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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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poll(2) System Calls Manual poll(2)
poll, ppoll - wait for some event on a file descriptor
Standard C library (libc, -lc)
#include <poll.h>
int poll(struct pollfd *fds, nfds_t nfds, int timeout);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <poll.h>
int ppoll(struct pollfd *fds, nfds_t nfds,
const struct timespec *_Nullable tmo_p,
const sigset_t *_Nullable sigmask);
poll() performs a similar task to select(2): it waits for one of
a set of file descriptors to become ready to perform I/O. The
Linux-specific epoll(7) API performs a similar task, but offers
features beyond those found in poll().
The set of file descriptors to be monitored is specified in the
fds argument, which is an array of structures of the following
form:
struct pollfd {
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
};
The caller should specify the number of items in the fds array in
nfds.
The field fd contains a file descriptor for an open file. If
this field is negative, then the corresponding events field is
ignored and the revents field returns zero. (This provides an
easy way of ignoring a file descriptor for a single poll() call:
simply set the fd field to its bitwise complement.)
The field events is an input parameter, a bit mask specifying the
events the application is interested in for the file descriptor
fd. This field may be specified as zero, in which case the only
events that can be returned in revents are POLLHUP, POLLERR, and
POLLNVAL (see below).
The field revents is an output parameter, filled by the kernel
with the events that actually occurred. The bits returned in
revents can include any of those specified in events, or one of
the values POLLERR, POLLHUP, or POLLNVAL. (These three bits are
meaningless in the events field, and will be set in the revents
field whenever the corresponding condition is true.)
If none of the events requested (and no error) has occurred for
any of the file descriptors, then poll() blocks until one of the
events occurs.
The timeout argument specifies the number of milliseconds that
poll() should block waiting for a file descriptor to become
ready. The call will block until either:
• a file descriptor becomes ready;
• the call is interrupted by a signal handler; or
• the timeout expires.
Being "ready" means that the requested operation will not block;
thus, poll()ing regular files, block devices, and other files
with no reasonable polling semantic always returns instantly as
ready to read and write.
Note that the 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 timeout means an infinite timeout. Specifying
a timeout of zero causes poll() to return immediately, even if no
file descriptors are ready.
The bits that may be set/returned in events and revents are
defined in <poll.h>:
POLLIN There is data to read.
POLLPRI
There is some exceptional condition on the file
descriptor. Possibilities include:
• There is out-of-band data on a TCP socket (see tcp(7)).
• A pseudoterminal master in packet mode has seen a state
change on the slave (see ioctl_tty(2)).
• A cgroup.events file has been modified (see
cgroups(7)).
POLLOUT
Writing is now possible, though a write larger than the
available space in a socket or pipe will still block
(unless O_NONBLOCK is set).
POLLRDHUP (since Linux 2.6.17)
Stream socket peer closed connection, or shut down writing
half of connection. The _GNU_SOURCE feature test macro
must be defined (before including any header files) in
order to obtain this definition.
POLLERR
Error condition (only returned in revents; ignored in
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.
POLLHUP
Hang up (only returned in revents; ignored in 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.
POLLNVAL
Invalid request: fd not open (only returned in revents;
ignored in events).
When compiling with _XOPEN_SOURCE defined, one also has the
following, which convey no further information beyond the bits
listed above:
POLLRDNORM
Equivalent to POLLIN.
POLLRDBAND
Priority band data can be read (generally unused on
Linux).
POLLWRNORM
Equivalent to POLLOUT.
POLLWRBAND
Priority data may be written.
Linux also knows about, but does not use POLLMSG.
ppoll()
The relationship between poll() and ppoll() is analogous to the
relationship between select(2) and pselect(2): like pselect(2),
ppoll() allows an application to safely wait until either a file
descriptor becomes ready or until a signal is caught.
Other than the difference in the precision of the timeout
argument, the following ppoll() call:
ready = ppoll(&fds, nfds, tmo_p, &sigmask);
is nearly equivalent to atomically executing the following calls:
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);
The above code segment is described as nearly equivalent because
whereas a negative timeout value for poll() is interpreted as an
infinite timeout, a negative value expressed in *tmo_p results in
an error from ppoll().
See the description of pselect(2) for an explanation of why
ppoll() is necessary.
If the sigmask argument is specified as NULL, then no signal mask
manipulation is performed (and thus ppoll() differs from poll()
only in the precision of the timeout argument).
The tmo_p argument specifies an upper limit on the amount of time
that ppoll() will block. This argument is a pointer to a
timespec(3) structure.
If tmo_p is specified as NULL, then ppoll() can block
indefinitely.
On success, poll() returns a nonnegative value which is the
number of elements in the pollfds whose 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.
On error, -1 is returned, and errno is set to indicate the error.
EFAULT fds points outside the process's accessible address space.
The array given as argument was not contained in the
calling program's address space.
EINTR A signal occurred before any requested event; see
signal(7).
EINVAL The nfds value exceeds the RLIMIT_NOFILE value.
