Operation on ... may be undefined?

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 for each node is
int trackFront[5] = {0, 0, 0, 0, 0}; // Array to keep track of which is the front of the array
int trackTail[5] = {0, 0, 0, 0, 0};


// Function to add to the array (CHANGE int frame)
void addFrame (int nodeNumber, FRAME frame)
{
	//Calc tail
	int tail = trackTail[nodeNumber-1];

	// Calc frames in buffer
	int framesinBuffer = trackBufferFull[nodeNumber-1];

	if (framesinBuffer == 10)
	{
		printf("Buffer is full\n");
	}
	else
	{
		
		// Add frame to frameArray
		frameArray[nodeNumber-1][tail] = frame; 
		printf("\nAdded a frame in node: %i to the buffer\n", nodeNumber);

		// Increment the count
		trackBufferFull[nodeNumber-1]++;
		trackTail[nodeNumber-1] = ++trackTail[nodeNumber-1] % 10;


	}  
}

The arrays I use for frameArray is a wrap-around/cyclic array of length 10, hence why I have the code

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

Everything works perfectly in a standalone file, however when run inside of a larger file, I get the following compile errors:

$ cnet GARETH -m 30
compiling gareth.c
gareth.c: In function ‘addFrame’:
gareth.c:77:27: error: operation on ‘trackTail[nodeNumber + -0x00000000000000001]’ may be undefined [-Werror=sequence-point]
gareth.c: In function ‘removeFirstFrame’:
gareth.c:98:28: error: operation on ‘trackFront[nodeNumber + -0x00000000000000001]’ may be undefined [-Werror=sequence-point]
gareth.c:105:1: error: control reaches end of non-void function [-Werror=return-type]
cc1: all warnings being treated as errors

Line 77 is the line

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

Help.

To see the code with line numbers and the errors side by side, I've uploaded an image to: http://i.imgur.com/wyO5a.png

> 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;

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;

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.