Advanced Programming in the UNIX® Environment

ptg10805159

Section 14.4 I/O Multiplexing 507

Withpoll,instead of building a set of descriptors for each condition (readability,

writability,and exception condition) as we did withselect, we build an array of

pollfdstructures, with each array element specifying a descriptor number and the

conditions that we’reinterested in for that descriptor:

struct pollfd {

int fd; /* file descriptor to check, or <0 to ignore */

short events; /* events of interest on fd */

short revents; /* events that occurred on fd */

};

The number of elements in thefdarrayarray is specified bynfds.

Historically,therehave been differences in how thenfdsparameter was declared. SVR3

specified the number of elements in the array as anunsigned long,which seems excessive.

In the SVR4 manual [AT&T 1990d], the prototype forpollshowed the data type of the second

argument assize_t.(Recall the primitive system data types from Figure2.21.) But the actual

prototype in the<poll.h>header still showed the second argument as anunsigned long.

The Single UNIX Specification defines the new typenfds_tto allow the implementation to

select the appropriate type and hide the details from applications. Note that this type has to be

large enough to hold an integer,since the return value represents the number of entries in the

array with satisfied events.

The SVID corresponding to SVR4 [AT&T 1989] showed the first argument topollasstruct

pollfdfdarray[],whereas the SVR4 manual page [AT&T 1990d] showed this argument as

struct pollfd *fdarray.Inthe C language, both declarations areequivalent. Weuse the

first declaration to reiterate thatfdarraypoints to an array of structures and not a pointer to

asingle structure.

To tell the kernel which events we’reinterested in for each descriptor, we have to set

theeventsmember of each array element to one or more of the values in Figure14.17.

On return, thereventsmember is set by the kernel, thereby specifying which events

have occurred for each descriptor.(Note that poll doesn’t change the events

member.This behavior differs from that ofselect,which modifies its arguments to

indicate what is ready.)

Input to Result from

Name events? revents? Description

POLLIN ••Data other than high priority data can be read without blocking

(equivalent toPOLLRDNORM|POLLRDBAND).

POLLRDNORM ••Normal data can be read without blocking.

POLLRDBAND ••Priority data can be read without blocking.

POLLPRI ••High-priority data can be read without blocking.

POLLOUT ••Normal data can be written without blocking.

POLLWRNORM ••Same asPOLLOUT.

POLLWRBAND ••Priority data can be written without blocking.

POLLERR • An error has occurred.

POLLHUP • Ahangup has occurred.

POLLNVAL • The descriptor does not reference an open file.

Figure 14.17 Theeventsandreventsflags forpoll