[Linux manuál]
Návod, kniha: POSIX Programmer's Manual
int poll(struct pollfd fds[], nfds_t
nfds , int timeout);
poll: multiplexování vstupu / výstupu
Originální popis anglicky: poll - input/output multiplexingNávod, kniha: POSIX Programmer's Manual
STRUČNĚ
#include <poll.h>POPIS / INSTRUKCE
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:- POLLIN
- Data other than high-priority data may be read without blocking.
- POLLRDNORM
- Normal data may be read without blocking.
- POLLRDBAND
- Priority data may be read without blocking.
- POLLPRI
- High-priority data may be read without blocking.
- POLLOUT
- Normal data may be written without blocking.
- POLLWRNORM
- Equivalent to POLLOUT.
- POLLWRBAND
- Priority data may be written.
- POLLERR
- An error has occurred on the device or stream. This flag is only valid in the revents bitmask; it shall be ignored in the events member.
- POLLHUP
- The device has been disconnected. This event and POLLOUT are mutually-exclusive; a stream can never be writable if a hangup has occurred. However, this event and POLLIN, POLLRDNORM, POLLRDBAND, or POLLPRI are not mutually-exclusive. This flag is only valid in the revents bitmask; it shall be ignored in the events member.
- POLLNVAL
- The specified fd value is invalid. This flag is only
valid in the revents member; it shall ignored in the events
member.
NÁVRATOVÁ HODNOTA
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.CHYBY / ERRORY
The poll() function shall fail if:- EAGAIN
- The allocation of internal data structures failed but a subsequent request may succeed.
- EINTR
- A signal was caught during poll().
- EINVAL
- The nfds argument is greater than {OPEN_MAX},
or one of the fd members refers to a STREAM or multiplexer
that is linked (directly or indirectly) downstream from a multiplexer.
PŘÍKLADY POUŽITÍ
Checking for Events on a Stream
The following example opens a pair of STREAMS devices and then waits for either one to become writable. This example proceeds as follows:- 1.
- Sets the timeout parameter to 500 milliseconds.
- 2.
- Opens the STREAMS devices /dev/dev0 and /dev/dev1, and then polls them, specifying POLLOUT and POLLWRBAND as the events of interest.
- 3.
- 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).
- 4.
- Checks the returned value of ret. If a positive value is returned, one of the following can be done:
- 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]).
- b.
- 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]).
- 5.
- If the returned value is not a positive value, permission to write data to the open STREAM (on any priority band) is denied.
- 6.
- 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. */
...
}
}
}
APPLICATION USAGE
None.RATIONALE
None.FUTURE DIRECTIONS
None.SOUVISEJÍCÍ
STREAMS , getmsg() , putmsg() , read() , select() , write() , the Base Definitions volume of IEEE Std 1003.1-2001, <poll.h>, <stropts.h>COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .| 2003 | IEEE/The Open Group |