MISRA Compliance

FreeRTOS-Kernel conforms to MISRA C:2012 guidelines, with the deviations listed below. Compliance is checked with Coverity static analysis. Since the FreeRTOS kernel is designed for small-embedded devices, it needs to have a very small memory footprint and has to be efficient. To achieve that and to increase the performance, it deviates from some MISRA rules. The specific deviations, suppressed inline, are listed below.

Additionally, MISRA configuration contains project wide deviations.

Suppressed with Coverity Comments

To find the violation references in the source files run grep on the source code with ( Assuming rule 8.4 violation; with justification in point 1 ):

grep 'MISRA Ref 8.4.1' . -rI

Rule 8.4

MISRA C:2012 Rule 8.4: A compatible declaration shall be visible when an object or function with external linkage is defined.

Ref 8.4.1

  • pxCurrentTCB(s) is defined with external linkage but it is only referenced from the assembly code in the port files. Therefore, adding a declaration in header file is not useful as the assembly code will still need to declare it separately.

Ref 8.4.2

  • xQueueRegistry is defined with external linkage because it is accessed by the kernel unit tests. It is not meant to be directly accessed by the application and therefore, not declared in a header file.

Rule 8.6

MISRA C:2012 Rule 8.6: An identifier with external linkage shall have exactly one external definition.

Ref 8.6.1

  • This rule prohibits an identifier with external linkage to have multiple definitions or no definition. FreeRTOS hook functions are implemented in the application and therefore, have no definition in the Kernel code.

Rule 11.1

MISRA C:2012 Rule 11.1: Conversions shall not be performed between a pointer to function and any other type.

Ref 11.1.1

  • The pointer to function is casted into void to avoid unused parameter compiler warning when Stream Buffer's Tx and Rx Completed callback feature is not used.

Rule 11.3

MISRA C:2012 Rule 11.3: A cast shall not be performed between a pointer to object type and a pointer to a different object type.

Ref 11.3.1

  • This rule prohibits casting a pointer to object into a pointer to a different object because it may result in an incorrectly aligned pointer, leading to undefined behavior. Even if the casting produces a correctly aligned pointer, the behavior may be still undefined if the pointer is used to access an object. FreeRTOS deliberately creates external aliases for all the kernel object types (StaticEventGroup_t, StaticQueue_t, StaticStreamBuffer_t, StaticTimer_t and StaticTask_t) for data hiding purposes. The internal object types and the corresponding external aliases are guaranteed to have the same size and alignment which is checked using configASSERT.

Rule 11.5

MISRA C:2012 Rule 11.5: A conversion should not be performed from pointer to void into pointer to object. This rule prohibits conversion of a pointer to void into a pointer to object because it may result in an incorrectly aligned pointer leading to undefined behavior.

Ref 11.5.1

  • The memory blocks returned by pvPortMalloc() are guaranteed to meet the architecture alignment requirements specified by portBYTE_ALIGNMENT. The casting of the pointer to void returned by pvPortMalloc() is, therefore, safe because it is guaranteed to be aligned.

Ref 11.5.2

  • The conversion from a pointer to void into a pointer to EventGroup_t is safe because it is a pointer to EventGroup_t, which is returned to the application at the time of event group creation for data hiding purposes.

Ref 11.5.3

  • The conversion from a pointer to void in list macros for list item owner is safe because the type of the pointer stored and retrieved is the same.

Ref 11.5.4

  • The conversion from a pointer to void into a pointer to EventGroup_t is safe because it is a pointer to EventGroup_t, which is passed as a parameter to the xTimerPendFunctionCallFromISR API when the callback is pended.

Ref 11.5.5

  • The conversion from a pointer to void into a pointer to uint8_t is safe because data storage buffers are implemented as uint8_t arrays for the ease of sizing, alignment and access.

Rule 21.6

MISRA C-2012 Rule 21.6: The Standard Library input/output functions shall not be used.

Ref 21.6.1

  • The Standard Library function snprintf is used in vTaskListTasks and vTaskGetRunTimeStatistics APIs, both of which are utility functions only and are not considered part of core kernel implementation.

MISRA configuration

Copy below content to misra.conf to run Coverity on FreeRTOS-Kernel.

// MISRA C-2012 Rules
{
    version : "2.0",
    standard : "c2012",
    title: "Coverity MISRA Configuration",
    deviations : [
        // Disable the following rules.
        {
            deviation: "Directive 4.8",
            reason: "HeapRegion_t and HeapStats_t are used only in heap files but declared in portable.h which is included in multiple source files. As a result, these definitions appear in multiple source files where they are not used."
        },
        {
            deviation: "Directive 4.9",
            reason: "FreeRTOS-Kernel is optimised to work on small micro-controllers. To achieve that, function-like macros are used."
        },
        {
            deviation: "Rule 1.2",
            reason: "The __attribute__ tags are used via macros which are defined in port files."
        },
        {
            deviation: "Rule 3.1",
            reason: "We post HTTP links in code comments which contain // inside comments blocks."
        },
        {
            deviation: "Rule 8.7",
            reason: "API functions are not used by the library outside of the files they are defined; however, they must be externally visible in order to be used by an application."
        }
    ]
}