 return ret;
89 }
90
92 As it can be seen on the previous listing, instead of allocating the
93 temporary context directly on mem_ctx, we created a new top level
94 context using NULL as the parameter for talloc_new() function. Take a
95 look at the following example:
96 char *create_user_filter(TALLOC_CTX *mem_ctx,
98 uid_t uid, const char *username)
99 {
100 char *filter = NULL;
101 char *sanitized_username = NULL;
102 /* tmp_ctx is a child of mem_ctx */
103 TALLOC_CTX *tmp_ctx = talloc_new(mem_ctx);
104 if (tmp_ctx == NULL) {
105 return NULL;
106 }
107 sanitized_username = sanitize_string(tmp_ctx, username);
109 if (sanitized_username == NULL) {
110 talloc_free(tmp_ctx);
111 return NULL;
112 }
113 filter = talloc_aprintf(tmp_ctx,"(|(uid=%llu)(uname=%s))",
115 uid, sanitized_username);
116 if (filter == NULL) {
117 return NULL; /* tmp_ctx is not freed */ (*@el{lst:tmp-ctx-3:leak}@*)
118 }
119 /* filter becomes a child of mem_ctx */
121 filter = talloc_steal(mem_ctx, filter);
122 talloc_free(tmp_ctx);
123 return filter;
124 }
125 We forgot to free tmp_ctx before the return statement in the filter ==
127 NULL condition. However, it is created as a child of mem_ctx context
128 and as such it will be freed as soon as the mem_ctx is freed.
129 Therefore, no detectable memory leak is created.
130 On the other hand, we do not have any way to access the allocated data
132 and for all we know mem_ctx may exist for the lifetime of our
133 application. For these reasons this should be considered as a memory
134 leak. How can we detect if it is unreferenced but still attached to its
135 parent context? The only way is to notice the mistake in the source
136 code.
137 But if we create the temporary context as a top level context, it will
139 not be freed and memory diagnostic tools (e.g. valgrind) are able to do
140 their job.
141
143 If we want to take the advantage of the talloc pool but also keep to
144 the pattern introduced in the previous section, we are unable to do it
145 directly. The best thing to do is to create a conditional build where
146 we can decide how do we want to create the temporary context. For
147 example, we can create the following macros:
148 #ifdef USE_POOL_CONTEXT
150 #define CREATE_POOL_CTX(ctx, size) talloc_pool(ctx, size)
151 #define CREATE_TMP_CTX(ctx) talloc_new(ctx)
152 #else
153 #define CREATE_POOL_CTX(ctx, size) talloc_new(ctx)
154 #define CREATE_TMP_CTX(ctx) talloc_new(NULL)
155 #endif
156 Now if our application is under development, we will build it with
158 macro USE_POOL_CONTEXT undefined. This way, we can use memory
159 diagnostic utilities to detect memory leaks.
160 The release version will be compiled with the macro defined. This will
162 enable pool contexts and therefore reduce the malloc() calls, which
163 will end up in a little bit faster processing.
164 int struct_foo_init(TALLOC_CTX *mem_ctx, struct foo **_foo)
166 {
167 int ret;
168 struct foo *foo = NULL;
169 TALLOC_CTX *tmp_ctx = CREATE_TMP_CTX(mem_ctx);
170 /* ... */
171 }
172 errno_t handle_request(TALLOC_CTX mem_ctx)
174 {
175 int ret;
176 struct foo *foo = NULL;
177 TALLOC_CTX *pool_ctx = CREATE_POOL_CTX(NULL, 1024);
178 ret = struct_foo_init(mem_ctx, &foo);
179 /* ... */
180 }
181Version 2.0 Tue Oct 30 2018 libtalloc_bestpractices(3)