Operation on … may be undefined?

Question

I have the following code

FRAME frameArray[5][10]; // Create the array of frames
int trackBufferFull[5] = {0, 0, 0, 0, 0};// Keeps track of how full the buffer f         


        

Answer 1

Line 77 is the line

trackTail[nodeNumber-1] = ++trackTail[nodeNumber-1] % 10;

You are changing trackTail[nodeNumber-1] twice between sequence points: once through ++, and once through assignment.

This is undefined behaviour.

The remedy is to rephrase the statement, for example like so:

trackTail[nodeNumber-1] = (trackTail[nodeNumber-1] + 1) % 10;

or like so:

trackTail[nodeNumber-1]++;
trackTail[nodeNumber-1] %= 10;

Answer 2

You're modifying trackTail[nodeNumber - 1] between sequence points. It's like you're assigning

i = ++i;

which is also undefined behaviour.

Change your code to something like this:

trackTail[nodeNumber - 1] = (trackTail[nodeNumber - 1] + 1) % 10;

Answer 3

trackTail[nodeNumber-1] = ++trackTail[nodeNumber-1] % 10;

Yep, that's undefined behavior just as the error message says. You're not allowed to modify the same value twice without a sequence point in between. In this case that means you're not allowed to both increment trackTail[nodeNumber-1] using ++ and reassign it using =.

If you just use + 1 instead of ++, it will work fine.