The exact implementation of mymalloc isn't relevant to the post. I have an old allocator published at https://github.com/joelsiks/jsmalloc that I did as part of my Master's thesis, which uses a similar debug-logging mechanism that is described in the post.

dlsym() with the RTLD_NEXT flag basically:

https://catonmat.net/simple-ld-preload-tutorial-part-two

There's actually a better way to hook GNUs malloc:

https://www.man7.org/linux/man-pages/man3/malloc_hook.3.html

This is better because you can disable the hook inside the callback, and therefore use malloc within your malloc hook (no recursion)

But you can't use this mechanism before main()