Tribblix: manual page: poll.4d

POLL(4D) Devices POLL(4D)

NAME

poll - driver for fast poll on many file descriptors

SYNOPSIS

#include <sys/devpoll.h>
int fd = open("/dev/poll", O_RDWR);
ssize_t n = write(int fd, struct pollfd buf[], int bufsize);
int n = ioctl(int fd, DP_POLL, struct dvpoll* arg);
int n = ioctl(int fd, DP_ISPOLLED, struct pollfd* pfd);

PARAMETERS

fd
Open file descriptor that refers to the /dev/poll
driver.

path
/dev/poll

buf
Array of pollfd structures.

bufsize
Size of buf in bytes.

arg
Pointer to pollcall structure.

pfd
Pointer to pollfd structure.

DESCRIPTION

The /dev/poll driver is a special driver that enables you to monitor
multiple sets of polled file descriptors. By using the /dev/poll
driver, you can efficiently poll large numbers of file descriptors.
Access to the /dev/poll driver is provided through open(2), write(2),
and ioctl(2) system calls.

Writing an array of pollfd struct to the /dev/poll driver has the
effect of adding these file descriptors to the monitored poll file
descriptor set represented by the fd. To monitor multiple file
descriptor sets, open the /dev/poll driver multiple times. Each fd
corresponds to one set. For each pollfd struct entry (defined in
sys/poll.h):

struct pollfd {
int fd;
short events;
short revents;
}

The fd field specifies the file descriptor being polled. The events
field indicates the interested poll events on the file descriptor. If
a pollfd array contains multiple pollfd entries with the same fd
field, the "events" field in each pollfd entry is OR'ed. A special
POLLREMOVE event in the events field of the pollfd structure removes
the fd from the monitored set. The revents field is not used. Write
returns the number of bytes written successfully or -1 when write
fails.

The DP_POLL ioctl is used to retrieve returned poll events occurred
on the polled file descriptors in the monitored set represented by
fd. arg is a pointer to the devpoll structures which are defined as
follows:

struct dvpoll {
struct pollfd* dp_fds;
int dp_nfds;
int dp_timeout;
}

The dp_fds points to a buffer that holds an array of returned pollfd
structures. The dp_nfds field specifies the size of the buffer in
terms of the number of pollfd entries it contains. The dp_nfds field
also indicates the maximum number of file descriptors from which poll
information can be obtained. If there is no interested events on any
of the polled file descriptors, the DP_POLL ioctl call will wait
dp_timeout milliseconds before returning. If dp_timeout is 0, the
ioctl call returns immediately. If dp_timeout is -1, the call blocks
until an interested poll events is available or the call is
interrupted. Upon return, if the ioctl call has failed, -1 is
returned. The memory content pointed by dp_fds is not modified. A
return value 0 means the ioctl is timed out. In this case, the memory
content pointed by dp_fds is not modified. If the call is
successful, it returns the number of valid pollfd entries in the
array pointed by dp_fds; the contents of the rest of the buffer is
undefined. For each valid pollfd entry, the fd field indicates the
file descriptor on which the polled events happened. The events
field is the user specified poll events. The revents field contains
the events occurred. -1 is returned if the call fails.

DP_ISPOLLED ioctl allows you to query if a file descriptor is already
in the monitored set represented by fd. The fd field of the pollfd
structure indicates the file descriptor of interest. The DP_ISPOLLED
ioctl returns 1 if the file descriptor is in the set. The events
field contains 0. The revents field contains the currently polled
events. The ioctl returns 0 if the file descriptor is not in the
set. The pollfd structure pointed by pfd is not modified. The ioctl
returns a -1 if the call fails.

EXAMPLES

The following example shows how /dev/poll may be used.

