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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |VERSIONS |STANDARDS |HISTORY |NOTES |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
poll(2) System Calls Manualpoll(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_tnfds, inttimeout);#define _GNU_SOURCE/* See feature_test_macros(7) */#include <poll.h>int ppoll(struct pollfd *fds, nfds_tnfds,const struct timespec *_Nullabletmo_p,const sigset_t *_Nullablesigmask);
poll() performs a similar task toselect(2): it waits for one of a set of file descriptors to become ready to perform I/O. The Linux-specificepoll(7) API performs a similar task, but offers features beyond those found inpoll(). The set of file descriptors to be monitored is specified in thefds 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 thefds array innfds. The fieldfd contains a file descriptor for an open file. If this field is negative, then the correspondingevents field is ignored and therevents field returns zero. (This provides an easy way of ignoring a file descriptor for a singlepoll() call: simply set thefd field to its bitwise complement.) The fieldevents is an input parameter, a bit mask specifying the events the application is interested in for the file descriptorfd. This field may be specified as zero, in which case the only events that can be returned inrevents arePOLLHUP,POLLERR, andPOLLNVAL(see below). The fieldrevents is an output parameter, filled by the kernel with the events that actually occurred. The bits returned inrevents can include any of those specified inevents, or one of the valuesPOLLERR,POLLHUP, orPOLLNVAL. (These three bits are meaningless in theevents field, and will be set in therevents field whenever the corresponding condition is true.) If none of the events requested (and no error) has occurred for any of the file descriptors, thenpoll() blocks until one of the events occurs. Thetimeout argument specifies the number of milliseconds thatpoll() 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 semanticalways returns instantly as ready to read and write. Note that thetimeout 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 intimeout means an infinite timeout. Specifying atimeout of zero causespoll() to return immediately, even if no file descriptors are ready. The bits that may be set/returned inevents andrevents are defined in<poll.h>:POLLINThere 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 (seetcp(7)). • A pseudoterminal master in packet mode has seen a state change on the slave (seeioctl_tty(2)). • Acgroup.events file has been modified (seecgroups(7)).POLLOUT Writing is now possible, though a write larger than the available space in a socket or pipe will still block (unlessO_NONBLOCKis set).POLLRDHUP(since Linux 2.6.17) Stream socket peer closed connection, or shut down writing half of connection. The_GNU_SOURCEfeature test macro must be defined (before includingany header files) in order to obtain this definition.POLLERR Error condition (only returned inrevents; ignored inevents). 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 inrevents; ignored inevents). 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 inrevents; ignored inevents). When compiling with_XOPEN_SOURCEdefined, one also has the following, which convey no further information beyond the bits listed above:POLLRDNORM Equivalent toPOLLIN.POLLRDBAND Priority band data can be read (generally unused on Linux).POLLWRNORM Equivalent toPOLLOUT.POLLWRBAND Priority data may be written. Linux also knows about, but does not usePOLLMSG.ppoll() The relationship betweenpoll() andppoll() is analogous to the relationship betweenselect(2) andpselect(2): likepselect(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 thetimeout argument, the followingppoll() call: ready = ppoll(&fds, nfds, tmo_p, &sigmask); is nearly equivalent toatomically 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 asnearly equivalent because whereas a negativetimeout value forpoll() is interpreted as an infinite timeout, a negative value expressed in*tmo_p results in an error fromppoll(). See the description ofpselect(2) for an explanation of whyppoll() is necessary. If thesigmask argument is specified as NULL, then no signal mask manipulation is performed (and thusppoll() differs frompoll() only in the precision of thetimeout argument). Thetmo_p argument specifies an upper limit on the amount of time thatppoll() will block. This argument is a pointer to atimespec(3) structure. Iftmo_p is specified as NULL, thenppoll() can block indefinitely.On success,poll() returns a nonnegative value which is the number of elements in thepollfds whoserevents 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, anderrno is set to indicate the error.
EFAULTfds points outside the process's accessible address space. The array given as argument was not contained in the calling program's address space.EINTRA signal occurred before any requested event; seesignal(7).EINVALThenfds value exceeds theRLIMIT_NOFILEvalue.EINVAL(ppoll()) The timeout value expressed in*tmo_p is invalid (negative).ENOMEMUnable to allocate memory for kernel data structures.
On some other UNIX systems,poll() can fail with the errorEAGAIN if the system fails to allocate kernel-internal resources, rather thanENOMEMas Linux does. POSIX permits this behavior. Portable programs may wish to check forEAGAINand loop, just as withEINTR. Some implementations define the nonstandard constantINFTIMwith the value -1 for use as atimeout forpoll(). This constant is not provided in glibc.C library/kernel differences The Linuxppoll() system call modifies itstmo_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 glibcppoll() function does not modify itstmo_p argument. The rawppoll() system call has a fifth argument,size_tsigsetsize, which specifies the size in bytes of thesigmask argument. The glibcppoll() wrapper function specifies this argument as a fixed value (equal tosizeof(kernel_sigset_t)). Seesigprocmask(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 glibcpoll() wrapper function provides emulation usingselect(2).ppoll() Linux 2.6.16, glibc 2.4.
The operation ofpoll() andppoll() is not affected by theO_NONBLOCKflag. For a discussion of what may happen if a file descriptor being monitored bypoll() is closed in another thread, seeselect(2).
See the discussion of spurious readiness notifications under the BUGS section ofselect(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 usingpoll() to monitor the file descriptors, printing the number of ready file descriptors on return. For each ready file descriptor, the program: • displays the returnedrevents 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 (presumablyPOLLHUP), 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 thatpoll() returned three times: • On the first return, the bits returned in therevents field werePOLLIN, indicating that the file descriptor is readable, andPOLLHUP, indicating that the other end of the FIFO has been closed. The program then consumed some of the available input. • The second return frompoll() also indicatedPOLLINandPOLLHUP; the program then consumed the last of the available input. • On the final return,poll() indicated onlyPOLLHUPon 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)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17poll(2)Pages that refer to this page:less(1), accept(2), connect(2), epoll_ctl(2), epoll_wait(2), eventfd(2), F_GETSIG(2const), FUTEX_FD(2const), 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), ldap.conf(5), 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)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |