.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved .TH "POLL" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual" .\" poll .SH NAME poll \- input/output multiplexing .SH SYNOPSIS .LP \fB#include .br .sp int poll(struct pollfd\fP \fIfds\fP\fB[], nfds_t\fP \fInfds\fP\fB, int\fP \fItimeout\fP\fB); \fP \fB .br \fP .SH DESCRIPTION .LP The \fIpoll\fP() 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 \fIfds\fP, \fIpoll\fP() shall examine the given file descriptor for the event(s) specified in \fIevents\fP. The number of \fBpollfd\fP structures in the \fIfds\fP array is specified by \fInfds\fP. The \fIpoll\fP() function shall identify those file descriptors on which an application can read or write data, or on which certain events have occurred. .LP The \fIfds\fP 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 \fBpollfd\fP structures within which \fIfd\fP specifies an open file descriptor and \fIevents\fP and \fIrevents\fP are bitmasks constructed by OR'ing a combination of the following event flags: .TP 7 POLLIN Data other than high-priority data may be read without blocking. .LP For STREAMS, this flag is set in \fIrevents\fP even if the message is of zero length. This flag shall be equivalent to POLLRDNORM | POLLRDBAND. .TP 7 POLLRDNORM Normal data may be read without blocking. .LP For STREAMS, data on priority band 0 may be read without blocking. This flag is set in \fIrevents\fP even if the message is of zero length. .TP 7 POLLRDBAND Priority data may be read without blocking. .LP For STREAMS, data on priority bands greater than 0 may be read without blocking. This flag is set in \fIrevents\fP even if the message is of zero length. .TP 7 POLLPRI High-priority data may be read without blocking. .LP For STREAMS, this flag is set in \fIrevents\fP even if the message is of zero length. .TP 7 POLLOUT Normal data may be written without blocking. .LP For STREAMS, data on priority band 0 may be written without blocking. .TP 7 POLLWRNORM Equivalent to POLLOUT. .TP 7 POLLWRBAND Priority data may be written. .LP 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. .TP 7 POLLERR An error has occurred on the device or stream. This flag is only valid in the \fIrevents\fP bitmask; it shall be ignored in the \fIevents\fP member. .TP 7 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 \fIrevents\fP bitmask; it shall be ignored in the \fIevents\fP member. .TP 7 POLLNVAL The specified \fIfd\fP value is invalid. This flag is only valid in the \fIrevents\fP member; it shall ignored in the \fIevents\fP member. .sp .LP The significance and semantics of normal, priority, and high-priority data are file and device-specific. .LP If the value of \fIfd\fP is less than 0, \fIevents\fP shall be ignored, and \fIrevents\fP shall be set to 0 in that entry on return from \fIpoll\fP(). .LP In each \fBpollfd\fP structure, \fIpoll\fP() shall clear the \fIrevents\fP member, except that where the application requested a report on a condition by setting one of the bits of \fIevents\fP listed above, \fIpoll\fP() shall set the corresponding bit in \fIrevents\fP if the requested condition is true. In addition, \fIpoll\fP() shall set the POLLHUP, POLLERR, and POLLNVAL flag in \fIrevents\fP if the condition is true, even if the application did not set the corresponding bit in \fIevents\fP. .LP If none of the defined events have occurred on any selected file descriptor, \fIpoll\fP() shall wait at least \fItimeout\fP milliseconds for an event to occur on any of the selected file descriptors. If the value of \fItimeout\fP is 0, \fIpoll\fP() shall return immediately. If the value of \fItimeout\fP is -1, \fIpoll\fP() shall block until a requested event occurs or until the call is interrupted. .LP 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. .LP The \fIpoll\fP() function shall not be affected by the O_NONBLOCK flag. .LP The \fIpoll\fP() function shall support regular files, terminal and pseudo-terminal devices, FIFOs, pipes, sockets and \ STREAMS-based files. The behavior of \fIpoll\fP() on elements of \fIfds\fP that refer to other types of file is unspecified. .LP Regular files shall always poll TRUE for reading and writing. .LP 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. .SH RETURN VALUE .LP Upon successful completion, \fIpoll\fP() 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 \fIrevents\fP member is non-zero). A value of 0 indicates that the call timed out and no file descriptors have been selected. Upon failure, \fIpoll\fP() shall return -1 and set \fIerrno\fP to indicate the error. .SH ERRORS .LP The \fIpoll\fP() function shall fail if: .TP 7 .B EAGAIN The allocation of internal data structures failed but a subsequent request may succeed. .TP 7 .B EINTR A signal was caught during \fIpoll\fP(). .TP 7 .B EINVAL The \fInfds\fP argument is greater than {OPEN_MAX}, \ or one of the \fIfd\fP members refers to a STREAM or multiplexer that is linked (directly or indirectly) downstream from a multiplexer. .sp .LP \fIThe following sections are informative.\fP .SH EXAMPLES .SS Checking for Events on a Stream .LP The following example opens a pair of STREAMS devices and then waits for either one to become writable. This example proceeds as follows: .IP " 1." 4 Sets the \fItimeout\fP parameter to 500 milliseconds. .LP .IP " 2." 4 Opens the STREAMS devices \fB/dev/dev0\fP and \fB/dev/dev1\fP, and then polls them, specifying POLLOUT and POLLWRBAND as the events of interest. .LP The STREAMS device names \fB/dev/dev0\fP and \fB/dev/dev1\fP 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. .LP .IP " 3." 4 Uses the \fIret\fP variable to determine whether an event has occurred on either of the two STREAMS. The \fIpoll\fP() function is given 500 milliseconds to wait for an event to occur (if it has not occurred prior to the \fIpoll\fP() call). .LP .IP " 4." 4 Checks the returned value of \fIret\fP. If a positive value is returned, one of the following can be done: .RS .IP " a." 4 Priority data can be written to the open STREAM on priority bands greater than 0, because the POLLWRBAND event occurred on the open STREAM ( \fIfds\fP[0] or \fIfds\fP[1]). .LP .IP " b." 4 Data can be written to the open STREAM on priority-band 0, because the POLLOUT event occurred on the open STREAM ( \fIfds\fP[0] or \fIfds\fP[1]). .LP .RE .LP .IP " 5." 4 If the returned value is not a positive value, permission to write data to the open STREAM (on any priority band) is denied. .LP .IP " 6." 4 If the POLLHUP event occurs on the open STREAM ( \fIfds\fP[0] or \fIfds\fP[1]), the device on the open STREAM has disconnected. .LP .sp .RS .nf \fB#include #include \&... struct pollfd fds[2]; int timeout_msecs = 500; int ret; int i; .sp /* 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; .sp ret = poll(fds, 2, timeout_msecs); .sp 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. */ \&... } } } \fP .fi .RE .SH APPLICATION USAGE .LP None. .SH RATIONALE .LP None. .SH FUTURE DIRECTIONS .LP None. .SH SEE ALSO .LP \fISTREAMS\fP , \fIgetmsg\fP() , \fIputmsg\fP() , \fIread\fP() , \fIselect\fP() , \fIwrite\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI\fP, \fI\fP .SH 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 .