[kevin_thibedeau]

He's targeting ANSI C, C89. Flexible array members are not supported officially until C99. 26 years later, it's time to stop clinging to outdated standards and crusty tooling. Even Linux had to read the room and adopt C11.

A C11 implementation could go one step further and use _Alignas(max_align_t) to keep the pool array aligned with no manual effort. The double allocation does this implicitly.

[listeria]

on the topic of alignment, the library (libpool) fails to align chunk_sz to allow storing a pointer when in the free_chunk list.

This issue is sidestepped in TFA by using a union, which ensures appropiate alignment for all it's members, but in the library there's nothing which prevents me from asking for a chunk size of, say, 9 bytes, which would store pointers at misaligned addresses when creating the free-list.

[liontwist]

The pool needs to align every allocation anyway.