FreeRTOS – Coding Standards Part Nine

This article introduces the <span>FreeRTOS</span> coding standards. Good coding practices are the foundation for writing quality code and collaborative software development.

Data Types

<span>FreeRTOS</span>‘ data types are defined in the <span>portmacrocommon.h</span> file. From the definitions, <span>FreeRTOS</span> has redefined the standard data types in <span>C</span>. For example, <span>char</span> is redefined as <span>portCHAR</span>.

    /**
     * @brief Type definitions.
     */
    #define portCHAR          char
    #define portFLOAT         float
    #define portDOUBLE        double
    #define portLONG          long
    #define portSHORT         short
    #define portSTACK_TYPE    uint32_t
    #define portBASE_TYPE     long
    
    typedef portSTACK_TYPE   StackType_t;
    typedef long             BaseType_t;
    typedef unsigned long    UBaseType_t;
    
    #if ( configTICK_TYPE_WIDTH_IN_BITS == TICK_TYPE_WIDTH_16_BITS )
        typedef uint16_t     TickType_t;
        #define portMAX_DELAY              ( TickType_t ) 0xffff
    #elif ( configTICK_TYPE_WIDTH_IN_BITS == TICK_TYPE_WIDTH_32_BITS )
        typedef uint32_t     TickType_t;
        #define portMAX_DELAY              ( TickType_t ) 0xffffffffUL
    
    /* 32-bit tick type on a 32-bit architecture, so reads of the tick count do
     * not need to be guarded with a critical section. */
        #define portTICK_TYPE_IS_ATOMIC    1
    #else
        #error configTICK_TYPE_WIDTH_IN_BITS set to unsupported tick type width.
    #endif

<span>IAR</span> defaults <span>char</span> as an unsigned type.

Several data types that need special mention may vary depending on the platform being ported.

• TickType_t

  For 32-bit architectures, <span>configUSE_16_BIT_TICKS</span> should always be set to 0.

• If <span>configUSE_16_BIT_TICKS</span> is set to non-zero (<span>true</span>), then <span>TickType_t</span> is defined as an unsigned 16-bit type.
• If <span>configUSE_16_BIT_TICKS</span> is set to zero (<span>false</span>), then <span>TickType_t</span> is defined as an unsigned 32-bit type.

• <span>BaseType_t </span>is defined as a 32-bit type on 32-bit architectures. On 16-bit architectures, <span>BaseType_t</span> is defined as a 16-bit type.

• <span>UBaseType_t</span> is an unsigned <span>BaseType_t</span>.

• <span>StackType_t</span> refers to the type used by the architecture to store stack items. Typically, it is a 16-bit type on 16-bit architectures and a 32-bit type on 32-bit architectures, but there are exceptions. It is used internally by <span>FreeRTOS</span>.

Naming

Variables

• <span>uint32_t</span> type variables are prefixed with <span>ul</span>, where “u” stands for “unsigned” and “l” stands for “long”.

• <span>uint16_t</span> type variables are prefixed with <span>us</span>, where “u” stands for “unsigned” and “s” stands for “short”.

• <span>uint8_t</span> type variables are prefixed with <span>uc</span>, where “u” stands for “unsigned” and “c” stands for “char”.

• <span>size_t</span> type variables are also prefixed with <span>ux</span>.

• Enumeration variables are prefixed with <span>e</span>.

• Pointers are prefixed with <span>p</span>, for example, a pointer to <span>uint16_t</span> will be prefixed with <span>pus</span>.

• Function names include the return type of the function, the filename where the function is located, and the function’s purpose. If it is a private function, a <span>prv (private)</span> prefix will be added. <span>FreeRTOS</span> has a good coding standard where the function name includes the filename, which greatly helps users improve the efficiency of finding function definitions and understanding the purpose of the function. For example:

<span>void vListInitialise( List_t * const pxList )</span>, v – void return type; List – located in list.c file; Initialise – function’s purpose.

<span>QueueHandle_t xQueueCreateStatic(UBaseType_t uxQueueLength, UBaseType_t uxItemSize, uint8_t *pucQueueStorage, StaticQueue_t *pxQueueBuffer) </span>

x – QueueHandle_t return type; Queue – located in queue.c file; CreateStatic – function’s purpose

Macros

• Macros are prefixed with the name of the file where the macro is defined. The prefix is in lowercase. For example, <span>configUSE_PREEMPTION</span> is defined in <span>FreeRTOSConfig.h</span>.
• All macros, except for the prefix, are written in uppercase letters and use underscores to separate words.

Style Guide

• Indentation uses four space characters.

• Comments. Use <span>FreeRTOS</span> recommended “block comments” (/* … */) or “line comments” (// …), avoid mixing; block comments are used for long multi-line texts (like function headers), and line comments are used for single-line explanations (like key code lines).

• Coding style. I prefer using <span>UTF-8</span>.