select

Function

Enable I/O multiplexing

totalFds = select(nfds, readfds, writefds, errorfds, timeout)

nfds - the range $[0, nfds)$ of file descriptors to check
readfds - a bit map, set bit $x$ to check whether file descriptor $x$ is ready for reading
writefds - set bit $y$ to check whether file descriptors $y$ is ready for writing
errorfds - bit map to check for errors
timeout - how long to wait before stop blocking the process
totalFds - number of ready descriptors, 0 for timeout, negative for error

When we call select, the process is put to sleep and the kernel will help us monitor the file descriptor we specify
Once a file descriptor is ready or timeout, the kernel will revise readfds, writefds, or errorfds making only the file descriptor which is ready to be 1
The kernel will then awake the process, then we can scan through readfds, writefds, and errorfds to find which file descriptor is ready
Eventually, the process can operate on the file

To tell the process which file descriptor is available, select need to modify readfds, writefds, …
If we only need to monitor fd 1000, we still need to set nfds to 1001