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. An enhanced facility with support for signal masking is . The select system call is similar to the newer 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 However, with the C10k problem, both select and poll have been superseded by the likes of , /dev/poll, and I/O completion ports.
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:
<code>fd_set type</code> arguments may be manipulated with four utility macros: , and .
Select returns the total number of bits set in and , 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 ' provides a more flexible solution.