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  77 >1.3. The hooks</A
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  79 ><P
  80 >&#13;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 >&#13;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 >&#13;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 >&#13;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 >&#13;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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