Unsequenced modification and access

As of this posting, searching Google for the title of this article will provide you with a Link to the patch for the -Wunsequenced flag introduced in C11, but it won’t tell you what this means. So I will.


int letters[15];
int index = 0;
...
if(((letters[index] & 0xF) - (letters[++index] & 0xF)) == 1) {
...


This will produce the warning “Unsequenced modification and access to ‘index'” on the if statement line. Why? Because the ‘-‘ operator is not a sequence point. Basically, the compiler is free to reorder anything and everything until it hits a “sequence point”. Things like return, if, assignment, variable declaration, etc are all sequence points.

But, when the compiler looks at addition (or subtraction), it might decide to evaluate the right side before the left, because subtraction is not a sequence point. This would result in evaluating letters[++index] before letters[index], which is not at all what you expected.

So yes, while this looks completely valid and deterministic read left to right, nobody forced the compiler to read it left to right, and ambiguity resulted. Now you know.