setpriorityWHICH,WHO,PRIORITY
This function sets the current priority for a process, a process group, or a user. See setpriority(2). Invoking setpriority will produce a fatal error if used on a machine that doesn't implement setpriority(2). To "nice" your process down by four units (the same as executing your program with nice(1)), try:
setpriority 0, 0, getpriority(0, 0) + 4;
The interpretation of a given priority may vary from one operating system to the next.