-.\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl)
-.\" and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com>
+.\" Copyright (C) 2006, 2019 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" 2006-07-01, mtk, Added POLLRDHUP + various other wording and
.\" formatting changes.
.\"
-.TH POLL 2 2014-01-31 "Linux" "Linux Programmer's Manual"
+.TH POLL 2 2020-04-11 "Linux" "Linux Programmer's Manual"
.SH NAME
poll, ppoll \- wait for some event on a file descriptor
.SH SYNOPSIS
.nf
.B #include <poll.h>
-.sp
+.PP
.BI "int poll(struct pollfd *" fds ", nfds_t " nfds ", int " timeout );
-.sp
+
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
+.B #include <signal.h>
.B #include <poll.h>
-.sp
+.PP
.BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ", "
-.BI " const struct timespec *" timeout_ts ", const sigset_t *" sigmask );
+.BI " const struct timespec *" tmo_p ", const sigset_t *" sigmask );
.fi
.SH DESCRIPTION
.BR poll ()
.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 ().
+.PP
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:
+.PP
.in +4n
-.nf
-
+.EX
struct pollfd {
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
};
+.EE
.in
-.fi
.PP
The caller should specify the number of items in the
.I fds
array in
.IR nfds .
-
+.PP
The field
.I fd
contains a file descriptor for an open file.
.BR poll ()
call: simply negate the
.I fd
-field.)
-
+field.
+Note, however, that this technique can't be used to ignore file descriptor 0.)
+.PP
The field
.I events
is an input parameter, a bit mask specifying the events the application
and
.B POLLNVAL
(see below).
-
+.PP
The field
.I revents
is an output parameter, filled by the kernel with the events that
field, and will be set in the
.I revents
field whenever the corresponding condition is true.)
-
+.PP
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.
-
+.PP
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 * 3
+.IP \(bu 2
a file descriptor becomes ready;
-.IP *
+.IP \(bu
the call is interrupted by a signal handler; or
-.IP *
+.IP \(bu
the timeout expires.
.PP
Note that the
of zero causes
.BR poll ()
to return immediately, even if no file descriptors are ready.
-
+.PP
The bits that may be set/returned in
.I events
and
.I revents
are defined in \fI<poll.h>\fP:
-.RS
.TP
.B POLLIN
There is data to read.
.TP
.B POLLPRI
-There is urgent data to read (e.g., out-of-band data on TCP socket;
-pseudoterminal master in packet mode has seen state change in slave).
+There is some exceptional condition on the file descriptor.
+Possibilities include:
+.RS
+.IP \(bu 2
+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 now will not block.
+Writing is now possible, though a write larger that 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,
in order to obtain this definition.
.TP
.B POLLERR
-Error condition (output only).
+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 (output only).
+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 (output only).
-.RE
+not open (only returned in
+.IR revents ;
+ignored in
+.IR events ).
.PP
When compiling with
.B _XOPEN_SOURCE
defined, one also has the following,
which convey no further information beyond the bits listed above:
-.RS
.TP
.B POLLRDNORM
Equivalent to
.TP
.B POLLWRBAND
Priority data may be written.
-.RE
.PP
Linux also knows about, but does not use
.BR POLLMSG .
argument, the following
.BR ppoll ()
call:
-.nf
-
- ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
-
-.fi
-is equivalent to
+.PP
+.in +4n
+.EX
+ready = ppoll(&fds, nfds, tmo_p, &sigmask);
+.EE
+.in
+.PP
+is nearly equivalent to
.I atomically
executing the following calls:
-.nf
-
- sigset_t origmask;
- int timeout;
+.PP
+.in +4n
+.EX
+sigset_t origmask;
+int timeout;
- timeout = (timeout_ts == NULL) ? \-1 :
- (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
- sigprocmask(SIG_SETMASK, &sigmask, &origmask);
- ready = poll(&fds, nfds, timeout);
- sigprocmask(SIG_SETMASK, &origmask, NULL);
-.fi
+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
+.PP
+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
+.IR *tmo_p
+results in an error from
+.BR ppoll ().
.PP
See the description of
.BR pselect (2)
for an explanation of why
.BR ppoll ()
is necessary.
-
+.PP
If the
.I sigmask
argument is specified as NULL, then
only in the precision of the
.I timeout
argument).
-
+.PP
The
-.I timeout_ts
+.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 structure of the following form:
+.PP
.in +4n
-.nf
-
+.EX
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
-.fi
+.EE
.in
-
+.PP
If
-.I timeout_ts
+.I tmo_p
is specified as NULL, then
.BR ppoll ()
can block indefinitely.
