Dynamic instrumentation of an executable program by means of causing a breakpoint at the entry point of a function and providing instrumentation code

Method and apparatus for dynamic instrumentation of an executable application program. The application program includes a plurality of functions, each function having an entry point and an endpoint. When the application is executed, a shared memory segment is created for an instrumentation program and the application program. Upon initial invocation of the original functions in the application program, corresponding substitute functions are created in the shared memory segment, the substitute versions including instrumentation code. Thereafter, the substitute functions are executed in lieu of the original functions in the application program.