Question

Consider the C code a = a = a. There’s no sequence point for assignment, so this code produces a warning when compiling about an undefined operation on a.

What are the possible values that a could have here? It seems like a couldn’t possibly change values. Is there actually undefined behavior here or are compilers just being lazy?

Answer 1

The rules of undefined behavior for sequence point violations do not make an exception for situations when “the value cannot change”. Nobody cares whether the value changes or not. What matters is that when you are making any sort of write access to the variable, you are modifying that variable. Even if you are assigning the variable a value that it already holds, you are still performing a modification of that variable. And if multiple modifications are not separated by sequence points, the behavior is undefined.

One can probably argue that such “non-modifying modifications” should not cause any problems. But the language specification does not concern itself with such details. In language terminology, again, every time you are writing something into a variable, you are modifying it.

Moreover, the fact that you use the word “ambiguous” in your question seems to imply that you believe the behavior is unspecified. I.e. as in “the resultant value of the variable is (or isn’t) ambiguous”. However, in sequence point violations the language specification does not restrict itself to stating that the result is unspecified. It goes much further and declares the behavior undefined. This means that the rationale behind these rules takes into consideration more than just an unpredictable final value of some variable. For example, on some imaginary hardware platform non-sequenced modification might result in invalid code being generated by the compiler, or something like that.