CODEWITHSUNDEEP
cc99operator-precedence
Multsens
Multsens

Reputation: 75

C operator Precedence == and &

I am pretty confused by a library doing something similar to:

while ( func() & DEFINED_CONST == DEFINED_CONST )
{
...
}

I am compiling C99 strict ansi. I thought that precedence of == is higher than &. If this is the case the evaluation of the DEFINED_CONST == DEFINED_CONST would be first. This would be totally nonsense. Since this example comes from a company which I think will not provide such bullshit, I am asking me, where is my lag of knowledge, what do I get wrong.

It would make sense to do: (func() & DEFINED_CONST) == DEFINED_CONST, but since the precedence of == is higher I thought it would get evaluated like func() & (DEFINED_CONST == DEFINED_CONST).

So what do I get wrong.

For background, func() is reading an register.

Upvotes: 0

Views: 184

Answers (2)

0___________
0___________

Reputation: 67546

if you compile yor code the compiler will tell you what the problem is :https://godbolt.org/z/61edYq

At the moment this expression checks if LSBit is set in the value returned by func.

What you can do:

  1. Add parentheses:
while ( (func() & DEFINED_CONST) == DEFINED_CONST )
  1. If DEFINED_CONST has only one bit set simply do not compare. It is enough to:
while ( func() & DEFINED_CONST)

Upvotes: 2

nos
nos

Reputation: 229108

Yes, you are right, that code is probably wrong and it is the same as

func() & (DEFINED_CONST == DEFINED_CONST)

Likely the code should be

(func() & DEFINED_CONST) == DEFINED_CONST

Upvotes: 2

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