Apache NuttX vs Sailfish OS

Apache NuttX is a real-time operating system for embedded systems, designed for a small footprint and standards-oriented development. It scales across 8-bit to 64-bit microcontroller environments and supports more than 15 CPU architectures, including ARM, RISC-V, x86, and Xtensa. The project lists support for more than 300 hardware boards and over 1,500 configuration templates for prototyping and production. NuttX uses POSIX and ANSI as its main standards, with POSIX serving as a development guideline rather than a licensed trademark claim. Optional filesystem support includes VFAT, ROMFS, littlefs, and NFS version 3 over UDP, and networking includes a BSD-compatible socket interface. It is free and open-source software, with separate release artifacts for the OS and integrated Apps. The configurable build system targets physical boards and devices as well as documented simulator targets. Its pseudo root filesystem can work without physical storage; mounting a true filesystem requires the relevant drivers and filesystem support. The project lists a main mailing list and Discord as community channels and is maintained by volunteers.

Sailfish OS has no write-up on Specifiction yet; its figures are above.