6.3.1. Coding Conventions

The style of RTEMS is generally consistent in the core areas. This section attempts to capture generally accepted practices. When in doubt, consult the code around you, look in the RTEMS sources in the directories cpukit/include/rtems/score and cpukit/score, or ask on the Developers Mailing List.

6.3.1.1. Source Documentation

  • Use Doxygen according to our Doxygen Guidelines.

  • Use the file templates, see File Templates.

  • Use /* */ comments.

  • Do not use // comments.

  • Use comments wisely within function bodies, to explain or draw attention without being verbose.

  • Use English prose and strive for good grammar, spelling, and punctuation.

  • Use TODO with a comment to indicate code that needs improvement. Make it clear what there is to do. Add a ticket and add a link to it.

  • Use XXX or FIXME to indicate an error/bug/broken code. Add a ticket and add a link to it.

6.3.1.2. Licenses

The RTEMS Project has strict requirements on the types of software licenses that apply to software it includes and distributes. Submissions will be summarily rejected that do not follow the correct license or file header requirements.

6.3.1.3. Language and Compiler

  • Use C99.

  • Treat warnings as errors: eliminate them.

  • Favor C, but when assembly language is required use inline assembly if possible.

  • Do not use compiler extensions.

  • Use the RTEMS macros defined in <rtems/score/basedefs.h> for abstracting compiler-specific features. For using attributes see the GCC attribute syntax. Prefer to place attributes in front of the declarator. Try to be in line with C++11 attributes and C11 keywords such as _Noreturn.

  • Use NULL for the null pointer, and prefer to use explicit checks against NULL, e.g.,

    if ( ptr != NULL )

    instead of

    if ( !ptr )

  • Use explicit checks for bits in variables.

    • Example 1: Use

      if ( XBITS == (var & XBITS) )

      to check for a set of defined bits.

    • Example 2: Use

      if ( (var & X_FLAGS) != 0) )

      instead of

      if ( !!(var & X_FLAGS) )

      to check for at least 1 defined bit in a set.

  • Use (void) unused; to mark unused parameters and set-but-unused variables immediately after being set.

  • Do not put function prototypes in C source files, any global functions should have a prototype in a header file and any private function should be declared static.

  • Declare global variables in exactly one header file. Define global variables in at most one source file. Include the header file declaring the global variable as the first include file if possible to make sure that the compiler checks the declaration and definition and that the header file is self-contained.

  • Do not cast arguments to any printf() or printk() variant. Use <inttypes.h> PRI constants for the types supported there. Use <rtems/inttypes.h> for the other POSIX and RTEMS types that have PRI constants defined there. This increases the portability of the printf() format.

  • Do not use the register keyword. It is deprecated since C++14.

6.3.1.4. Readability

  • Understand and follow the naming rules..

  • Use typedef to remove ‘struct’, but do not use typedef to hide pointers or arrays. * Exception: typedef can be used to simplify function pointer types.

  • Do not mix variable declarations and code.

  • Declare variables at the start of a block.

  • Only use primitive initialization of variables at their declarations. Avoid complex initializations or function calls in variable declarations.

  • Do not put unrelated functions or data in a single file.

  • Do not declare functions inside functions.

  • Avoid deep nesting by using early exits e.g. return, break, continue. * Parameter checking should be done first with early error returns. * Avoid allocation and critical sections until error checking is done. * For error checks that require locking, do the checks early after acquiring locks. * Use of ‘goto’ requires good reason and justification.

  • Test and action should stay close together.

  • Avoid complex logic in conditional and loop statements.

  • Put conditional and loop statements on the line after the expression.

  • Favor inline functions to hide compile-time feature-conditioned compilation..

  • Define non-inline functions in a .c source file.

  • Declare all global (non-static) functions in a .h header file.

  • Declare and define inline functions in one place. Usually, this is a *impl.h header file.

