The Portable Operating System Interface (POSIX) was developed to provide a unified interface for interacting with operating systems, making code more portable between systems.
At the time of writing, FreeRTOS has a beta implementation for a subset of the POSIX API. The POSIX headers that have been (partly) ported are listed here:
Generally speaking, threading, queues, mutexes, semaphores, timers, sleep, and some clock functions are implemented by the port. This feature set sometimes covers enough of a real-world use case to enable porting applications that have been written to be POSIX-compliant to an MCU supporting FreeRTOS. Keep in mind that FreeRTOS does not supply a filesystem on its own without additional middleware, so any application requiring filesystem access...