[peff]

On the page you linked, you can see that `ctype.h` reserves all prefixes of `is[a-z]` and `to[a-z]`, and `string.h` reserves `str[a-z]`. These come from the C standard (in C99, it's 7.26 "Future Library Directions"), though I don't think they use the word "reserved" there.

[aw1621107]

The "reserved" bit is from section 7.1.3:

> Each header declares or defines all identifiers listed in its associated subclause, and optionally declares or defines identifiers listed in its associated future library directions subclause and identifiers which are always reserved either for any use or for use as file scope identifiers.