MISRA.md · chummy2015/FreeRTOS-Kernel

# MISRA Compliance

FreeRTOS-Kernel conforms to [MISRA C:2012](https://www.misra.org.uk/misra-c)
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](#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

_Ref 8.4.1_

- MISRA C:2012 Rule 8.4: A compatible declaration shall be visible when an
        object or function with external linkage is defined.
        This rule requires that a compatible declaration is made available
        in a header file when an object with external linkage is defined.
        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.

#### Rule 11.3

_Ref 11.3.1_

- 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.
        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.

### 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."
        },
        {
            deviation: "Rule 11.5",
            reason: "Allow casts from `void *`. List owner, pvOwner, is stored as `void *` and are cast to various types for use in functions."
        }
    ]
}
```

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