protobuf/upb/stdc/README

This directory contains code that is ANSI C but uses parts of the
standard library that are not available to very limited environments
like Linux Kernel modules.  The standard calls environments like this
"freestanding implementations."

This does *not* imply that the upb core can be compiled directly on a
freestanding implementation.  Even the core uses library functions
that are not directly available on freestanding implementations
(notably malloc()/free(), vsnprintf(), and assert()).  So compiling on
freestanding implementations may require implementing compatibility
versions of functions like malloc().

Also, Linux is not technically a freestanding implementation either,
since it does not accept functions that return float or double on
x86-64 (these use SSE registers which are disabled in kernel mode).