Using the Python API's to create custom breakpoints
+Another use of the Python API's in lldb is to create a custom breakpoint resolver. This facility + was added in r342259. +
++ It allows you to provide the algorithm which will be used in the breakpoint's + search of the space of the code in a given Target + to determine where to set the breakpoint locations - the actual places where the breakpoint will trigger. + To understand how this works you need to know a little about how lldb handles breakpoints. +
++ In lldb, a breakpoint is composed of three parts: the Searcher, the Resolver, and the Stop Options. The Searcher and + Resolver cooperate to determine how breakpoint locations are set and differ between each breakpoint type. + Stop options determine what happens when a location triggers and includes the commands, conditions, ignore counts, etc. + Stop options are common between all breakpoint types, so for our purposes only the Searcher and Resolver are relevant. +
++ The Searcher's job is to traverse in a structured way the code in the current target. It + proceeds from the Target, to search all the Modules in the Target, in each Module it can recurse + into the Compile Units in that module, and within each Compile Unit it can recurse over the Functions + it contains. +
++ The Searcher can be provided with a SearchFilter that it will use to restrict this search. For instance, if the + SearchFilter specifies a list of Modules, the Searcher will not recurse into Modules that aren't on the list. + When you pass the -s modulename flag to break set you are creating a Module-based search filter. + When you pass -f filename.c to break set -n you are creating a file based search filter. If neither + of these is specified, the breakpoint will have a no-op search filter, so all parts of the program are searched + and all locations accepted. +
++ The Resolver has two functions. The most important one is the callback it provides. This will get called at the appropriate time + in the course of the search. The callback is where the job of adding locations to the breakpoint gets done. +
++ The other function is specifying to the Searcher at what depth in the above described recursion it wants to be + called. Setting a search depth also provides a stop for the recursion. For instance, if you request a Module depth + search, then the callback will be called for each Module as it gets added to the Target, but the searcher will not recurse into the + Compile Units in the module. +
++ One other slight sublety is that the depth at which you get called back is not necessarily the depth at which the + the SearchFilter is specified. For instance, if you are doing symbol searches, it is convenient to use the Module + depth for the search, since symbols are stored in the module. + But the SearchFilter might specify some subset of CompileUnits, so not all the symbols you might find in each module + will pass the search. You don't need to + handle this situation yourself, since SBBreakpoint::AddLocation will only add locations that pass the Search Filter. + This API returns an SBError to inform you whether your location was added. +
++ When the breakpoint is originally created, its Searcher will process all the currently loaded modules. + The Searcher will also visit any new modules as they are added to the target. This happens, for instance, when + a new shared library gets added to the target in the course of running, or on rerunning if any of the currently + loaded modules have been changed. Note, in the latter case, all the locations set in the old module will get + deleted and you will be asked to recreate them in the new version of the module when your callback gets called + with that module. For this reason, you shouldn't + try to manage the locations you add to the breakpoint yourself. Note that the Breakpoint takes care of + deduplicating equal addresses in AddLocation, so you shouldn't need to worry about that anyway. +
++ At present, when adding a scripted Breakpoint type, you can only provide a custom Resolver, not a custom SearchFilter. +
++ The custom Resolver is provided as a Python class with the following methods: +
+ +| Name | +Arguments | +Description | +
| + __init__ + | ++ bkpt: lldb.SBBreakpoint + extra_args: lldb.SBStructuredData + | +
+ + This is the constructor for the new Resolver. + ++ bkpt is the breakpoint owning this Resolver. + ++ extra_args is an SBStructuredData object that the user can pass in when creating instances of this + breakpoint. It is not required, but is quite handy. For instance if you were implementing a breakpoint on some + symbol name, you could write a generic symbol name based Resolver, and then allow the user to pass + in the particular symbol in the extra_args + |
+
| + __callback__ + | ++ sym_ctx: lldb.SBSymbolContext + | +
+ This is the Resolver callback.
+ The sym_ctx argument will be filled with the current stage
+ of the search.
+
+ + For instance, if you asked for a search depth of lldb.eSearchDepthCompUnit, then the + target, module and compile_unit fields of the sym_ctx will be filled. The callback should look just in the + context passed in sym_ctx for new locations. If the callback finds an address of interest, it + can add it to the breakpoint with the SBBreakpoint::AddLocation method, using the breakpoint passed + in to the __init__ method. + |
+
| + __get_depth__ + | ++ None + | +
+ Specify the depth at which you wish your callback to get called. The currently supported options are:
+
|
+
| + get_short_help + | ++ None + | ++ This is an optional method. If provided, the returned string will be printed at the beginning of + the description for this breakpoint. + | +
To define a new breakpoint command defined by this class from the lldb command line, use the command:
+ +(lldb) breakpoint set -P MyModule.MyResolverClass
+
You can also populate the extra_args SBStructuredData with a dictionary of key/value pairs with:
+ +(lldb) breakpoint set -P MyModule.MyResolverClass -k key_1 -v value_1 -k key_2 -v value_2
+
Although you can't write a scripted SearchFilter, both the command line and the SB API's for adding a + scripted resolver allow you to specify a SearchFilter restricted to certain modules or certain compile + units. When using the command line to create the resolver, you can specify a Module specific SearchFilter + by passing the -s ModuleName option - which can be specified multiple times. + You can also specify a SearchFilter restricted to certain + compile units by passing in the -f CompUnitName option. This can also be specified more than + once. And you can mix the two to specify "this comp unit in this module". So, for instance, +
+ +(lldb) breakpoint set -P MyModule.MyResolverClass -s a.out
+
+ will use your resolver, but will only recurse into or accept new locations in the module a.out. +
+ +Another option for creating scripted breakpoints is to use the SBTarget.CreateBreakpointFromScript API. + This one has the advantage that you can pass in an arbitrary SBStructuredData object, so you can + create more complex parametrizations. + SBStructuredData has a handy SetFromJSON method which you can use for this purpose. + Your __init__ function gets passed this SBStructuredData object. + This API also allows you to directly provide the list of Modules and the list of CompileUnits that will + make up the SearchFilter. If you pass in empty lists, the breakpoint will use the default "search everywhere,accept + everything" filter. +
+ +