1libtalloc_dts(3) talloc libtalloc_dts(3)
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6 libtalloc_dts - Chapter 3: Dynamic type system
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10 Generic programming in the C language is very difficult. There is no
11 inheritance nor templates known from object oriented languages. There
12 is no dynamic type system. Therefore, generic programming in this
13 language is usually done by type-casting a variable to void* and
14 transferring it through a generic function to a specialized callback as
15 illustrated on the next listing.
16 void generic_function(callback_fn cb, void *pvt)
18 {
19 /* do some stuff and call the callback */
20 cb(pvt);
21 }
22 void specific_callback(void *pvt)
24 {
25 struct specific_struct *data;
26 data = (struct specific_struct*)pvt;
27 /* ... */
28 }
29 void specific_function()
31 {
32 struct specific_struct data;
33 generic_function(callback, &data);
34 }
35 Unfortunately, the type information is lost as a result of this type
37 cast. The compiler cannot check the type during the compilation nor are
38 we able to do it at runtime. Providing an invalid data type to the
39 callback will result in unexpected behaviour (not necessarily a crash)
40 of the application. This mistake is usually hard to detect because it
41 is not the first thing which comes the mind.
42 As we already know, every talloc context contains a name. This name is
44 available at any time and it can be used to determine the type of a
45 context even if we lose the type of a variable.
46 Although the name of the context can be set to any arbitrary string,
48 the best way of using it to simulate the dynamic type system is to set
49 it directly to the type of the variable.
50 It is recommended to use one of talloc() and talloc_array() (or its
52 variants) to create the context as they set its name to the name of the
53 given type automatically.
54 If we have a context with such as a name, we can use two similar
56 functions that do both the type check and the type cast for us:
57 · talloc_get_type()
59 · talloc_get_type_abort()
61
63 The following example will show how generic programming with talloc is
64 handled - if we provide invalid data to the callback, the program will
65 be aborted. This is a sufficient reaction for such an error in most
66 applications.
67 void foo_callback(void *pvt)
69 {
70 struct foo *data = talloc_get_type_abort(pvt, struct foo);
71 /* ... */
72 }
73 int do_foo()
75 {
76 struct foo *data = talloc_zero(NULL, struct foo);
77 /* ... */
78 return generic_function(foo_callback, data);
79 }
80 But what if we are creating a service application that should be
82 running for the uptime of a server, we may want to abort the
83 application during the development process (to make sure the error is
84 not overlooked) and try to recover from the error in the customer
85 release. This can be achieved by creating a custom abort function with
86 a conditional build.
87 void my_abort(const char *reason)
89 {
90 fprintf(stderr, "talloc abort: %s0, reason);
91 #ifdef ABORT_ON_TYPE_MISMATCH
92 abort();
93 #endif
94 }
95 The usage of talloc_get_type_abort() would be then:
97 talloc_set_abort_fn(my_abort);
99 TALLOC_CTX *ctx = talloc_new(NULL);
101 char *str = talloc_get_type_abort(ctx, char);
102 if (str == NULL) {
103 /* recovery code */
104 }
105 /* talloc abort: ../src/main.c:25: Type mismatch:
106 name[talloc_new: ../src/main.c:24] expected[char] */
107Version 2.0 Sat May 11 2019 libtalloc_dts(3)