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