6.8 Using Trace Filters
A filter is a specialized client that receives trace messages and, unlike regular clients, also forwards these messages to other clients or filters.
To avoid losing trace messages, always connect all the filters and clients to each other first, before making a final connection from the first filter to the manager (see
Section 6.6.1).
6.8.1 Using the Predefined Filters
The Execution Tracing package provides two concrete filters,
RWTraceLevelFilter and
RWTraceMultiClientFilter.
6.8.1.1 Using RWTraceLevelFilter
The
RWTraceLevelFilter filter is used to dynamically filter trace events at runtime. The
setLevel() function can be used by your program to reset the filter’s cut-off level at any time during your program’s execution. This filter and two other ways of filtering trace events are compared in
Section 6.10, “Controlling Trace Output.”Example 54 shows how to use the
RWTraceLevelFilter class.
Example 54 – Implementing dynamic filtering of trace events
#define RW_USER_TRACE_LEVEL 8 //1
#include <rw/trace/trace.h>
int main()
{
// create trace client that logs messages to cerr
RWTraceOstreamClient myTraceClient(cerr); //2
// create filter that only lets info and more important events
// pass through
RWTraceLevelFilter myTraceLevelFilter(RW_TRACE_LEVEL_INFO); //3
// connect client to level filter
myTraceClient.connect(myTraceLevelFilter); //4
// connect level filter to manager
myTraceLevelFilter.connectToManager(); //5
RW_USER_TRACEABLE_FUNCTION("main”); //6
// generate some trace information
RW_USER_TRACE_INFO("This info level message should be seen.");
RW_USER_TRACE_WARNING("This warning level message should be seen.");
RW_USER_TRACE_ERROR("This error level message should be seen.");
// change the level
myTraceLevelFilter.setLevel(RW_TRACE_LEVEL_ERROR); //7
// generate some more trace information
RW_USER_TRACE_INFO("This info level message should not be seen.");
RW_USER_TRACE_WARNING("This warning level message should”
"not be seen.");
RW_USER_TRACE_ERROR("This error level message should be seen.");
return 0;
}
The symbolic constants referred to in line //3 are defined in the file rw/trace/RWTraceEventSeverity.h. They cannot be used when setting the RW_USER_TRACE_LEVEL macro, because this macro needs to be set before including any trace header files.
The constants are: RW_TRACE_LEVEL_FATAL, RW_TRACE_LEVEL_ERROR, RW_TRACE_LEVEL_WARNING, RW_TRACE_LEVEL_INFO, RW_TRACE_LEVEL_TEST, RW_TRACE_LEVEL_DEBUG, RW_TRACE_LEVEL_ENTRY and RW_TRACE_LEVEL_NONE. They all map directly to the appropriate severity level. RW_TRACE_LEVEL_NONE filters out all trace messages. It is equivalent to zero.
6.8.1.2 Using RWTraceMultiClientFilter
Sometimes it is desirable to connect multiple clients to the manager or a filter. The
RWTraceMultiClientFilter class facilitates this.
Example 55 creates two clients; one displays messages on the screen, and the other saves them to a file. This example can be found in
buildspace\examples\trace\example2.cpp.
Example 55 – Using two trace clients
#define RW_USER_TRACE_LEVEL 8
#include <rw/trace/trace.h>
#include <fstream.h>
int main()
{
ofstream traceLog("trace.log");
RWTraceOstreamClient myFileTraceClient(traceLog);
RWTraceOstreamClient myCerrTraceClient(cerr);
RWTraceMultiClientFilter myMultiFilter; // 1
// first: connect two clients to multi client filter
myFileTraceClient.connect(myMultiFilter); // 2
myCerrTraceClient.connect(myMultiFilter);
// last: connect filter to singleton manager
myMultiFilter.connectToManager(); // 3
RW_USER_TRACEABLE_FUNCTION("main”); // 4
RW_USER_TRACE_DEBUG("Picked up pencil.");
RW_USER_TRACE_TEST("Visual inspection of pencil complete.");
RW_USER_TRACE_INFO("Doodling!");
...
return 0;
}
6.8.2 Chaining Multiple Filters
Instead of using only one filter attached directly to the trace manager, you can chain together several filters. To connect them, use the downstream filter’s connect() member function and pass it the upstream filter as an argument, similar to this:
myMultiFilter2.connect(myMultiFilter1);
6.8.3 Creating User-defined Filters
Creating a user-defined filter follows the same pattern as creating a user-defined client. You must create a derived body class, where you specify the rules for filtering. If your filter’s body class has any new public members (in addition to those inherited from its base classes), you must also create a corresponding derived handle class. The predefined Trace Level filter was created in the same way that you would create your own filter, so the source code for
RWTraceLevelFilter and
RWTraceLevelFilterImp is an example.
6.8.3.1 Creating the Filter’s Body Class
The filter's implementation must derive from
RWTraceSingleClientFilterImp, for a single-client filter, or
RWTraceMultiClientFilterImp, for a multiple-client filter.
The doTrace() function. You specify the rules for filtering by redefining the
doTrace() member function, which is inherited from
RWTraceEventClientImp. The
RWTraceEventClientImp::doTrace() function is declared pure-virtual, so a derived body class must provide an implementation of this function.
For a single-client filter, check that the event meets your filter’s criteria and forward it to the connected client, using the
trace() member function.
Example 56 is taken from the source code for
RWTraceLevelFilterImp.
Example 56 – Creating a single-client filter
void
RWTraceLevelFilterImp::doTrace(const RWTraceEvent& ev) {
if (getClient() && (level_ >= ev.getSeverity()) )
getClient()->trace(ev);
}
For a multi-client filter, iterate over the connected clients and selectively forward the trace events, based on your filter's criteria.
Example 57 is taken from the source code for
RWTraceMultiClientFilterImp.
Example 57 – Creating a multi-client filter
void
RWTraceMultiClientFilterImp::doTrace(const RWTraceEvent& ev) {
{
RWGUARD(getMutex());
for(unsigned i=0; i < clientPs_.length(); i++ )
clientPs_(i)->trace(ev);
}
}
Do not embed trace macros in the filter’s trace event processing code. To prevent infinite recursion, the trace manager ignores any trace events generated by a filter.
The static make() function. If you are using the predefined handle class, your body class must supply a static
make() function. (If you are creating a derived handle class, you can skip to
Section 6.8.3.2.) The
make() function constructs a body and returns a handle to it. This example is taken from the source code for
RWTraceLevelFilterImp.
Example 58 – Creating a filter body class to use with the predefined handle class
RWTraceSingleClientFilter
RWTraceLevelFilterImp::make(RWTraceEventSeverity level) {
return new RWTraceLevelFilterImp(level);
}
The returned handle's type is actually the parent class of your derived handle.
6.8.3.2 Creating the Filter’s Handle Class
If your filter’s body class has any new public members (in addition to those inherited from its base classes), you must create a derived handle class that forwards those calls to your body. Derive your handle from
RWTraceMultiClientFilter, for a multiple-client filter, or
RWTraceSingleClientFilter, for a single-client filter. For an example, see the source code for
RWTraceLevelFilter:
6.8.3.3 Instantiating Your Derived Filter
If you created your own derived handle class, you can instantiate your derived filter like this:
MyMultiClientFilter myMultiFilter;
If you did not create your own derived handle class, you must explicitly instantiate a body with your body class’s
make() function and pass the body to a handle of type
RWTraceEventFilter, as shown in this example:
RWTraceEventFilter myTraceFilter ( Test2TraceFilterImp::make() );
Use your filter as you would any predefined filter, as discussed in
Section 6.8.1.