Question

I have been writing some code to create a byte array I will be sending over a socket to another process. However noticed some really odd behavior regarding my byte[].

The cout at the end prints out 99, however looking at my code, I couldn’t find where the value is being set. I create a char array of size sendingSize which is a constant. I don’t set the value 307200* 3 so I don’t understand how it prints out with a value…

char tosend[sendingSize];
//Send over the frame
for(int i = 0; i < 307200; i++)
{
    tosend[i * 3] = (byte)imCopy[i/640][i%640].red;
    tosend[i * 3+1] = (byte)imCopy[i/640][i%640].green;
    tosend[i * 3+2] = (byte)imCopy[i/640][i%640].blue;
}

char *bytePointer = tosend;

cout<<(int)tosend[307200* 3]<<endl;

Answer 1

Your code does not write any value into index 307200*3 (because the highest index your for loop reaches is 307199). So you are reading some byte from memory beyond the declared size of your array. This is undefined behaviour and anything could happen.

Some other programming languages (such as Java) do automatic range checking on arrays and would throw an exception in this case. In C++, you are expected to do the right thing and the compiler doesn’t generate range checking code for you.