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poll - input/output multiplexing
#include <poll.h>
int poll(struct pollfd fds[], nfds_t nfds,
int timeout);
The poll() function provides applications with a mechanism for multiplexing input/output over a set of file descriptors. For each member of the array pointed to by fds, poll() shall examine the given file descriptor for the event(s) specified in events. The number of pollfd structures in the fds array is specified by nfds. The poll() function shall identify those file descriptors on which an application can read or write data, or on which certain events have occurred.
The fds argument specifies the file descriptors to be examined and the events of interest for each file descriptor. It is a pointer to an array with one member for each open file descriptor of interest. The array's members are pollfd structures within which fd specifies an open file descriptor and events and revents are bitmasks constructed by OR'ing a combination of the following event flags:
For STREAMS, this flag is set in revents even if the message is of zero length. This flag shall be equivalent to POLLRDNORM | POLLRDBAND.
For STREAMS, data on priority band 0 may be read without blocking. This flag is set in revents even if the message is of zero length.
For STREAMS, data on priority bands greater than 0 may be read without blocking. This flag is set in revents even if the message is of zero length.
For STREAMS, this flag is set in revents even if the message is of zero length.
For STREAMS, data on priority band 0 may be written without blocking.
For STREAMS, data on priority bands greater than 0 may be written without blocking. If any priority band has been written to on this STREAM, this event only examines bands that have been written to at least once.
The significance and semantics of normal, priority, and high-priority data are file and device-specific.
If the value of fd is less than 0, events shall be ignored, and revents shall be set to 0 in that entry on return from poll().
In each pollfd structure, poll() shall clear the revents member, except that where the application requested a report on a condition by setting one of the bits of events listed above, poll() shall set the corresponding bit in revents if the requested condition is true. In addition, poll() shall set the POLLHUP, POLLERR, and POLLNVAL flag in revents if the condition is true, even if the application did not set the corresponding bit in events.
If none of the defined events have occurred on any selected file descriptor, poll() shall wait at least timeout milliseconds for an event to occur on any of the selected file descriptors. If the value of timeout is 0, poll() shall return immediately. If the value of timeout is -1, poll() shall block until a requested event occurs or until the call is interrupted.
Implementations may place limitations on the granularity of timeout intervals. If the requested timeout interval requires a finer granularity than the implementation supports, the actual timeout interval shall be rounded up to the next supported value.
The poll() function shall not be affected by the O_NONBLOCK flag.
The poll() function shall support regular files, terminal and pseudo-terminal devices, FIFOs, pipes, sockets and STREAMS-based files. The behavior of poll() on elements of fds that refer to other types of file is unspecified.
Regular files shall always poll TRUE for reading and writing.
A file descriptor for a socket that is listening for connections shall indicate that it is ready for reading, once connections are available. A file descriptor for a socket that is connecting asynchronously shall indicate that it is ready for writing, once a connection has been established.
Upon successful completion, poll() shall return a non-negative value. A positive value indicates the total number of file descriptors that have been selected (that is, file descriptors for which the revents member is non-zero). A value of 0 indicates that the call timed out and no file descriptors have been selected. Upon failure, poll() shall return -1 and set errno to indicate the error.
The poll() function shall fail if:
The following sections are informative.
The following example opens a pair of STREAMS devices and then waits for either one to become writable. This example proceeds as follows:
Sets the timeout parameter to 500 milliseconds.
Opens the STREAMS devices /dev/dev0 and /dev/dev1, and then polls them, specifying POLLOUT and POLLWRBAND as the events of interest.
The STREAMS device names /dev/dev0 and /dev/dev1 are only examples of how STREAMS devices can be named; STREAMS naming conventions may vary among systems conforming to the IEEE Std 1003.1-2001.
Uses the ret variable to determine whether an event has occurred on either of the two STREAMS. The poll() function is given 500 milliseconds to wait for an event to occur (if it has not occurred prior to the poll() call).
Checks the returned value of ret. If a positive value is returned, one of the following can be done: <ol type="a">
Priority data can be written to the open STREAM on priority bands greater than 0, because the POLLWRBAND event occurred on the open STREAM ( fds[0] or fds[1]).
Data can be written to the open STREAM on priority-band 0, because the POLLOUT event occurred on the open STREAM ( fds[0] or fds[1]).
If the returned value is not a positive value, permission to write data to the open STREAM (on any priority band) is denied.
If the POLLHUP event occurs on the open STREAM ( fds[0] or fds[1]), the device on the open STREAM has disconnected.
#include <stropts.h>
#include <poll.h>
...
struct pollfd fds[2];
int timeout_msecs = 500;
int ret;
int i;
/* Open STREAMS device. */
fds[0].fd = open("/dev/dev0", ...);
fds[1].fd = open("/dev/dev1", ...);
fds[0].events = POLLOUT | POLLWRBAND;
fds[1].events = POLLOUT | POLLWRBAND;
ret = poll(fds, 2, timeout_msecs);
if (ret > 0) {
/* An event on one of the fds has occurred. */
for (i=0; i<2; i++) {
if (fds[i].revents & POLLWRBAND) {
/* Priority data may be written on device number i. */
...
}
if (fds[i].revents & POLLOUT) {
/* Data may be written on device number i. */
...
}
if (fds[i].revents & POLLHUP) {
/* A hangup has occurred on device number i. */
...
}
}
}
STREAMS , getmsg() , putmsg() , read() , select() , write() , the Base Definitions volume of IEEE Std 1003.1-2001, <poll.h>, <stropts.h>
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