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-.\" Copyright (C) 2006, 2019 Michael Kerrisk <mtk.manpages@gmail.com>
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.\" Additions from Richard Gooch <rgooch@atnf.CSIRO.AU> and aeb, 971207
-.\" 2006-03-13, mtk, Added ppoll() + various other rewordings
-.\" 2006-07-01, mtk, Added POLLRDHUP + various other wording and
-.\" formatting changes.
-.\"
-.TH poll 2 2023-10-31 "Linux man-pages 6.7"
-.SH NAME
-poll, ppoll \- wait for some event on a file descriptor
-.SH LIBRARY
-Standard C library
-.RI ( libc ", " \-lc )
-.SH SYNOPSIS
-.nf
-.B #include <poll.h>
-.P
-.BI "int poll(struct pollfd *" fds ", nfds_t " nfds ", int " timeout );
-.P
-.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
-.B #include <poll.h>
-.P
-.BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ,
-.BI " const struct timespec *_Nullable " tmo_p ,
-.BI " const sigset_t *_Nullable " sigmask );
-.fi
-.SH DESCRIPTION
-.BR poll ()
-performs a similar task to
-.BR select (2):
-it waits for one of a set of file descriptors to become ready
-to perform I/O.
-The Linux-specific
-.BR epoll (7)
-API performs a similar task, but offers features beyond those found in
-.BR poll ().
-.P
-The set of file descriptors to be monitored is specified in the
-.I fds
-argument, which is an array of structures of the following form:
-.P
-.in +4n
-.EX
-struct pollfd {
- int fd; /* file descriptor */
- short events; /* requested events */
- short revents; /* returned events */
-};
-.EE
-.in
-.P
-The caller should specify the number of items in the
-.I fds
-array in
-.IR nfds .
-.P
-The field
-.I fd
-contains a file descriptor for an open file.
-If this field is negative, then the corresponding
-.I events
-field is ignored and the
-.I revents
-field returns zero.
-(This provides an easy way of ignoring a
-file descriptor for a single
-.BR poll ()
-call: simply set the
-.I fd
-field to its bitwise complement.)
-.P
-The field
-.I events
-is an input parameter, a bit mask specifying the events the application
-is interested in for the file descriptor
-.IR fd .
-This field may be specified as zero,
-in which case the only events that can be returned in
-.I revents
-are
-.BR POLLHUP ,
-.BR POLLERR ,
-and
-.B POLLNVAL
-(see below).
-.P
-The field
-.I revents
-is an output parameter, filled by the kernel with the events that
-actually occurred.
-The bits returned in
-.I revents
-can include any of those specified in
-.IR events ,
-or one of the values
-.BR POLLERR ,
-.BR POLLHUP ,
-or
-.BR POLLNVAL .
-(These three bits are meaningless in the
-.I events
-field, and will be set in the
-.I revents
-field whenever the corresponding condition is true.)
-.P
-If none of the events requested (and no error) has occurred for any
-of the file descriptors, then
-.BR poll ()
-blocks until one of the events occurs.
-.P
-The
-.I timeout
-argument specifies the number of milliseconds that
-.BR poll ()
-should block waiting for a file descriptor to become ready.
-The call will block until either:
-.IP \[bu] 3
-a file descriptor becomes ready;
-.IP \[bu]
-the call is interrupted by a signal handler; or
-.IP \[bu]
-the timeout expires.
-.P
-Being "ready" means that the requested operation will not block; thus,
-.BR poll ()ing
-regular files,
-block devices,
-and other files with no reasonable polling semantic
-.I always
-returns instantly as ready to read and write.
-.P
-Note that the
-.I timeout
-interval will be rounded up to the system clock granularity,
-and kernel scheduling delays mean that the blocking interval
-may overrun by a small amount.
-Specifying a negative value in
-.I timeout
-means an infinite timeout.
-Specifying a
-.I timeout
-of zero causes
-.BR poll ()
-to return immediately, even if no file descriptors are ready.
-.P
-The bits that may be set/returned in
-.I events
-and
-.I revents
-are defined in \fI<poll.h>\fP:
-.TP
-.B POLLIN
-There is data to read.
-.TP
-.B POLLPRI
-There is some exceptional condition on the file descriptor.
-Possibilities include:
-.RS
-.IP \[bu] 3
-There is out-of-band data on a TCP socket (see
-.BR tcp (7)).
