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QNX Software Development Platform
QNX SDP is a cross-compiling and debugging environment, including an IDE and command-line tools, for building binary images and programs for target boards running the QNX OS 8.0.
System Architecture
The System Architecture guide accompanies the QNX OS and is intended for both application developers and end-users.
The QNX OS Microkernel
The microkernel implements the core POSIX features used in embedded realtime systems, along with the fundamental QNX OS message-passing services.
Thread scheduling
When and how are scheduling decisions made?
External code invoked by the scheduler
The scheduler may use code that's external to the kernel to perform certain functions. This code can affect thread scheduling and, thus, the deterministic behavior of the QNX OS.

QNX Momentics IDE User's Guide
This User's Guide describes version 8.0 of the Integrated Development Environment (IDE) that's part of the QNX Tool Suite.
QNX Software Development Platform
QNX SDP is a cross-compiling and debugging environment, including an IDE and command-line tools, for building binary images and programs for target boards running the QNX OS 8.0.
- Quickstart Guide
- System Architecture
  The System Architecture guide accompanies the QNX OS and is intended for both application developers and end-users.
  - The Philosophy of the QNX OS
    The primary goal of the QNX OS is to deliver the open systems POSIX API in a robust, scalable form suitable for a wide range of systems—from tiny, resource-constrained embedded systems to high-end distributed computing environments. The OS supports several processor families, including x86 and ARM.
  - The QNX OS Microkernel
    The microkernel implements the core POSIX features used in embedded realtime systems, along with the fundamental QNX OS message-passing services.
    - Design goals of the QNX OS
      Historically, the application pressure on BlackBerry QNX's operating systems has been from both ends of the computing spectrum—from memory-limited embedded systems all the way up to high-end SMP (symmetrical multiprocessing) machines with gigabytes of physical memory.
    - System services
    - Threads and processes
      When building an application (realtime, embedded, graphical, or otherwise), the developer may want several algorithms within the application to execute concurrently. This concurrency is achieved by using the POSIX thread model, which defines a process as containing one or more threads of execution.
    - Thread scheduling
      When and how are scheduling decisions made?
      - Scheduling priority
        Every thread is assigned a priority. The thread scheduler selects the next thread to run by looking at the priority assigned to every thread that's READY (i.e., capable of using the CPU).
      - Scheduling policies
      - IPC issues
        Since all the threads in a process have unhindered access to the shared data space, wouldn't this execution model trivially solve all of our IPC problems? Can't we just communicate the data through shared memory and dispense with any other execution models and IPC mechanisms?
      - Thread complexity issues
        Although threads are very appropriate for some system designs, it's important to respect the Pandora's box of complexities their use unleashes.
      - External code invoked by the scheduler
        The scheduler may use code that's external to the kernel to perform certain functions. This code can affect thread scheduling and, thus, the deterministic behavior of the QNX OS.
    - Synchronization services
      The QNX OS provides the POSIX-standard thread-level synchronization primitives, some of which are useful even between threads in different processes.
    - Clock and timer services
      Clock services are used to maintain the time of day, which is in turn used by the kernel timer calls to implement interval timers.
    - Interrupt handling
      No matter how much we wish it were so, computers are not infinitely fast. In a realtime system, it's absolutely crucial that CPU cycles aren't unnecessarily spent. But, it is important to balance this with safety, predictable behaviour, and an avoidance of run-time allocation or queueing. QNX OS implements a low-latency path to schedule user threads to handle hardware interrupts.
  - Interprocess Communication (IPC)
    Interprocess Communication (IPC) plays a fundamental role in the transformation of the microkernel from an embedded realtime kernel into a full-scale POSIX operating system. As various service-providing processes are added to the microkernel, IPC is the glue that connects those components into a cohesive whole.
  - The Microkernel Instrumentation
    The microkernel (procnto-smp-instr) is equipped with a sophisticated tracing and profiling mechanism that lets you monitor your system's execution in real time. This instrumentation works on both single-CPU and SMP systems.
  - Process Manager
    The Process Manager can create and manage multiple POSIX processes, each of which may contain multiple POSIX threads.
  - Dynamic Linking
    In a typical system, a number of programs will be running. Each program relies on a number of functions, some of which will be standard C library functions, like printf(), malloc(), write(), etc.
  - Resource Managers
    To give the QNX OS a great degree of flexibility, to minimize the runtime memory requirements of the final system, and to cope with the wide variety of devices that may be found in a custom embedded system, the OS allows user-written processes to act as resource managers that can be started and stopped dynamically.
  - Filesystems
    The QNX OS provides a rich variety of filesystems. Like most service-providing processes in the OS, these filesystems execute outside the kernel; applications use them by communicating via messages generated by the shared-library implementation of the POSIX API.
  - Character I/O
    A key requirement of any realtime operating system is high-performance character I/O.
  - Networking Architecture
    As with other service-providing processes in the QNX OS, the networking services execute outside the kernel. Developers are presented with a single unified interface, regardless of the configuration and number of networks involved.
  - TCP/IP Networking
    Our TCP/IP stack is included in the io-sock networking stack and uses the common FreeBSD API.
  - High Availability
    The term High Availability (HA) is commonly used in telecommunications and other industries to describe a system's ability to remain up and running without interruption for extended periods of time.
  - What is Real Time and Why Do I Need It?
    Real time is an often misunderstood—and misapplied—property of operating systems. This appendix provides a summary of some of the critical elements of realtime computing and discusses a few design considerations and benefits.
  - Glossary
- OS Components & Operations
- Graphics
- Programming
- System Security Guide
  The QNX System Security Guide is intended for both system integrators who are responsible for the security of a QNX OS system and developers who want to create a QNX OS resource manager free from vulnerabilities.
- Utilities & Libraries
- Migrating to QNX OS 8.0
QNX Software in the Cloud
QNX Toolkit for Visual Studio Code
This User's Guide describes the QNX^® Toolkit for Visual Studio Code. The guide introduces you to the QNX Toolkit by explaining the QNX development environment and how to build, run, and debug your QNX^® Operating System (OS) applications and systems.
Typographical Conventions, Support, and Licensing
This section describes the typographical conventions used throughout the documentation, explains how to obtain technical support, and provides some information about licensing.

External code invoked by the scheduler

QNX SDP8.0System ArchitectureDeveloperUser

The scheduler may use code that's external to the kernel to perform certain functions. This code can affect thread scheduling and, thus, the deterministic behavior of the QNX OS.

The external code is sometimes in kernel callouts, which are stand-alone code segments that perform hardware-specific functions. For instance, the scheduler may invoke a callout to send an interprocessor interrupt (IPI) to force another processor in the system to make a scheduling decision. The callout implementation is hardware-specific, but it is expected that the hardware can send IPIs in constant time. During this time, another interrupt may occur, a high-priority thread may become unblocked (i.e., ready), or an asynchronous event may result in a thread's cancellation (e.g., a signal or timeout). These occurrences can alter the subsequent scheduling decision.

For more information about kernel callouts, refer to Kernel Callouts in the Building Embedded Systems guide.

If tracing is enabled, the scheduler also invokes QNX OS tracing code (which is not in a kernel callout) when the associated thread state change occurs. Thread state events are emitted from the scheduler to preserve the correct event order. The tracing calls are expected to take constant time and have negligible impact on run time when tracing is off, but they can influence scheduling when tracing is on.

The System Analysis Toolkit User's Guide describes the QNX OS tracing mechanism and the various events related to scheduling and other system state changes.

Page updated: August 09, 2024