  • Declare and define static functions in one place. Usually, this is toward the start of a .c file. Minimize forward declarations of static functions.

  • Function declarations should include variable names.

  • Avoid excess parentheses. Learn the operator precedence. rules.

  • Always use parentheses with sizeof. This is an exception to the rule about excess parentheses.

6.3.1.5. Robustness

  • Check all return statuses.

  • Validate input parameters.

  • Use debug assertions (assert).

  • Use const when appropriate for read-only function parameters and compile-time constant values.

  • Do not hard code limits such as maximum instances into your code.

  • Prefer to use sizeof(variable) instead of sizeof(type).

  • Favor C automatic variables over global or static variables.

  • Use global variables only when necessary and ensure atomicity of operations.

  • Do not shadow variables.

  • Avoid declaring large buffers or structures on the stack.

  • Avoid using zero (0) as a valid value. Memory often defaults to being zero.

  • Favor mutual exclusion primitives over disabling preemption.

  • Avoid unnecessary dependencies, such as by not calling ‘’printf()’’ on error paths.

  • Avoid inline functions and macros with complicated logic and decision points.

  • Prefer inline functions, enum, and const variables instead of CPP macros.

  • CPP macros should use a leading underscore for parameter names and avoid macro pitfalls..

6.3.1.6. Portability

  • Think portable! RTEMS supports a lot of target hardware.

  • For integer primitives, prefer to use precise-width integer types from C99 stdint.h.

  • Write code that is 16-bit, 32-bit, and 64-bit friendly.

6.3.1.7. Maintainability

  • Minimize modifications to third-party code.

  • Keep it simple! Simple code is easier to debug and easier to read than clever code.

  • Share code with other architectures, CPUs, and BSPs where possible.

  • Do not duplicate standard OS or C Library routines.

6.3.1.8. Performance

  • Prefer algorithms with the lowest order of time and space. for fast, deterministic execution times with small memory footprints.

  • Understand the constraints of real-time programming.. Limit execution times in interrupt contexts and critical sections, such as Interrupt and Timer Service Routines (TSRs).

  • Prefer to ++preincrement instead of postincrement++.

  • Avoid using floating point except where absolutely necessary.

6.3.1.9. Miscellaneous

  • If you need to temporarily change the execution mode of a task/thread, restore it.

  • If adding code to ‘’cpukit’’ be sure the filename is unique since all files under that directory get merged into a single library.

6.3.1.10. Header Files

  • Do not add top-level header files. Place the header files in a directory, for example #include <rtems/*>, #include <bsp/*>, #include <dev/*>, etc.

  • Use the extension .h for C header files.

  • Use the extension .hpp for C++ header files.

  • Use the file template for header files, see C/C++ Header File Template.

  • Use separate header files for the API and the implementation.

  • Use foobar.h for the header file of the foobar module which defines API components.

  • Use foobardata.h for the header file of the foobar module which defines interfaces used by the application configuration.

  • Use foobarimpl.h for the header file of the foobar module which defines interfaces, macros, and inline functions used by the implementation.

  • Do not place inline functions which are only used in one implementation source file into the implementation header file. Add these inline functions directly to the corresponding source file.

  • Document all elements in header files with comments in Doxygen markup, see Doxygen Guidelines.

  • Only place header files which should be directly included by the user with an @file Doxygen directive into the API documentation group. Place internal API header files with an @file Doxygen command into the implementation documentation group even if they define API elements. The API documentation group should only list public header files and no internal header files.

6.3.1.11. Layering

6.3.1.12. Exceptions to the Rules

  • Minimize reformatting existing code in RTEMS unless the file undergoes substantial non-style changes.

  • Third-party code. should not be reformatted to fit RTEMS style. Exception: unmaintained third-party code adopted and maintained by RTEMS may be reformatted, subject to the above rules.

6.3.1.13. Tools

Some of the above can be assisted by tool support. Feel free to add more tools, configurations, etc here.