-.IP \[bu]
-A pseudoterminal master in packet mode has seen a state change on the slave
-(see
-.BR ioctl_tty (2)).
-.IP \[bu]
-A
-.I cgroup.events
-file has been modified (see
-.BR cgroups (7)).
-.RE
-.TP
-.B POLLOUT
-Writing is now possible, though a write larger than the available space
-in a socket or pipe will still block (unless
-.B O_NONBLOCK
-is set).
-.TP
-.BR POLLRDHUP " (since Linux 2.6.17)"
-Stream socket peer closed connection,
-or shut down writing half of connection.
-The
-.B _GNU_SOURCE
-feature test macro must be defined
-(before including
-.I any
-header files)
-in order to obtain this definition.
-.TP
-.B POLLERR
-Error condition (only returned in
-.IR revents ;
-ignored in
-.IR events ).
-This bit is also set for a file descriptor referring
-to the write end of a pipe when the read end has been closed.
-.TP
-.B POLLHUP
-Hang up (only returned in
-.IR revents ;
-ignored in
-.IR events ).
-Note that when reading from a channel such as a pipe or a stream socket,
-this event merely indicates that the peer closed its end of the channel.
-Subsequent reads from the channel will return 0 (end of file)
-only after all outstanding data in the channel has been consumed.
-.TP
-.B POLLNVAL
-Invalid request:
-.I fd
-not open (only returned in
-.IR revents ;
-ignored in
-.IR events ).
-.P
-When compiling with
-.B _XOPEN_SOURCE
-defined, one also has the following,
-which convey no further information beyond the bits listed above:
-.TP
-.B POLLRDNORM
-Equivalent to
-.BR POLLIN .
-.TP
-.B POLLRDBAND
-Priority band data can be read (generally unused on Linux).
-.\" POLLRDBAND is used in the DECnet protocol.
-.TP
-.B POLLWRNORM
-Equivalent to
-.BR POLLOUT .
-.TP
-.B POLLWRBAND
-Priority data may be written.
-.P
-Linux also knows about, but does not use
-.BR POLLMSG .
-.SS ppoll()
-The relationship between
-.BR poll ()
-and
-.BR ppoll ()
-is analogous to the relationship between
-.BR select (2)
-and
-.BR pselect (2):
-like
-.BR pselect (2),
-.BR ppoll ()
-allows an application to safely wait until either a file descriptor
-becomes ready or until a signal is caught.
-.P
-Other than the difference in the precision of the
-.I timeout
-argument, the following
-.BR ppoll ()
-call:
-.P
-.in +4n
-.EX
-ready = ppoll(&fds, nfds, tmo_p, &sigmask);
-.EE
-.in
-.P
-is nearly equivalent to
-.I atomically
-executing the following calls:
-.P
-.in +4n
-.EX
-sigset_t origmask;
-int timeout;
-\&
-timeout = (tmo_p == NULL) ? \-1 :
- (tmo_p\->tv_sec * 1000 + tmo_p\->tv_nsec / 1000000);
-pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
-ready = poll(&fds, nfds, timeout);
-pthread_sigmask(SIG_SETMASK, &origmask, NULL);
-.EE
-.in
-.P
-The above code segment is described as
-.I nearly
-equivalent because whereas a negative
-.I timeout
-value for
-.BR poll ()
-is interpreted as an infinite timeout, a negative value expressed in
-.I *tmo_p
-results in an error from
-.BR ppoll ().
-.P
-See the description of
-.BR pselect (2)
-for an explanation of why
-.BR ppoll ()
-is necessary.
-.P
-If the
-.I sigmask
-argument is specified as NULL, then
-no signal mask manipulation is performed
-(and thus
-.BR ppoll ()
-differs from
-.BR poll ()
-only in the precision of the
-.I timeout
-argument).
-.P
-The
-.I tmo_p
-argument specifies an upper limit on the amount of time that
-.BR ppoll ()
-will block.
-This argument is a pointer to a
-.BR timespec (3)
-structure.
-.P
-If
-.I tmo_p
-is specified as NULL, then
-.BR ppoll ()
-can block indefinitely.
-.SH RETURN VALUE
-On success,
-.BR poll ()
-returns a nonnegative value which is the number of elements in the
-.I pollfds
-whose
-.I revents
-fields have been set to a nonzero value (indicating an event or an error).
