如果两个函数都返回布尔值，使用按位运算符将它们组合起来是否安全？

Question

Suppose I have two functions which may have side effects, and return boolean values. (with bool as defined in <stdbool.h> , so that defines bool as the _Bool type)

bool tweedledee(MyState *pmystate);
bool tweedledum(MyState *pmystate);

Is it safe to use bitwise operators to combine them?

bool both_tweedles = tweedledee(&mystate) & tweedledum(&mystate);

or

bool either_tweedle = tweedledee(&mystate) | tweedledum(&mystate);

I would traditionally use the logical operators && or || ; I'm working on a project where my team members are using the bitwise operators, since the functions may have side effects, and we want both function calls to occur. (The logical operators are short-circuiting .)

My only reservation is that I'm not sure whether a function returning bool can safely be assumed to return 1 and 0, rather than something else as an alternate value for true .

---

Just as an example: what if someone evil implemented them as follows?

bool tweedledee(MyState *pmystate)
{
   return 66;
}

bool tweedledum(MyState *pmystate)
{
   return 33;
}

then 66 & 33 is 0, and 66 | 33 66 | 33 is 99.

Answer 1

From section 6.3.1.2/1 of the C99 standard:

When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1.

If you're using stdbool.h , then bool (which is a preprocessor macro defined to be _Bool ) therefore must be only 0 or 1, your tweedledee and tweedledum functions cannot return values outside of that range, and using bitwise operations on them should do what you expect.

If, however, tweedledee and tweedledum are using some other bool type that isn't _Bool , then all bets are off.

Answer 2

To some extent, the safety of the sample that you posted will depend on the compiler/version/platform in use.

A potentially safer implementation for the use of '&' would be something like the following:

bool either_tweedle = ( tweedledee(&mystate) != 0 ) & ( tweedledum(&mystate) != 0 );

This defends against the possibility of either tweedledee or tweedledum shoving a value other than 0 or 1 into their bool return value.

The '|' case you provided should be perfectly safe.

Answer 3

I'm not sure whether a function returning bool can safely be assumed to return 1 and 0, rather than something else as an alternate value for true.

Yes. You can safely assume this and use bitwise operations.

From C99 (ISO 9899:1999) 7.16.3:

The remaining three macros are suitable for use in #if preprocessing directives. They are:

true which expands to the integer constant 1,

false which expands to the integer constant 0, and

__bool_true_false_are_defined which expands to the integer constant 1.

Answer 4

If you are using the result of the logical operation as the condition of an if-statement, it is possible that one of the both functions will not be executed at all. If this is not intended, then you'd better save the results in two variables and then do logical operation directly on the result variables.