The example that I am looking at is like this:
#define CONTROL_MEM_SIZE ((CONTROL_ITEM_SIZE * CONTROL_QUEUE_SIZE) + \\
portQ
It's not an operator, really. It's just a line extension - it tells the preprocessor that the #define replacement text continues on the next line of the file.
Check out #3 at this link:
Continued lines are merged into one long line. A continued line is a line which ends with a backslash,
\. The backslash is removed and the following line is joined with the current one.