The author recommends -std=c99 (-pedantic) default flags, yet suggests K&R (2nd edition, presumably) as a reference...
Most microcontrollers are ARM Cortex-M and are programmed with a recent GCC toolchain
https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-...
(it has a solid subset of C99 since around VS2015 though, just _Generic was missing - and ignoring VLAs, which have been "dropped" in later C standards anyway).