poll(2) System Calls Manual poll(2)
NAME
poll, ppoll - wait for some event on a file descriptor
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
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 ar-
gument, 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 be-
low).
The field revents is an output parameter, filled by the kernel with the
events that actually occurred. The bits returned in revents can in-
clude 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 oc-
curs.
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 reason-
able 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 inter-
val 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 avail-
able space in a socket or pipe will still block (unless O_NON-
BLOCK 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 de-
fined (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 rela-
tionship between select(2) and pselect(2): like pselect(2), ppoll() al-
lows an application to safely wait until either a file descriptor be-
comes 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 infi-
nite 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 ma-
nipulation 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) struc-
ture.
If tmo_p is specified as NULL, then ppoll() can block indefinitely.
RETURN VALUE
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 indi-
cates 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.
ERRORS
EFAULT fds points outside the process's accessible address space. The
array given as argument was not contained in the calling pro-
gram'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 (neg-
ative).
ENOMEM Unable to allocate memory for kernel data structures.
VERSIONS
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 pro-
vided 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 vari-
able 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 discus-
sion on the differences between the kernel and the libc notion of the
sigset.
STANDARDS
poll() POSIX.1-2008.
ppoll()
Linux.
HISTORY
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.
NOTES
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 moni-
tored by poll() is closed in another thread, see select(2).
BUGS
See the discussion of spurious readiness notifications under the BUGS
section of select(2).
EXAMPLES
The program below opens each of the files named in its command-line ar-
guments 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 oc-
curred (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 POLL-
HUP, 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 termi-
nated.
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 <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);
}
SEE ALSO
restart_syscall(2), select(2), select_tut(2), timespec(3), epoll(7),
time(7)
Linux man-pages 6.7 2023-10-31 poll(2)
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