-A return value of zero indicates that the system call timed out
-before any file descriptors became ready.
-.P
-On error, \-1 is returned, and
-.I errno
-is set to indicate the error.
-.SH ERRORS
-.TP
-.B EFAULT
-.I fds
-points outside the process's accessible address space.
-The array given as argument was not contained in the calling program's
-address space.
-.TP
-.B EINTR
-A signal occurred before any requested event; see
-.BR signal (7).
-.TP
-.B EINVAL
-The
-.I nfds
-value exceeds the
-.B RLIMIT_NOFILE
-value.
-.TP
-.B EINVAL
-.RB ( ppoll ())
-The timeout value expressed in
-.I *tmo_p
-is invalid (negative).
-.TP
-.B ENOMEM
-Unable to allocate memory for kernel data structures.
-.SH VERSIONS
-On some other UNIX systems,
-.\" Darwin, according to a report by Jeremy Sequoia, relayed by Josh Triplett
-.BR poll ()
-can fail with the error
-.B EAGAIN
-if the system fails to allocate kernel-internal resources, rather than
-.B ENOMEM
-as Linux does.
-POSIX permits this behavior.
-Portable programs may wish to check for
-.B EAGAIN
-and loop, just as with
-.BR EINTR .
-.P
-Some implementations define the nonstandard constant
-.B INFTIM
-with the value \-1 for use as a
-.I timeout
-for
-.BR poll ().
-This constant is not provided in glibc.
-.SS C library/kernel differences
-The Linux
-.BR ppoll ()
-system call modifies its
-.I tmo_p
-argument.
-However, the glibc wrapper function hides this behavior
-by using a local variable for the timeout argument that
-is passed to the system call.
-Thus, the glibc
-.BR ppoll ()
-function does not modify its
-.I tmo_p
-argument.
-.P
-The raw
-.BR ppoll ()
-system call has a fifth argument,
-.IR "size_t sigsetsize" ,
-which specifies the size in bytes of the
-.I sigmask
-argument.
-The glibc
-.BR ppoll ()
-wrapper function specifies this argument as a fixed value
-(equal to
-.IR sizeof(kernel_sigset_t) ).
-See
-.BR sigprocmask (2)
-for a discussion on the differences between the kernel and the libc
-notion of the sigset.
-.SH STANDARDS
-.TP
-.BR poll ()
-POSIX.1-2008.
-.TP
-.BR ppoll ()
-Linux.
-.\" FIXME .
-.\" ppoll() is proposed for inclusion in POSIX:
-.\" https://www.austingroupbugs.net/view.php?id=1263
-.\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
-.SH HISTORY
-.TP
-.BR poll ()
-POSIX.1-2001.
-Linux 2.1.23.
-.IP
-On older kernels that lack this system call,
-the glibc
-.BR poll ()
-wrapper function provides emulation using
-.BR select (2).
-.TP
-.BR ppoll ()
-Linux 2.6.16,
-glibc 2.4.
-.SH NOTES
-The operation of
-.BR poll ()
-and
-.BR ppoll ()
-is not affected by the
-.B O_NONBLOCK
-flag.
-.P
-For a discussion of what may happen if a file descriptor being monitored by
-.BR poll ()
-is closed in another thread, see
-.BR select (2).
-.SH BUGS
-See the discussion of spurious readiness notifications under the
-BUGS section of
-.BR select (2).
-.SH EXAMPLES
-The program below opens each of the files named in its command-line
-arguments and monitors the resulting file descriptors for readiness to read
-.RB ( POLLIN ).
-The program loops, repeatedly using
-.BR poll ()
-to monitor the file descriptors,
-printing the number of ready file descriptors on return.
-For each ready file descriptor, the program:
-.IP \[bu] 3
-displays the returned
-.I revents
-field in a human-readable form;
-.IP \[bu]
-if the file descriptor is readable, reads some data from it,
-and displays that data on standard output; and
-.IP \[bu]
-if the file descriptor was not readable,
-but some other event occurred (presumably
-.BR POLLHUP ),
-closes the file descriptor.
