select (Unix)
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.1 An enhanced facility with support for signal masking is pselect. The select system call is similar to the newer poll facility introduced in UNIX System V and later operating systems. However, with the C10k problem, both select and poll have been superseded by the likes of kqueue,/dev/poll, epoll and I/O completion ports.2
One common use of select outside of its stated use of waiting on filehandles is to implement a portable sub-second sleep. This can be achieved by passing NULL for all three fd_set arguments, and the duration of the desired sleep as the timeout argument.
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:
int select(int nfds, fd_set﹡ readfds, fd_set﹡ writefds, fd_set﹡ errorfds, struct timeval﹡ timeout);
fd_set typearguments 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.
Printed 2026-10-13.
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