Real-time C++ Programming Guidelines

The Execute method in a C++ program must not contain any code that could compromise the strict deterministic performance of the system. The Execute method must complete within a "reasonable" period of time. This must be considered this when using time-consuming program constructs like loops.

Perhaps the most common mistake made in a developer's first real-time C++ program is to treat the Execute method like the main function in a C++ application, and include something like a while(true) loop in that method. Not only is this not necessary (the Execute method will be called at a fixed frequency from a cyclic task), this type of endless loop will immediately "crash" the PLC because the Execute method will never exit, and either the task watchdog timer or the system watchdog timer will be exceeded.

Here are some other tips for real-time C++ programming:

  • Never allocate or re-allocate memory in the Execute method. e.g.

    • don't use malloc
    • don't use new
    • don't assign a value to a string variable unless you know it will not result in memory re-allocation.
    • be careful when using classes like std::vector, since these can allocate memory automatically.

  • Never use std::mutex in a real-time C++ program. If mutexes are required, use Arp::Mutex.

  • Don't use third-party code (e.g. libraries) if that code does any of the above. Since third-party code is generally not designed with these limitations in mind, this means it is very unlikely that you will be able to use any third-party code directly in a C++ program without modification.

  • Do not call methods on RSC services from the Execute method.

To help make sure you keep within these guidelines:

  • Perform all memory allocation in the program constructor, e.g. allocating memory for the maximum expected size of all string and vector variables.
  • Check any operations that might re-allocate memory in the Execute method (e.g. vector resizing).
  • Consider doing non-real time work in the component that is associated with the program, and use Arp::Mutex to synchronise variable access between the component and the program.
  • If in doubt, don't use third party code in the Execute method.