A method instruments a function in an executable file so that the instrumented function calls a generic preprocessor prior to execution of the body of the function. After the preprocessor modifies the original function's incoming parameters, the body of the function itself is executed. Finally, execution is directed to a generic postprocessor prior to returning from the function. The postprocessor modifies the outgoing parameters and return value. In one implementation, the parameters of an instrumented function are described and packaged into a descriptor data structure. The descriptor data structure is passed to the generic preprocessor and postprocessor. A generic processor uses the descriptor to select changed behaviors based on the calling context.