Native profiling

Version 0.4+ is able to profile native functions (routines written in a different language like C) on Mac OS X and Linux. See below for a technical overview.

By default this feature is enabled. To disable native profiling add --no-native as a command line switch.

NOTE be sure to provide debugging symbols for your native functions, otherwise you will not see the symbol name of your e.g. C program.

Technical Design

Native sampling utilizes libunwind in the signal handler to unwind the stack.

Each stack frame is inspected until the frame evaluation function is encountered. Then the stack walking switches back to the traditional Python frame walking. Callbacks (Python frame -> ... C frame ... -> Python frame ->

C frame)

will not display intermediate native functions. It would give the impression that the first C frame was never called, but it will show the second C frame.

Earlier Implementation

Prior to 0.4.3 the following logic was implemented (see e.g. commit 3912330b509d). It was removed because it could not be implemented on Mac OS X (libunwind misses register/cancel functions for generated machine code).

To find the corresponding PyFrameObject during stack unwinding vmprof inserts a trampoline on CPython (called vmprof_eval) and places it just before PyEval_EvalFrameEx. It is a callee trampoline saving the PyFrameObject in the callee saved register %rbx. On Python 3.6+ the frame evaluation PEP 523 is utilized as trampoline.