-.P
-Suppose we run the program in one terminal, asking it to open a FIFO:
-.P
-.in +4n
-.EX
-$ \fBmkfifo myfifo\fP
-$ \fB./poll_input myfifo\fP
-.EE
-.in
-.P
-In a second terminal window, we then open the FIFO for writing,
-write some data to it, and close the FIFO:
-.P
-.in +4n
-.EX
-$ \fBecho aaaaabbbbbccccc > myfifo\fP
-.EE
-.in
-.P
-In the terminal where we are running the program, we would then see:
-.P
-.in +4n
-.EX
-Opened "myfifo" on fd 3
-About to poll()
-Ready: 1
- fd=3; events: POLLIN POLLHUP
- read 10 bytes: aaaaabbbbb
-About to poll()
-Ready: 1
- fd=3; events: POLLIN POLLHUP
- read 6 bytes: ccccc
-\&
-About to poll()
-Ready: 1
- fd=3; events: POLLHUP
- closing fd 3
-All file descriptors closed; bye
-.EE
-.in
-.P
-In the above output, we see that
-.BR poll ()
-returned three times:
-.IP \[bu] 3
-On the first return, the bits returned in the
-.I revents
-field were
-.BR POLLIN ,
-indicating that the file descriptor is readable, and
-.BR POLLHUP ,
-indicating that the other end of the FIFO has been closed.
-The program then consumed some of the available input.
-.IP \[bu]
-The second return from
-.BR poll ()
-also indicated
-.B POLLIN
-and
-.BR POLLHUP ;
-the program then consumed the last of the available input.
-.IP \[bu]
-On the final return,
-.BR poll ()
-indicated only
-.B POLLHUP
-on the FIFO,
-at which point the file descriptor was closed and the program terminated.
-.\"
-.SS Program source
-\&
-.\" SRC BEGIN (poll_input.c)
-.EX
-/* poll_input.c
-\&
- Licensed under GNU General Public License v2 or later.
-*/
-#include <fcntl.h>
-#include <poll.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <unistd.h>
-\&
-#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
- } while (0)
-\&
-int
-main(int argc, char *argv[])
-{
- int ready;
- char buf[10];
- nfds_t num_open_fds, nfds;
- ssize_t s;
- struct pollfd *pfds;
-\&
- if (argc < 2) {
- fprintf(stderr, "Usage: %s file...\en", argv[0]);
- exit(EXIT_FAILURE);
- }
-\&
- num_open_fds = nfds = argc \- 1;
- pfds = calloc(nfds, sizeof(struct pollfd));
- if (pfds == NULL)
- errExit("malloc");
-\&
- /* Open each file on command line, and add it to \[aq]pfds\[aq] array. */
-\&
- for (nfds_t j = 0; j < nfds; j++) {
- pfds[j].fd = open(argv[j + 1], O_RDONLY);
- if (pfds[j].fd == \-1)
- errExit("open");
-\&
- printf("Opened \e"%s\e" on fd %d\en", argv[j + 1], pfds[j].fd);
-\&
- pfds[j].events = POLLIN;
- }
-\&
- /* Keep calling poll() as long as at least one file descriptor is
- open. */
-\&
- while (num_open_fds > 0) {
- printf("About to poll()\en");
- ready = poll(pfds, nfds, \-1);
- if (ready == \-1)
- errExit("poll");
-\&
- printf("Ready: %d\en", ready);
-\&
- /* Deal with array returned by poll(). */
-\&
- for (nfds_t j = 0; j < nfds; j++) {
- if (pfds[j].revents != 0) {
- printf(" fd=%d; events: %s%s%s\en", pfds[j].fd,
- (pfds[j].revents & POLLIN) ? "POLLIN " : "",
- (pfds[j].revents & POLLHUP) ? "POLLHUP " : "",
- (pfds[j].revents & POLLERR) ? "POLLERR " : "");
-\&
- if (pfds[j].revents & POLLIN) {
- s = read(pfds[j].fd, buf, sizeof(buf));
- if (s == \-1)
- errExit("read");
- printf(" read %zd bytes: %.*s\en",
- s, (int) s, buf);
- } else { /* POLLERR | POLLHUP */
- printf(" closing fd %d\en", pfds[j].fd);
- if (close(pfds[j].fd) == \-1)
- errExit("close");
- num_open_fds\-\-;
- }
- }
- }
- }
-\&
- printf("All file descriptors closed; bye\en");
- exit(EXIT_SUCCESS);
-}
-.EE
-.\" SRC END
-.SH SEE ALSO
-.BR restart_syscall (2),
-.BR select (2),
-.BR select_tut (2),
-.BR timespec (3),
-.BR epoll (7),
-.BR time (7)