.SH RETURN VALUE
-On success, a positive number is returned; this is
-the number of structures which have nonzero
+On success,
+.BR poll ()
+returns a nonnegative value which is the number of elements in the
+.I pollfds
+whose
.I revents
-fields (in other words, those descriptors with events or errors reported).
-A value of 0 indicates that the call timed out and no file
-descriptors were ready.
+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 read.
+.PP
On error, \-1 is returned, and
.I errno
-is set appropriately.
+is set to indicate the cause of 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 RLIMIT_NOFILE
value.
.TP
+.B EINVAL
+.RB ( ppoll ())
+The timeout value expressed in
+.IR *ip
+is invalid (negative).
+.TP
.B ENOMEM
-There was no space to allocate file descriptor tables.
+Unable to allocate memory for kernel data structures.
.SH VERSIONS
The
.BR poll ()
system call was introduced in Linux 2.1.23.
On older kernels that lack this system call,
-.\" library call was introduced in libc 5.4.28
-the glibc (and the old Linux libc)
+the glibc
.BR poll ()
wrapper function provides emulation using
.BR select (2).
-
+.PP
The
.BR ppoll ()
system call was added to Linux in kernel 2.6.16.
library call was added in glibc 2.4.
.SH CONFORMING TO
.BR poll ()
-conforms to POSIX.1-2001.
+conforms to POSIX.1-2001 and POSIX.1-2008.
.BR ppoll ()
is Linux-specific.
+.\" 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 NOTES
+The operation of
+.BR poll ()
+and
+.BR ppoll ()
+is not affected by the
+.BR O_NONBLOCK
+flag.
+.PP
+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 .
+.PP
Some implementations define the nonstandard constant
.B INFTIM
with the value \-1 for use as a
for
.BR poll ().
This constant is not provided in glibc.
-
+.PP
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).
-.SS Linux notes
+.SS C library/kernel differences
The Linux
.BR ppoll ()
system call modifies its
-.I timeout_ts
+.I tmo_p
argument.
However, the glibc wrapper function hides this behavior
by using a local variable for the timeout argument that
Thus, the glibc
.BR ppoll ()
function does not modify its
-.I timeout_ts
+.I tmo_p
+argument.
+.PP
+The raw
+.BR ppoll ()
+system call has a fifth argument,
+.IR "size_t sigsetsize" ,
+which specifies the size in bytes of the
+.IR 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 BUGS
See the discussion of spurious readiness notifications under the
BUGS section of
.BR select (2).
+.SH EXAMPLE
+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 2
+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 descriptors was not readable,
+but some other event occurred (presumably
+.BR POLLHUP ),
+closes the file descriptor.
+.PP
+Suppose we run the program in one terminal, asking it to open a FIFO:
+.PP
+.in +4n
+.EX
+$ \fBmkfifo myfifo\fP
+$ \fB./poll_input myfifo\fP
+.EE
+.in
+.PP
+In a second terminal window, we then open the FIFO for writing,
+write some data to it, and close the FIFO:
+.PP
+.in +4n
+.EX
+$ \fBecho aaaaabbbbbccccc > myfifo\fP
+.EE
+.in
+.PP
+In the terminal where we are running the program, we would then see:
+.PP
+.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
+.PP
+In the above output, we see that
+.BR poll ()
+returned three times:
+.IP \(bu 2
+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
+.BR POLLIN
+and
+.BR POLLHUP ;
+the program then consumed the last of the available input.
+.IP \(bu
+On the final return,
+.BR poll ()
+indicated only
+.BR POLLHUP
+on the FIFO,
+at which point the file descriptor was closed and the program terminated.
+.\"
+.SS Program source
+\&
+.nf
+/* poll_input.c
+
+ Licensed under GNU General Public License v2 or later.
+*/
+#include <poll.h>
+#include <fcntl.h>
+#include <sys/types.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 nfds, num_open_fds;
+ 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 \(aqpfds\(aq array */
+
+ for (int 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) {
+ int ready;
+
+ 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 (int j = 0; j < nfds; j++) {
+ char buf[10];
+
+ 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) {
+ ssize_t 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);
+}
+.fi
.SH SEE ALSO
.BR restart_syscall (2),
.BR select (2),
.BR select_tut (2),
+.BR epoll (7),
.BR time (7)
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