Event Loop - File Interface

File Interface

Under Unix, the "everything is a file" paradigm naturally leads to a file-based event loop. Reading from and writing to files, inter-process communication, network communication and device control are all achieved using file I/O, with the target identified by a file descriptor. The select and poll system calls allow a set of file descriptors to be monitored for a change of state, e.g. when data becomes available to be read.

For example, consider a program that reads from a continuously updated file and displays its contents in the X Window System, which communicates with clients over a socket (either Unix domain or Berkeley):

main: file_fd = open ("logfile") x_fd = open_display construct_interface while changed_fds = select ({file_fd, x_fd}): if file_fd in changed_fds: data = read_from (file_fd) append_to_display (data) send_repaint_message if x_fd in changed_fds: process_x_messages