Ternary operator argument evaluation in C

Question

I've been working with some code to ensure MISRA compliance. The issue with this piece of code is

Operands shall not be of an inappropriate essential type. The operand of the ? operator is of an inappropriate essential type category unsigned.

I assume the issue is with the first argument being an unsigned data type, instead of boolean, which means the fix bellow would work.

The original,

 return (uint32Var & 0x80000000u) ? 0u : 1u;

My change to the code,

 return ( (uint32Var & 0x80000000u)!=0u ) ? 0u : 1u;

Is this a correct change to make? I'm concerned about changing the functionality of the code but as far as I know, at least in if logic, the operand is evaluated as numVar != 0 inside the if ( numVar ) operator.

You could avoid the ternary operator altogether, e.g. `return 0 == (uint32Var & 0x80000000u);`. (I have no idea if that complies with the MISRA(ble) rules.) — Ian Abbott, Jul 23 '19 at 12:16

score 1 · Accepted Answer · answered Jul 23 '19 at 10:36

That is safe.

You'll be comparing the unsigned 32-bit:

(uint32Var & 0x80000000u)

To the unsigned int:

0u

Usual arithmetic conversions apply to ensure that regardless of the actual types involved here, you'll be comparing values of types that are at least large enough to contains unsigned 32-bit.

The value (uint32Var & 0x80000000u) is a false one if it is equal to 0, otherwise it is a true one. Comparing the value to 0 has the effect, and yielding 0 if the comparison is equal, otherwise yielding 1 is equivalent behavior.

As another note, the value that you end up using for the first operand of the ternary operator isn't a bool, it's an int. The != operator yields an int value of either 0 or 1.

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

I assume the issue is with the first argument being an unsigned data type, instead of boolean, which means the fix bellow would work. /--/

Is this a correct change to make?

Yes. As in, it will make the code MISRA compliant. The warning is indeed about using a non-boolean type for the 1st operand.

However, assuming that the function returns uint32_t, you may as well just write:

return (uint32_t) !(uint32Var & mask);

(mask since MISRA, like everyone else, discourages the use of "magic numbers".)

Equivalent and also MISRA-C compatible:

return ~uint32Var >> bits; // where bits in this case is 31

Normally the ~ operator is a nasty piece when writing MISRA compilant code, but it is perfectly safe when you use an unsigned integer type which is not a small integer type, such as uint32_t.

Ternary operator argument evaluation in C

2 Answers2