poll, devpoll.h - Device driver for a fast poll on many
file descriptors
#include <sys/devpoll.h>
The /dev/poll driver supports the monitoring of one or
more sets of file descriptors. Access to the /dev/poll
driver is supported through the open(), write(), and
ioctl() functions. By using open(), write(), and ioctl()
calls with the /dev/poll driver, an application can poll
large numbers of file descriptors more efficiently than by
using poll() and select() calls.
Creating a Poll Set [Toc] [Back]
Applications create a set of file descriptors to be monitored
by opening the /dev/poll driver and then writing an
array of pollfd structures to the driver. The open() call
on the driver returns the file descriptor that identifies
the poll set. To monitor multiple sets of file descriptors,
the application must open the driver multiple times
to retrieve different poll set descriptors.
Each entry in the array written to the driver is a pollfd
structure, which is defined in sys/poll.h:
struct pollfd {
int fd;
short events;
short revents; }
In this structure: Specifies the file descriptor being
polled. Specifies the events to be monitored for this
file descriptor. Flags that can be specified for the
events field are the same as those used with the poll()
system call. See poll(2) for information about these
flags.
The /dev/poll driver supports one additional flag,
POLLREMOVE, which is described in Removing a File
Descriptor From a Poll Set.
If the buffer array written to the driver contains
more than one pollfd entry with the same fd value,
the last pollfd entry for that fd will be used for
the value of events. If a previous write operation
created a pollfd entry that contains the same fd
value as an entry written by a new write operation,
the events value from the new write operation overwrites
the old events value. Not used for poll set
creation.
This field is used with the ioctl() function's
DP_POLL request. For more information, see Monitoring
Events for Poll Set Members (DP_POLL Ioctl).
Removing a File Descriptor From a Poll Set [Toc] [Back]
To remove a file descriptor from a poll set, the application
writes to the driver a pollfd entry in which fd is
set to the file descriptor being removed and events is set
to POLLREMOVE.
Monitoring Events for Poll Set Members (DP_POLL Ioctl) [Toc] [Back]
Applications retrieve events for file descriptors in a
poll set by using an ioctl() call that contains the following
arguments: The file descriptor for the poll set
(returned when the driver was opened). DP_POLL A pointer
to the dvpoll structure
The dvpoll structure is defined in <sys/devpoll.h> as follows:
struct dvpoll {
struct pollfd * dp_fds;
int dp_nfds;
int dp_timeout; }
In this structure: Points to an array of returned pollfd
structures as described in Creating a Poll Set. Specifies
the number of pollfd structures to be returned.
The application can set the dp_nfds value lower
than the number of file descriptors in the poll set
when there is a need to limit the number of file
descriptors for which information is gathered. The
number of miliseconds to wait before returning if
none of the events being monitored for the file
descriptors in the poll set have occurred.
If the application sets dp_timeout to -1, the
ioctl() call blocks until an event occurs or the
call in interrupted. If the application sets
dp_timeout to 0, the call returns immediately.
The ioctl() call with a DP_POLL request returns the following:
Success. This value is the number of valid pollfd
entries that are returned into the array pointed to by
dp_fds. For each valid entry in this array: Indicates the
file descriptor polled. Indicates the events being monitored
by the application for that file descriptor. Indicates
which of those events, if any, occurred. See poll(2)
for descriptions of flag values that can be returned to
this field.
The contents of the rest of the buffer are undefined.
The call timed out. In this case, the content
of the array pointed to by dp_fds is not modified.
An error occurred. In this case, errno is
set to indicate the error.
Querying a Poll Set for a File Descriptor (DP_ISPOLLED Ioctl) [Toc] [Back]
To determine if a file descriptor is already a member of a
poll set, applications call ioctl() with DP_ISPOLLED as
the request argument. The fildes and arg arguments are the
same as for the DP_POLL request as described in Monitoring
Events for Poll Set Members (DP_POLL Ioctl). Before the
call is made, the application sets the fd field of the
pollfd entry in the array pointed to by arg to the value
of the file descriptor being queried.
An ioctl() call with a DP_ISPOLLED request returns the
following: The file descriptor is a member of the poll
set. In this case, the events field of the pollfd entry is
modified to indicate the events being monitored for that
file descriptor. The file descriptor is not a member of
the poll set. In this case, the pollfd entry for the file
descriptor is not modified. An error occurred. In this
case, errno is set to indicate the error.
The poll() and select() functions cannot be used with the
/dev/poll driver.
The following code fragment shows how to create a poll set
and query events that occurred for its members:
{
...
/*
* open the driver
*/
if ((dfd = open("/dev/poll", O_RDWR)) < 0) {
exit(-1);
}
pollfd = (struct pollfd* )malloc(sizeof(struct
pollfd) * MAXBUF);
if (pollfd == NULL) {
close(dfd);
exit(-1);
}
/*
* initialize buffer
*/
for (i = 0; i < MAXBUF; i++) {
pollfd[i].fd = fds[i];
pollfd[i].events = POLLIN;
pollfd[i].revents = 0;
}
if (write(dfd, &pollfd[0], sizeof(struct
pollfd) * MAXBUF) !=
sizeof(struct pollfd) *
MAXBUF) {
perror("failed to write all pollfds");
close (dfd);
free(pollfd);
exit(-1);
}
/*
* read from the /dev/poll driver
*/
dopoll.dp_timeout = -1;
dopoll.dp_nfds = MAXBUF;
dopoll.dp_fds = pollfd;
result = ioctl(dfd, DP_POLL, &dopoll);
if (result < 0) {
perror("/dev/poll ioctl DP_POLL
failed");
close (dfd);
free(pollfd);
exit(-1);
}
for (i = 0; i < result; i++) {
read(dopoll.dp_fds[i].fd, rbuf,
STRLEN);
}
... } The following code fragment shows how
to determine if a file descriptor is a member of a
poll set and how to remove the file descriptor from
the set:
... main(argc,argv ) char **argv[]; int argc; {
int fid;
int fid2;
struct pollfd upoll;
int i;
int nfid;
int error;
struct dvpoll dvp;
fid = open( "/dev/poll", O_RDWR, 0);
fid2 = open( "/dev/poll", O_RDWR, 0);
for (i=0; i< 512; i++) {
nfid = open( "/dev/null", O_RDWR, 0);
upoll.fd=nfid;
upoll.events=POLLNORM;
if (write(fid, &upoll, sizeof(struct
pollfd)) < 0)
perror("write");
if (write(fid2, &upoll, sizeof(struct
pollfd)) < 0)
perror("write");
}
upoll.fd= 400;
upoll.events= 888;
upoll.revents= 999;
error = ioctl(fid , DP_ISPOLLED, &upoll);
printf("%d %d %d %d\n", error, upoll.fd,
upoll.events, upoll.revents);
upoll.fd=400;
upoll.events=POLLREMOVE;
if (write(fid, &upoll, sizeof(struct
pollfd)) < 0)
perror("write");
. . .
close(fid); }
For error information, see the reference pages for the
system calls discussed in this reference page.
Device special file for the fast poll driver.
Functions: ioctl(2), open(2), poll(2), select(2), write(2)
poll(7)
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