hello,
I have experience in many RTOS, all based on posix OS, but rather in the tiny Linux-based kernel if they are compatible, but some developers prefer not to use OS API and create your own scheduler, memory access, mutex, threading, etc
and actually get more speed and this is important in a system SRT,
but basically many companies of micro architecture, implemented in its basic functions that are common libraries on POSIX systems, such as all functions of architecture "core-MIPS" preved using "pthread and mutex", "heap FIB", ect, but ultimately people do not use jejejejej
regards.