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| 56 | >Chapter 1. Linux Trace Toolkit Viewer Text Module Tutorial</TD |
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| 77 | >1.3. The hooks</A |
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| 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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