Advanced Programming in the UNIX® Environment

ptg10805159

502 Advanced I/O Chapter 14

I/O multiplexing was provided with theselectfunction in 4.2BSD. This function has

always worked with any descriptor,although its main use has been for terminal I/O and

network I/O. SVR3 added thepollfunction when the STREAMS mechanism was added.

Initially,pollworked only with STREAMS devices. In SVR4, support was added to allow

pollto work on any descriptor.

14.4.1 selectandpselect Functions

The select function lets us do I/O multiplexing under all POSIX-compatible

platforms. The arguments we pass toselecttell the kernel

•Which descriptors we’reinterested in.

•Which conditions we’reinterested in for each descriptor.(Do we want to read

from a given descriptor? Do we want to write to a given descriptor? Arewe

interested in an exception condition for a given descriptor?)

•How long we want to wait. (Wecan wait forever,wait a fixed amount of time,

or not wait at all.)

On the return fromselect,the kernel tells us

•The total count of the number of descriptors that areready

•Which descriptors areready for each of the three conditions (read, write, or

exception condition)

With this return information, we can call the appropriate I/O function (usuallyreador

write)and know that the function won’t block.

#include <sys/select.h>

int select(intmaxfdp1,fd_set *restrict readfds,

fd_set *restrict writefds,fd_set *restrict exceptfds,

struct timeval *restricttvptr);

Returns: count of ready descriptors, 0 on timeout,−1 on error

Let’s look at the last argument first. It specifies how long we want to wait in terms

of seconds and microseconds (recall Section 4.20). Thereare three conditions.

tvptr== NULL

Wait forever.This infinite wait can be interrupted if we catch a signal. Return

is made when one of the specified descriptors is ready or when a signal is

caught. Ifasignal is caught,selectreturns−1witherrnoset toEINTR.

tvptr−>tv_sec== 0 &&tvptr−>tv_usec== 0

Don’t wait at all. All the specified descriptors aretested, and return is made

immediately.This is a way to poll the system to find out the status of multiple

descriptors without blocking in theselectfunction.