运算符'！'不允许的操作数 [MISRA 2012规则10.1，必填]

Question

Observing below Misra warning for below part of code.

Unpermitted operand to operator '!' [MISRA 2012 Rule 10.1, required]

Not sure what could be the fix here to get rid of this warning.

#define C_BYTE unsigned char
C_BYTE SessionStatus;
#define DISCONNECTED   0x10

if((!(SessionStatus & (C_BYTE) DISCONNECTED)))
{
   //Do something
}

I tried a few chances but didn't work as below.

1)

if((~(SessionStatus & (C_BYTE) DISCONNECTED)))
{
       //Do something
}

2)

if((!(SessionStatus & (C_BYTE) DISCONNECTED)) != 0u)
{
       //Do something
}

Answer 1

The reason for the warning is that MISRA-C rule 10.1 expects the operand to the ! && || ! && || operators to be "essentially boolean". In your case it is actually an int , because of implicit type promotion rules .

Your 2nd example almost solved it, but you must convert to essentially boolean before applying ! . That is:

if(!( (SessionStatus & (C_BYTE)DISCONNECTED) != 0u )) 

This is ok since the result of the != operator is to be regarded as essentially boolean. So that code is MISRA-C compliant, but a bit hard to read. I would instead recommend this:

#define DISCONNECTED   0x10u // u suffix makes this essentially unsigned
...

bool disconnected = (bool) (SessionStatus & DISCONNECTED);
if(!disconnected)

By adding the u suffix to the integer constant, it is essentially unsigned, same type category as unsigned char. Therefore the & operation is valid without using any casts. However, we are not allowed to implicitly convert from essentially unsigned to essentially boolean, therefore add the cast to bool .

EDIT

Since SessionStatus & DISCONNECTED is a "composite expression", MISRA doesn't allow the result to be assigned or cast to a different or wider essential type. The rationale is that they fear incompetent programmers who believe that the calculation in for example (uint32_t)(u16a + u16b) is carried out with uint32_t because of the cast, which is of course nonsense. (It would be better to educate the programmers about basic C than to come up with artificial rules, but anyway...)

I'd advise to ignore this rule entirely, but if you can't for whatever reason, here's an alternative fix:

#define DISCONNECTED   0x10u
...

unsigned char disconnected = SessionStatus & DISCONNECTED;
if(!(bool)disconnected)

But of course this is worse code than my first example.