In a process where multiple I/O operations can occur it is necessary for the program to be able to wait on one or several of the operations to complete. For example, consider a program that issues a READ to multiple sockets whose blocking mode is set. Because the socket would block on a READ macro, only one socket could be read at a time. Setting the sockets to nonblocking would solve this problem, but would require polling each socket repeatedly until data becomes available. The SELECT macro allows you to test several sockets and to process a later I/O macro only when one of the tested sockets is ready. This ensures that the I/O macro does not block.