POSIX (Portable Operating System Interface) is a standardized API for system calls and services, designed to ensure compatibility between operating systems. Originally developed for Unix-like operating systems, POSIX plays a significant role in embedded software by providing a common interface that enhances portability, reusability, and interoperability across different platforms.

Embedded systems often rely on real-time operating systems (RTOS) for deterministic behavior, reliability, and resource efficiency. POSIX compliance in an RTOS enables developers to write portable applications that can be reused across multiple systems with minimal changes. This is particularly important in industries such as Automotive, Avionics, Industrial Automation, and Telecommunications, where long product life cycles and strict certification requirements exist.

POSIX Standards and Real-Time Embedded Systems

The POSIX standard defines various interfaces, including:

• Process and thread management: pthread_create(), pthread_join()
• Inter-process communication (IPC): Message queues, semaphores, shared memory
• File system and I/O management: Standard file operations, like open(), read(), write()
• Signals and timers: signal(), timer_create()
• Networking (Sockets API): socket(), bind(), listen(), accept()

However, embedded real-time systems often do not require the full POSIX standard. Instead, a subset, POSIX.1b (Real-Time Extensions), is used to enable deterministic execution. Key real-time POSIX features include:

• Priority scheduling (SCHED_FIFO, SCHED_RR)
• High-resolution timers
• Real-time signals
• Shared memory and semaphores

A full-fledged Unix-like POSIX environment may introduce overhead, which is why embedded RTOS solutions usually implement only a minimal or configurable POSIX layer.

POSIX in SYSGO’s PikeOS

PikeOS by SYSGO is a real-time operating system (RTOS) and hypervisor that supports multiple execution environments, including POSIX. It is designed primarily for Safety- and Security-critical applications, particularly in Avionics (DO-178C), Automotive (ISO 26262), Industrial Automation (IEC 61508), and Railway (EN 50128/50657).

The integration of POSIX within PikeOS follows a partitioned architecture, where POSIX applications run alongside other execution environments, such as ARINC 653 (for Avionics), AUTOSAR (for Automotive), or even native PikeOS APIs.

1. PikeOS Hypervisor and Partitioning

PikeOS implements a separation kernel that ensures strong isolation between partitions, which can host:

• POSIX applications
• Legacy RTOS applications
• Bare-metal code
• Guest OS instances (e.g., ELinOS, Linux, Android)

This partitioning enables mixed-criticality systems, allowing safety-critical POSIX tasks to coexist with general-purpose applications without interfering with each other.

2. POSIX API in PikeOS

PikeOS provides a subset of POSIX designed for real-time performance, including:

• Threading model based on POSIX pthreads
• Priority-based real-time scheduling
• POSIX message queues and semaphores for deterministic IPC
• Timers and signals adapted for real-time constraints
• File system support, but optimized for embedded use

This allows developers to port existing POSIX-based applications (e.g., from Linux) to PikeOS with minimal effort.

3. Real-Time Capabilities and Certification

Unlike full Linux implementations that provide a soft real-time POSIX layer, PikeOS guarantees hard real-time determinism. It is certification-ready for DO-178C, ISO 26262, IEC 61508, and other standards, making it ideal for Aerospace, Automotive, and Industrial Automation systems.

Key advantages of PikeOS’ POSIX implementation include:

• Deterministic behavior with bounded execution times
• Static partitioning ensuring resource isolation
• Security mechanisms (MILS architecture, Multiple Independent Levels of Security) for separation of Safety and non-Safety functions
• Certifiable runtime environment, reducing certification costs

4. PikeOS vs. Linux RT

While real-time Linux (PREEMPT-RT) also provides a POSIX API, it does not guarantee hard real-time determinism. Linux is a monolithic kernel, meaning all components share the same memory space, leading to potential interference. PikeOS, on the other hand, uses a microkernel architecture that strictly enforces process isolation.

Thus, for mission-critical embedded systems, PikeOS is preferred over Linux RT because:

• It provides strong partitioning and Security separation
• It supports hard real-time scheduling
• It has formal certification pathways for Safety-critical applications

Conclusion

POSIX is a crucial standard in the embedded world, allowing software portability and efficient development across different platforms. However, traditional POSIX environments often lack the real-time guarantees required for Safety-critical embedded applications.

SYSGO’s PikeOS bridges the gap by offering a certifiable real-time POSIX API within a partitioned, hypervisor-based RTOS environment. This allows developers to leverage POSIX compatibility while maintaining strict deterministic execution and Safety certification compliance. By supporting mixed-criticality applications and secure isolation, PikeOS is an ideal choice for industries that demand both real-time performance and Security.