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77 | >1.3. The hooks</A |
78 | ></H1 |
79 | ><P |
80 | > The before and after trace hooks only exists to be able to generate a report at |
81 | the end of a trace computation. The effective computation is done by the event |
82 | hooks. |
83 | </P |
84 | ><P |
85 | > These hooks does particular computation on data arriving as argument, a |
86 | call_data. The type of the call_data, when a hook is called during the trace |
87 | read, is a traceset context. It contains all the necessary information about the |
88 | read in progress. This is the base class from which inherits trace set |
89 | state, and trace set/trace/tracefile state is the base classe of trace |
90 | set/trace/tracefile statistics. All these types can be casted to another without |
91 | problem (a TracesetState, for example, can be casted to a TracesetContext, but |
92 | it's not true for the casting between a TraceContext and a TracesetContext, see |
93 | the chapter "How to use the trace reading context" for details). They offer the |
94 | input data and they give a container (the attributes of the trace set/trace/tracefile |
95 | statistics) to write the output of this hook. |
96 | </P |
97 | ><P |
98 | > The idea behind writing in the attributes container is to provide an extensible |
99 | way of storing any type of information. For example, a specific module that adds |
100 | statistics to a trace can store them there, and the statistic printout will |
101 | automatically include the results produced by the specific module. |
102 | </P |
103 | ><P |
104 | > Output data does not necessarily need to be stored in such a global container |
105 | though. If we think of data of which we need to keed track during the execution, |
106 | an event counter for example, we should create our own data structure that |
107 | contains this counter, and pass the address of the allocated structure as the |
108 | hook_data parameter of the hook list creation function. That way, the hook will |
109 | be called with its hook_data as first parameter, which it can read and write. We |
110 | can think of this structure as the data related to the function that persists |
111 | between each call to the hook. You must make sure that you cast the hook_data to |
112 | the type of the structure before you use it in the hook function. |
113 | </P |
114 | ><P |
115 | > The detail about how to access the different fields of the reading context (the |
116 | hook's call_data) will be discussed in the chapter "How to use the trace |
117 | reading context". |
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