| 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:
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>:
When compiling with _XOPEN_SOURCE defined, one also has the following, which convey no further information beyond the bits listed above:
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 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.
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.
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.
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:
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:
/* 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); }
restart_syscall(2), select(2), select_tut(2), timespec(3), epoll(7), time(7)
| 2023-07-08 | Linux man-pages 6.05.01 |