EINVAL (ppoll()) The timeout value expressed in *tmo_p is invalid
(negative).
ENOMEM Unable to allocate memory for kernel data structures.
On some other UNIX systems, poll() can fail with the error EAGAIN
if the system fails to allocate kernel-internal resources, rather
than ENOMEM as Linux does. POSIX permits this behavior.
Portable programs may wish to check for EAGAIN and loop, just as
with EINTR.
Some implementations define the nonstandard constant INFTIM with
the value -1 for use as a timeout for poll(). This constant is
not provided in glibc.
C library/kernel differences
The Linux ppoll() system call modifies its 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 ppoll() function does not modify
its tmo_p argument.
The raw ppoll() system call has a fifth argument, size_t
sigsetsize, which specifies the size in bytes of the sigmask
argument. The glibc ppoll() wrapper function specifies this
argument as a fixed value (equal to sizeof(kernel_sigset_t)).
See sigprocmask(2) for a discussion on the differences between
the kernel and the libc notion of the sigset.
poll() POSIX.1-2008.
ppoll()
Linux.
poll() POSIX.1-2001. Linux 2.1.23.
On older kernels that lack this system call, the glibc
poll() wrapper function provides emulation using
select(2).
ppoll()
Linux 2.6.16, glibc 2.4.
The operation of poll() and ppoll() is not affected by the
O_NONBLOCK flag.
For a discussion of what may happen if a file descriptor being
monitored by poll() is closed in another thread, see select(2).
See the discussion of spurious readiness notifications under the
BUGS section of select(2).
The program below opens each of the files named in its command-
line arguments and monitors the resulting file descriptors for
readiness to read (POLLIN). The program loops, repeatedly using
poll() to monitor the file descriptors, printing the number of
ready file descriptors on return. For each ready file
descriptor, the program:
• displays the returned revents field in a human-readable form;
• if the file descriptor is readable, reads some data from it,
and displays that data on standard output; and
• if the file descriptor was not readable, but some other event
occurred (presumably POLLHUP), closes the file descriptor.
Suppose we run the program in one terminal, asking it to open a
FIFO:
$ mkfifo myfifo
$ ./poll_input myfifo
In a second terminal window, we then open the FIFO for writing,
write some data to it, and close the FIFO:
$ echo aaaaabbbbbccccc > myfifo
In the terminal where we are running the program, we would then
see:
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
In the above output, we see that poll() returned three times:
• On the first return, the bits returned in the revents field
were POLLIN, indicating that the file descriptor is readable,
and POLLHUP, indicating that the other end of the FIFO has
been closed. The program then consumed some of the available
input.
• The second return from poll() also indicated POLLIN and
POLLHUP; the program then consumed the last of the available
input.
• On the final return, poll() indicated only POLLHUP on the
FIFO, at which point the file descriptor was closed and the
program terminated.
Program source
/* 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 <sys/types.h>
#include <unistd.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} 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...\n", 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 'pfds' 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 \"%s\" on fd %d\n", 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()\n");
ready = poll(pfds, nfds, -1);
if (ready == -1)
errExit("poll");
printf("Ready: %d\n", 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\n", 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\n",
s, (int) s, buf);
} else { /* POLLERR | POLLHUP */
printf(" closing fd %d\n", pfds[j].fd);
if (close(pfds[j].fd) == -1)
errExit("close");
num_open_fds--;
}
}
}
}
printf("All file descriptors closed; bye\n");
exit(EXIT_SUCCESS);
}
restart_syscall(2), select(2), select_tut(2), timespec(3),
epoll(7), time(7)
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Linux man-pages 6.9.1 2024-06-15 poll(2)
Pages that refer to this page: less(1), accept(2), connect(2), epoll_ctl(2), epoll_wait(2), eventfd(2), fcntl(2), futex(2), io_uring_enter2(2), io_uring_enter(2), open(2), perf_event_open(2), perfmonctl(2), personality(2), pidfd_open(2), PR_SET_TIMERSLACK(2const), ptrace(2), recv(2), restart_syscall(2), seccomp_unotify(2), select(2), select_tut(2), sigaction(2), signalfd(2), syscalls(2), timerfd_create(2), TIOCPKT(2const), userfaultfd(2), io_uring_prep_poll_add(3), io_uring_prep_poll_multishot(3), ldap_get_option(3), pcap(3pcap), pcap_get_required_select_timeout(3pcap), pcap_get_selectable_fd(3pcap), rtime(3), sctp_connectx(3), sd_bus_get_fd(3), sd_bus_wait(3), sd_journal_get_fd(3), sd_login_monitor_new(3), sd_notify(3), random(4), proc_pid_mounts(5), slapd-asyncmeta(5), slapd-ldap(5), slapd-meta(5), systemd.exec(5), cgroups(7), epoll(7), fanotify(7), inotify(7), mq_overview(7), pipe(7), signal(7), signal-safety(7), socket(7), spufs(7), system_data_types(7), tcp(7), udp(7), setarch(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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