{
...
/*
* open the driver
*/
if ((wfd = open("/dev/poll", O_RDWR)) < 0) {
exit(-1);
}
pollfd = (struct pollfd* )malloc(sizeof(struct pollfd) * MAXBUF);
if (pollfd == NULL) {
close(wfd);
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(wfd, &pollfd[0], sizeof(struct pollfd) * MAXBUF) !=
sizeof(struct pollfd) * MAXBUF) {
perror("failed to write all pollfds");
close (wfd);
free(pollfd);
exit(-1);
}
/*
* read from the devpoll driver
*/
dopoll.dp_timeout = -1;
dopoll.dp_nfds = MAXBUF;
dopoll.dp_fds = pollfd;
result = ioctl(wfd, DP_POLL, &dopoll);
if (result < 0) {
perror("/dev/poll ioctl DP_POLL failed");
close (wfd);
free(pollfd);
exit(-1);
}
for (i = 0; i < result; i++) {
read(dopoll.dp_fds[i].fd, rbuf, STRLEN);
}
...
}

The following example is part of a test program which shows how
DP_ISPOLLED() ioctl may be used.

{
...

loopcnt = 0;
while (loopcnt < ITERATION) {
rn = random();
rn %= RANGE;
if (write(fds[rn], TESTSTRING, strlen(TESTSTRING)) !=
strlen(TESTSTRING)) {
perror("write to fifo failed.");
close (wfd);
free(pollfd);
error = 1;
goto out1;
}
dpfd.fd = fds[rn];
dpfd.events = 0;
dpfd.revents = 0;
result = ioctl(wfd, DP_ISPOLLED, &dpfd);
if (result < 0) {
perror("/dev/poll ioctl DP_ISPOLLED failed");
printf("errno = %d\n", errno);
close (wfd);
free(pollfd);
error = 1;
goto out1;
}
if (result != 1) {
printf("DP_ISPOLLED returned incorrect result: %d.\n",
result);
close (wfd);
free(pollfd);
error = 1;
goto out1;
}
if (dpfd.fd != fds[rn]) {
printf("DP_ISPOLLED returned wrong fd %d, expect %d\n",
dpfd.fd, fds[rn]);
close (wfd);
free(pollfd);
error = 1;
goto out1;
}
if (dpfd.revents != POLLIN) {
printf("DP_ISPOLLED returned unexpected revents %d\n",
dpfd.revents);
close (wfd);
free(pollfd);
error = 1;
goto out1;
}
if (read(dpfd.fd, rbuf, strlen(TESTSTRING)) !=
strlen(TESTSTRING)) {
perror("read from fifo failed");
close (wfd);
free(pollfd);
error = 1;
goto out1;
}
loopcnt++;
}

ERRORS

EACCES
A process does not have permission to access the content
cached in /dev/poll.

EINTR
A signal was caught during the execution of the ioctl(2)
function.

EFAULT
The request argument requires a data transfer to or from a
buffer pointed to by arg, but arg points to an illegal
address.

EINVAL
The request or arg parameter is not valid for this
device, or field of the dvpoll struct pointed by arg is
not valid (for example, when using write/pwrite dp_nfds
is greater than {OPEN_MAX}, or when using the DPPOLL ioctl
dp_nfds is greater than or equal to {OPEN_MAX}}.

ENXIO
The O_NONBLOCK flag is set, the named file is a FIFO, the
O_WRONLY flag is set, and no process has the file open for
reading; or the named file is a character special or block
special file and the device associated with this special
file does not exist.

ATTRIBUTES

NOTES

The /dev/poll API is particularly beneficial to applications that
poll a large number of file descriptors repeatedly. Applications
will exhibit the best performance gain if the polled file descriptor
list rarely change.

When using the /dev/poll driver, you should remove a closed file
descriptor from a monitored poll set. Failure to do so may result in
a POLLNVAL revents being returned for the closed file descriptor.
When a file descriptor is closed but not removed from the monitored
set, and is reused in subsequent open of a different device, you will
be polling the device associated with the reused file descriptor. In
a multithreaded application, careful coordination among threads doing
close and DP_POLL ioctl is recommended for consistent results.

The /dev/poll driver caches a list of polled file descriptors, which
are specific to a process. Therefore, the /dev/poll file descriptor
of a process will be inherited by its child process, just like any
other file descriptors. But the child process will have very limited
access through this inherited /dev/poll file descriptor. Any attempt
to write or do ioctl by the child process will result in an EACCES
error. The child process should close the inherited /dev/poll file
descriptor and open its own if desired.

The /dev/poll driver does not yet support polling. Polling on a
/dev/poll file descriptor will result in POLLERR being returned in
the revents field of pollfd structure.

January 10, 2020 POLL(4D)