How Fil works

Fil uses the LD_PRELOAD/DYLD_INSERT_LIBRARIES mechanism to preload a shared library at process startup. This is why Fil can’t be used as regular library and needs to be started in a special way: it requires setting up the correct environment before Python starts.

This shared library intercepts all the low-level C memory allocation and deallocation API calls, and keeps track of the corresponding allocation. For example, instead of a malloc() memory allocation going directly to your operating system, Fil will intercept it, keep note of the allocation, and then call the underlying implementation of malloc().

At the same time, the Python tracing infrastructure (the same infrastructure used by cProfile and is used to figure out which Python callstack/backtrace is responsible for each allocation.