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MISRA Compliance

The FreeRTOS-Plus-TCP library files conform to the MISRA C:2012 guidelines, with the deviations listed below. Compliance is checked with Coverity static analysis. Since the FreeRTOS-Plus-TCP library 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 performace of the IP-stack, it deviates from some MISRA rules. The specific deviations, suppressed inline, are listed below.

Additionally, MISRA configuration file contains the 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 11.4 violation; with justification in point 2 ):

grep 'MISRA Ref 11.4.2' . -rI

Rule 2.2

Ref 2.2.1

  • MISRA C-2012 Rule 2.2 Unions are used for checksum computation to speed up the process by utilizing the full length of registers (32-bits). After this, the 16-bit union members are used to then compute the final checksum. Doing this is considered as 'overwriting the variable' by Coverity. Thus, it marks some statements as dead code. This is a false positive.

Rule 8.9

Ref 8.9.1

  • MISRA C-2012 Rule 8.9 For unit-tests to be repeatable and independent of the order of execution, some variables have file scope definitions rather than function scope.

Rule 11.3

Ref 11.3.1

  • MISRA C-2012 Rule 11.3 The data received/sent by the IP stack is represent as a byte stream. This byte stream needs to be casted to various data structures to access certain fields of the packet. However, when casting a byte stream to a structure, MISRA warns us that it can lead to unaligned access. But, in case of FreeRTOS+TCP, the buffer in which the packets are stored are always aligned to a 4 byte word boundary with an offset of 2 bytes. The reason for this 2 byte offset is that the ethernet header is of 14 (12 + 2) bytes. Thus, everything except the ethernet header is properly aligned. There is one alignment exception, which is the sender protocol address in the ARP Header. To combat that, the sender protocol address field is declared as an array of 4 bytes instead of a uint32_t. More details can be found here.

Rule 11.4

Ref 11.4.1

  • MISRA c-2012 Rule 11.4 Warns about conversion between a pointer and an integer. Whenever a socket is created using the FreeRTOS_Socket API, either a valid socket (a valid non-NULL pointer) is returned; or FREERTOS_INVALID_SOCKET is returned (which is essentially ~0U) to depict an error in the socket creation process. This conversion from ~0U to a pointer is used to convey the error to various functions. If the pointer is equal to FREERTOS_INVALID_SOCKET, then it is not dereferenced. Thus, this violation can be safely suppressed.

Ref 11.4.2

  • MISRA Rule 11.4 The following statement may trigger a: warning: cast increases required alignment of target type [-Wcast-align]. It has been programatically checked that the pointer is well aligned before this point.

Ref 11.4.3

  • MISRA Rule 11.4 warns about casting pointer to an integer and vice versa. Here, the poiner to the starting byte of the packet is cast to an integer which is then used to see whether the pointer is well aligned or not. It is not used to access any pointer values. Thus, this violation can be safely suppressed.

Rule 11.6

Ref 11.6.1

  • When sending and receiving a DHCP event to the IP-stack, the events are converted to a void pointer and sent to the IP-task. The function used to send the events handles various events for the IP-task and thus only accepts void pointers. The IP-task converts the void pointer back to the original event. Thus, this rule can be safely suppressed.

Ref 11.6.2

  • MISRA Rule 11.6 uintptr_t is guaranteed by the implementation to fit a pointer size of the platform. The pointer has to be moved backward by a constant offset to get to a 'hidden' pointer which is not available for the user to use. This conversion is done to achieve that while avoiding pointer arithmetic.

Rule 11.8

Ref 11.8.1

  • MISRA c-2012 Rule 11.8 warns about removing the const qualifier when assigning one value to another. In this case however, a function pointer is being copied. It doesn't make sense in case of function pointers for the pointee to be const or mutable. Thus, this rule is safe to suppress. 1

Rule 14.3

Ref 14.3.1

  • MISRA C-2012 Rule 14.3 False positive as the value might be changed depending on the conditionally compiled code

Rule 21.6

Ref 21.6.1

  • MISRA C-2012 Rule 21.6 warns about the use of standard library input/output functions as they might have implementation defined or undefined behaviour. The function snprintf is used to insert information in a logging string. This is only used in a utility function which aids in debugging and is not part of the 'core' code governing the functionality of the TCP/IP stack.

Rule 17.2

Ref 17.2.1

  • MISRA C-2012 Rule 17.2 warns about using recursion in software as that can have severe implications on the stack usage and can lead to a serious issue. In this case however, the number of recursions are limited by design. Any socket spawned (child) by a socket in listening state (parent) cannot be in listening state. Thus it is not possible for the child to have a secondary child socket thereby limiting the number of recursive calls to one.

Rule 20.10

Ref 20.10.1

  • MISRA C-2012 Rule 20.10 warns against the use of ## concatination operator. However, in this case, it must be used to support compile time assertions in case the preprocessor does not suppport sizeof. This operation (assert) has no runtime execution.