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In legacy RTLinux, timing-critical application code was commonly written in C and loaded into the kernel as a module—not run as an ordinary Linux process. That put real-time tasks under kernel-space constraints and made programming errors capable of destabilizing the whole machine. For new Linux real-time development, distinguish that historical RTLinux model from upstream PREEMPT_RT, which improves Linux scheduling and interrupt behavior through a different implementation.
What “application code” meant in legacy RTLinux
RTLinux was designed around a real-time execution environment that ran alongside Linux, with Linux treated as a lower-priority guest operating system. The RTLinux HOWTO describes its subject as real-time kernel programming and introduces both real-time-kernel concepts and basic module programming.
The Debian RTLinux 2.0 guide says real-time programs were loaded as Linux modules. A module source file could be an ordinary C file, but its entry points were init_module() and cleanup_module() rather than an application’s usual main(). Those names mark the code as a kernel module; they do not make it a normal user-space program.
As the guide cautions, “Since Real-Time programs in RTL are executed in the kernel space, special care must be taken when programming real-time tasks.” A fault in this code can affect the entire system rather than just terminate one application.
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How to approach a legacy RTLinux application
For an existing RTLinux system, work from the exact RTLinux release, kernel, and hardware environment it was built for. The historical module model and its project-specific APIs should not be assumed to work unchanged with a modern Linux installation.
- Establish the target environment. Identify the compatible kernel and RTLinux setup for the hardware before building or changing code.
- Separate the real-time task from supervisory software. Put only timing-critical work in the real-time portion. Keep user interfaces, databases, logging, and network-facing control functions in ordinary Linux user space, communicating across a deliberately designed interface.
- Study the target release’s module examples and APIs. The RTLinux HOWTO recommends learning module programming and examining example programs before attempting a larger application. The Debian guide points to examples for measurement and floating-point use, as well as POSIX-thread references. Do not substitute APIs from another real-time project without checking compatibility.
- Implement module startup and cleanup. In the historical model, the C module provides
init_module()andcleanup_module()entry points and uses the task, timer, synchronization, and communication facilities available in that RTLinux environment. - Keep timing-sensitive execution predictable. Avoid operations that can block or allocate unpredictably on the real-time path. Validate timing on the target system with the environment’s examples and measurement tools before relying on the behavior.
Why kernel-space real-time code needs special care
A kernel module is not isolated like an ordinary user-space process. A bug can crash or destabilize the whole machine, so keep the real-time portion small and treat its boundary with Linux user space as a core design decision. Put functions that can tolerate ordinary operating-system scheduling—such as the UI, logging, database work, and network-facing control—in user space rather than adding them to a timing-critical task.
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For portability, also distinguish the stable interface between user-space applications and the kernel from internal in-kernel interfaces. Greg Kroah-Hartman’s kernel-interface guidance identifies the syscall interface as the stable kernel-to-user-space interface; it also warns that in-kernel driver interfaces do not offer a stable binary interface. A module can therefore depend on kernel-version-specific internals even when a user-space program uses the stable syscall boundary.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Legacy RTLinux and PREEMPT_RT are different approaches
PREEMPT_RT is not a modern name for the old RTLinux module API. Linux kernel documentation describes PREEMPT_RT as changing Linux’s own locking and interrupt behavior to make more of the kernel preemptible and reduce delays before a high-priority runnable task executes. The Real-Time Linux project describes it as an upstream kernel configuration; it reports real-time support in Linux 6.12 for x86, ARM64, and RISC-V on the original Linux kernel.
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| Design question | Legacy RTLinux | PREEMPT_RT |
|---|---|---|
| Where does the real-time work run? | Real-time programs were commonly C modules executing in kernel space, in a dedicated real-time environment alongside Linux. (Debian RTLinux 2.0 guide) | Within the upstream Linux kernel configured for PREEMPT_RT. (Linux kernel documentation; Real-Time Linux project) |
| What is the scheduling and interrupt approach? | A separate real-time execution environment treated Linux as a lower-priority guest. (RTLinux HOWTO) | Threaded interrupts, more preemption points, and priority-inheritance-aware locking; the documented aim is to reduce latency between a high-priority task becoming runnable and its execution. (Linux kernel documentation) |
| What kind of code should a sample represent? | A module using APIs for the particular RTLinux environment—not a standard user-space C program. | Code intended for the configured Linux environment; whether a sample is a user-space POSIX/scheduling program or an in-kernel component must still be made explicit. |
| What is the main maintenance concern? | Legacy APIs and modules are tied to their environment and should not be presumed compatible with modern kernels. | Kernel configuration and architecture support matter; the project reports Linux 6.12 real-time support for x86, ARM64, and RISC-V. |
These approaches should not be treated as interchangeable. If maintaining an old RTLinux system, use documentation and examples for its actual release. If starting new work, current PREEMPT_RT documentation is the relevant starting point for real-time support in upstream Linux, rather than the old RTLinux HOWTO’s module workflow.
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How to judge code examples and compatibility claims
- Check the execution model. A user-space POSIX or scheduling example, a kernel module, and a project-specific RTLinux API example are different kinds of code.
- Check the target version and architecture. The RTLinux HOWTO and Debian RTLinux 2.0 guide document historical environments; their instructions are not proof of compatibility with a modern kernel.
- Check the exact API claim. RTAI documentation describes a compatibility API using headers, macros, and inline functions to compile source for both RTAI and NMT RTLinux. That is evidence of a specific historical compatibility arrangement, not evidence that legacy code runs unchanged on a modern distribution.
- Check the failure boundary. Kernel-space real-time code has broader failure consequences than an ordinary user-space process. Consider what belongs in the timing-critical path and what should remain outside it.
- Check the latency and maintenance requirements. Compare expected worst-case latency, interrupt and scheduling behavior, portability across kernel releases, debugging needs, hardware architecture, and ongoing maintenance before choosing an approach.
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