The Linux Programming Interface

Alternative I/O Models 1331

We can use select() and poll() to monitor file descriptors for regular files, termi-

nals, pseudoterminals, pipes, FIFOs, sockets, and some types of character devices.

Both system calls allow a process either to block indefinitely waiting for file descrip-

tors to become ready or to specify a timeout on the call.

63.2.1 The select() System Call

The select() system call blocks until one or more of a set of file descriptors becomes

ready.

The nfds, readfds, writefds, and exceptfds arguments specify the file descriptors that

select() is to monitor. The timeout argument can be used to set an upper limit on the

time for which select() will block. We describe each of these arguments in detail below.

In the prototype for select() shown above, we include <sys/time.h> because that

was the header specified in SUSv2, and some UNIX implementations require

this header. (The <sys/time.h> header is present on Linux, and including it

does no harm.)

File descriptor sets

The readfds, writefds, and exceptfds arguments are pointers to file descriptor sets, repre-

sented using the data type fd_set. These arguments are used as follows:

z readfds is the set of file descriptors to be tested to see if input is possible;

z writefds is the set of file descriptors to be tested to see if output is possible; and

z exceptfds is the set of file descriptors to be tested to see if an exceptional condi-

tion has occurred.

The term exceptional condition is often misunderstood to mean that some sort of

error condition has arisen on the file descriptor. This is not the case. An exceptional

condition occurs in just two circumstances on Linux (other UNIX implementations

are similar):

z A state change occurs on a pseudoterminal slave connected to a master that is

in packet mode (Section 64.5).

z Out-of-band data is received on a stream socket (Section 61.13.1).

Typically, the fd_set data type is implemented as a bit mask. However, we don’t

need to know the details, since all manipulation of file descriptor sets is done via

four macros: FD_ZERO(), FD_SET(), FD_CLR(), and FD_ISSET().

#include <sys/time.h> /* For portability */

#include <sys/select.h>

int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,

struct timeval *timeout);

Returns number of ready file descriptors, 0 on timeout, or –1 on error