This invention provides a system and method for instrumenting an executable file by replacing an original function in the executable file with a user-supplied function and enabling the user-supplied function to invoke the original function. In particular, the executable file includes an access to the original function. To instrument the executable file, the access to the original function is replaced with an access to the user-supplied function. Access information associated with the original function is retained, which enables the user-supplied function to locate the original function.