.\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl) .\" and Copyright (C) 2006, Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Additions from Richard Gooch and aeb, 971207 .\" 2006-03-13, mtk, Added ppoll() + various other rewordings .\" 2006-07-01, mtk, Added POLLRDHUP + various other wording and .\" formatting changes. .\" .TH POLL 2 2017-03-13 "Linux" "Linux Programmer's Manual" .SH NAME poll, ppoll \- wait for some event on a file descriptor .SH SYNOPSIS .nf .B #include .sp .BI "int poll(struct pollfd *" fds ", nfds_t " nfds ", int " timeout ); .sp .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .B #include .sp .BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ", " .BI " const struct timespec *" tmo_p ", const sigset_t *" sigmask ); .fi .SH DESCRIPTION .BR poll () performs a similar task to .BR select (2): it waits for one of a set of file descriptors to become ready to perform I/O. The set of file descriptors to be monitored is specified in the .I fds argument, which is an array of structures of the following form: .in +4n .nf struct pollfd { int fd; /* file descriptor */ short events; /* requested events */ short revents; /* returned events */ }; .in .fi .PP The caller should specify the number of items in the .I fds array in .IR nfds . The field .I fd contains a file descriptor for an open file. If this field is negative, then the corresponding .I events field is ignored and the .I revents field returns zero. (This provides an easy way of ignoring a file descriptor for a single .BR poll () call: simply negate the .I fd field. Note, however, that this technique can't be used to ignore file descriptor 0.) The field .I events is an input parameter, a bit mask specifying the events the application is interested in for the file descriptor .IR fd . This field may be specified as zero, in which case the only events that can be returned in .I revents are .BR POLLHUP , .BR POLLERR , and .B POLLNVAL (see below). The field .I revents is an output parameter, filled by the kernel with the events that actually occurred. The bits returned in .I revents can include any of those specified in .IR events , or one of the values .BR POLLERR , .BR POLLHUP , or .BR POLLNVAL . (These three bits are meaningless in the .I events field, and will be set in the .I 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 .BR poll () blocks until one of the events occurs. The .I timeout argument specifies the number of milliseconds that .BR poll () should block waiting for a file descriptor to become ready. The call will block until either: .IP * 3 a file descriptor becomes ready; .IP * the call is interrupted by a signal handler; or .IP * the timeout expires. .PP Note that the .I 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 .I timeout means an infinite timeout. Specifying a .I timeout of zero causes .BR poll () to return immediately, even if no file descriptors are ready. The bits that may be set/returned in .I events and .I revents are defined in \fI\fP: .RS .TP .B POLLIN There is data to read. .TP .B POLLPRI There is urgent data to read (e.g., out-of-band data on TCP socket; pseudoterminal master in packet mode has seen state change in slave). .TP .B POLLOUT Writing is now possible, though a write larger that the available space in a socket or pipe will still block (unless .B O_NONBLOCK is set). .TP .BR POLLRDHUP " (since Linux 2.6.17)" Stream socket peer closed connection, or shut down writing half of connection. The .B _GNU_SOURCE feature test macro must be defined (before including .I any header files) in order to obtain this definition. .TP .B POLLERR Error condition (only returned in .IR revents ; ignored in .IR events ). .TP .B POLLHUP Hang up (only returned in .IR revents ; ignored in .IR 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. .TP .B POLLNVAL Invalid request: .I fd not open (only returned in .IR revents ; ignored in .IR events ). .RE .PP When compiling with .B _XOPEN_SOURCE defined, one also has the following, which convey no further information beyond the bits listed above: .RS .TP .B POLLRDNORM Equivalent to .BR POLLIN . .TP .B POLLRDBAND Priority band data can be read (generally unused on Linux). .\" POLLRDBAND is used in the DECnet protocol. .TP .B POLLWRNORM Equivalent to .BR POLLOUT . .TP .B POLLWRBAND Priority data may be written. .RE .PP Linux also knows about, but does not use .BR POLLMSG . .SS ppoll() The relationship between .BR poll () and .BR ppoll () is analogous to the relationship between .BR select (2) and .BR pselect (2): like .BR pselect (2), .BR ppoll () allows an application to safely wait until either a file descriptor becomes ready or until a signal is caught. .PP Other than the difference in the precision of the .I timeout argument, the following .BR ppoll () call: .nf ready = ppoll(&fds, nfds, tmo_p, &sigmask); .fi is equivalent to .I atomically executing the following calls: .nf 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); .fi .PP See the description of .BR pselect (2) for an explanation of why .BR ppoll () is necessary. If the .I sigmask argument is specified as NULL, then no signal mask manipulation is performed (and thus .BR ppoll () differs from .BR poll () only in the precision of the .I timeout argument). The .I tmo_p argument specifies an upper limit on the amount of time that .BR ppoll () will block. This argument is a pointer to a structure of the following form: .in +4n .nf struct timespec { long tv_sec; /* seconds */ long tv_nsec; /* nanoseconds */ }; .fi .in If .I tmo_p is specified as NULL, then .BR ppoll () can block indefinitely. .SH RETURN VALUE On success, a positive number is returned; this is the number of structures which have nonzero .I revents fields (in other words, those descriptors with events or errors reported). A value of 0 indicates that the call timed out and no file descriptors were ready. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EFAULT The array given as argument was not contained in the calling program's address space. .TP .B EINTR A signal occurred before any requested event; see .BR signal (7). .TP .B EINVAL The .I nfds value exceeds the .B RLIMIT_NOFILE value. .TP .B ENOMEM There was no space to allocate file descriptor tables. .SH VERSIONS The .BR poll () system call was introduced in Linux 2.1.23. On older kernels that lack this system call, .\" library call was introduced in libc 5.4.28 the glibc (and the old Linux libc) .BR poll () wrapper function provides emulation using .BR select (2). The .BR ppoll () system call was added to Linux in kernel 2.6.16. The .BR ppoll () library call was added in glibc 2.4. .SH CONFORMING TO .BR poll () conforms to POSIX.1-2001 and POSIX.1-2008. .BR ppoll () is Linux-specific. .\" NetBSD 3.0 has a pollts() which is like Linux ppoll(). .SH NOTES On some other UNIX systems, .\" Darwin, according to a report by Jeremy Sequoia, relayed by Josh Triplett .BR poll () can fail with the error .B EAGAIN if the system fails to allocate kernel-internal resources, rather than .B ENOMEM as Linux does. POSIX permits this behavior. Portable programs may wish to check for .B EAGAIN and loop, just as with .BR EINTR . Some implementations define the nonstandard constant .B INFTIM with the value \-1 for use as a .IR timeout for .BR poll (). This constant is not provided in glibc. For a discussion of what may happen if a file descriptor being monitored by .BR poll () is closed in another thread, see .BR select (2). .SS C library/kernel differences The Linux .BR ppoll () system call modifies its .I 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 .BR ppoll () function does not modify its .I tmo_p argument. The raw .BR ppoll () system call has a fifth argument, .IR "size_t sigsetsize" , which specifies the size in bytes of the .IR sigmask argument. The glibc .BR ppoll () wrapper function specifies this argument as a fixed value (equal to .IR sizeof(kernel_sigset_t) ). See .BR sigprocmask (2) for a discussion on the differences between the kernel and the libc notion of the sigset. .SH BUGS See the discussion of spurious readiness notifications under the BUGS section of .BR select (2). .SH SEE ALSO .BR restart_syscall (2), .BR select (2), .BR select_tut (2), .BR epoll (7), .BR time (7) .SH COLOPHON This page is part of release 4.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.