RTEMS

Free

Real-Time Executive for Multiprocessor Systems or RTEMS is an open source Real Time Operating System (RTOS) that supports open standard application programming interfaces (API) such as POSIX. It is used in space flight, medical, networking and many more embedded devices using processor architectures

FreeFree tier

Type

Open Source

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About RTEMS

RTEMS (Real-Time Executive for Multiprocessor Systems) is an open-source real-time operating system (RTOS) designed for deeply embedded systems. It supports open standard APIs including POSIX, the Classic API (RTEID/ORKID), uITRON 3.0, and BSD sockets. RTEMS powers space flight, medical devices, networking equipment, and countless other embedded applications. It currently supports 18 processor architectures and approximately 200 Board Support Packages (BSPs), covering ARM, PowerPC, Intel, SPARC, RISC-V, MIPS, and more. Key capabilities include symmetric multiprocessing (SMP), a high-performance FreeBSD-derived TCP/IP stack (IPv4/IPv6), multiple file systems (IMFS, FAT32, RFS, NFS, etc.), dynamic loading, an embedded shell, and a GNU-based toolchain for C, C++, and Ada. The latest release (6.2, December 2025) features a Python-based Waf build system and a transition to the 2-Clause BSD License. RTEMS is developed openly with public Git repositories, mailing lists, and Discord, and commercial support is available from multiple companies.

Key Features

Multitasking kernel with priority-based preemptive scheduling and optional rate-monotonic scheduling

Supports POSIX, Classic API (RTEID/ORKID), uITRON 3.0, and BSD sockets

High-performance TCP/IP stack from FreeBSD with IPv4/IPv6, UDP, ICMP, DHCP, and PPP

Multiple file systems: IMFS, Mini-IMFS, Device FS, MS-DOS FAT12/16/32, RTEMS File System (RFS), NFS, TFTP client

Symmetric multiprocessing (SMP) for homogeneous and heterogeneous systems

18 processor architectures and ~200 Board Support Packages (BSPs)

GNU toolchain with ISO/ANSI C, C++ (including STL), and Ada (GNAT/RTEMS) support

Thread-aware debugging over Ethernet or serial using GDB and DDD

Dynamic memory allocation and high user configurability

Open development with public Git, mailing lists, and Discord community

Pros & Cons

Pros

Fully open source with no licensing fees, available under permissive licenses (2-Clause BSD for recent code)
Mature project with over 30 years of active development and long-term support
Supports a wide range of processor architectures and board support packages
Rich set of standard APIs (POSIX, BSD sockets) eases porting of applications
High-performance networking stack derived from FreeBSD
Active community and availability of commercial support from multiple companies
Public and transparent development process with version-controlled sources

Cons

Steep learning curve for developers new to real-time operating systems
Documentation quality varies and may be less extensive than commercial RTOS alternatives
Not intended for general-purpose computing; primarily targets deeply embedded systems
Limited ecosystem of third-party libraries and middleware compared to Linux

Best For

Space flight systemsMedical devicesNetworking equipment and infrastructureDeeply embedded systems requiring deterministic real-time behaviorIndustrial control and automationLong-life embedded deployments needing 30+ year open-source stability

FAQ

What does RTEMS stand for?

Real-Time Executive for Multiprocessor Systems.

Is RTEMS free to use?

Yes, RTEMS is open source and available under various free licenses. The latest code base is relicensed under the 2-Clause BSD License.

What processor architectures does RTEMS support?

RTEMS supports 18 processor architectures, including ARM, PowerPC, Intel, SPARC, RISC-V, MIPS, Blackfin, Microblaze, and more.

What APIs are supported by RTEMS?

RTEMS supports POSIX 1003.1b, the Classic API (similar to pSOS+), uITRON 3.0, and BSD sockets for networking.

How long has RTEMS been an open source project?

RTEMS has been an open source project for over 30 years.