select is a system call and application programming interface (API) in Unix-like and POSIX-compliant operating systems for examining the status of file descriptors of open input/output channels. The select system call is similar to the poll facility introduced in UNIX System V and later operating systems.
In the C programming language, the select system call is declared in the header file sys/select.h or unistd.h, and has the following syntax:
fd_set type arguments may be manipulated with four utility macros: FD_SET(), FD_CLR(), FD_ZERO(), and FD_ISSET().
Select returns the total number of bits set in readfds, writefds and errorfds, or zero if the timeout expired, and -1 on error.
The sets of file descriptor used in select are finite in size, depending on the operating system. The newer system call poll provides a more flexible solution.