Advanced Programming in the UNIX® Environment

ptg10805159

506 Advanced I/O Chapter 14

It is important to realize that whether a descriptor is blocking or not doesn’t affect

whetherselectblocks. That is, if we have a nonblocking descriptor that we want to

read from and we callselectwith a timeout value of 5 seconds,selectwill block for

up to 5 seconds. Similarly, if we specify an infinite timeout,selectblocks until data is

ready for the descriptor or until a signal is caught.

If we encounter the end of file on a descriptor,that descriptor is considered readable

byselect.Wethen callreadand it returns 0—the way to signify end of file on UNIX

systems. (Many people incorrectly assume that select indicates an exception

condition on a descriptor when the end of file is reached.)

POSIX.1 also defines a variant ofselectcalledpselect.

#include <sys/select.h>

int pselect(intmaxfdp1,fd_set *restrictreadfds,

fd_set *restrict writefds,fd_set *restrictexceptfds,

const struct timespec *restrict tsptr,

const sigset_t *restrictsigmask);

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

Thepselectfunction is identical toselect,with the following exceptions.

•The timeout value forselectis specified by atimevalstructure, but for

pselect,atimespecstructure is used. (Recall the definition of thetimespec

structure in Section 4.2.) Instead of seconds and microseconds, thetimespec

structurerepresents the timeout value in seconds and nanoseconds. This

provides a higher-resolution timeout if the platform supports that fine a level of

granularity.

•The timeout value forpselectis declaredconst,and we areguaranteed that

its value will not change as a result of callingpselect.

•Anoptional signal mask argument is available withpselect.Ifsigmaskis

NULL,pselectbehaves asselectdoes with respect to signals. Otherwise,

sigmaskpoints to a signal mask that is atomically installed whenpselectis

called. Onreturn, the previous signal mask is restored.

14.4.2 pollFunction

Thepollfunction is similar toselect,but the programmer interface is different. This

function was originally introduced in System V to support the STREAMS subsystem,

but we areable to use it with any type of file descriptor.

#include <poll.h>

int poll(struct pollfdfdarray[], nfds_t nfds,int timeout);

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