poll, ppoll - wait for some event on a file descriptor
#include <poll.h>
int poll(struct pollfd *fds, nfds_t nfds, int timeout);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <signal.h>
#include <poll.h>
int ppoll(struct pollfd *fds, nfds_t nfds,
const struct timespec *tmo_p, const sigset_t *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(2).
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 negate the
fd
field. Note, however, that this technique can't be used to ignore file
descriptor 0.)
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.
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 that 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.
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 structure of the
following form:
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
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
read.
On error, -1 is returned, and
errno is set to indicate the cause of 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 *ip is invalid
(negative).
- ENOMEM
- Unable to allocate memory for kernel data structures.
The
poll() system call was introduced in Linux 2.1.23. On older kernels
that lack this system call, the glibc
poll() wrapper function provides
emulation using
select(2).
The
ppoll() system call was added to Linux in kernel 2.6.16. The
ppoll() library call was added in glibc 2.4.
poll() conforms to POSIX.1-2001 and POSIX.1-2008.
ppoll() is
Linux-specific.
The operation of
poll() and
ppoll() is not affected by the
O_NONBLOCK flag.
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.
For a discussion of what may happen if a file descriptor being monitored by
poll() is closed in another thread, see
select(2).
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.
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 descriptors 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.
/* poll_input.c */
#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); \
} while (0)
int
main(int argc, char *argv[])
{
int nfds, num_open_fds;
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 'pfds' 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 \"%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) {
int ready;
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 (int j = 0; j < nfds; j++) {
char buf[10];
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) {
ssize_t 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),
epoll(7),
time(7)