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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".
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