.\" Hey Emacs! This file is -*- nroff -*- source. .\" .\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl) .\" and Copyright (C) 2006, Michael Kerrisk .\" .\" 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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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. .\" .\" 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. .\" .\" FIXME . 2.6.17 has a definition for POLLREMOVE, but this .\" flag is not used in the code. Check later to see if it .\" does get a use. 2.6.21 still shows no use. .\" .TH POLL 2 2006-03-13 "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 .B #define _GNU_SOURCE .B #include .sp .BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ", " .BI " const struct timespec *" timeout ", 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 .I nfds structures of the following form: .nf struct pollfd { int fd; /* file descriptor */ short events; /* requested events */ short revents; /* returned events */ }; .fi The field .I fd contains a file descriptor for an open file. The field .I events is an input parameter, a bitmask specifying the events the application is interested in. 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 an upper limit on the time for which .BR poll () will block, in milliseconds. Specifying a negative value in .I timeout means an infinite timeout. 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; pseudo-terminal master in packet mode has seen state change in slave). .TP .B POLLOUT Writing now will not block. .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 in order to obtain this definition. .TP .B POLLERR Error condition (output only). .TP .B POLLHUP Hang up (output only). .TP .B POLLNVAL Invalid request: .I fd not open (output only). .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 .I timeout argument, the following .BR ppoll () call: .nf ready = ppoll(&fds, nfds, timeout, &sigmask); .fi is equivalent to .I atomically executing the following calls: .nf sigset_t origmask; sigprocmask(SIG_SETMASK, &sigmask, &origmask); ready = poll(&fds, nfds, timeout); sigprocmask(SIG_SETMASK, &origmask, NULL); .fi .PP See the description of .BR pselect (2) for an explanation of why .BR ppoll () is necessary. The .I timeout 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 +0.25i .nf struct timespec { long tv_sec; /* seconds */ long tv_nsec; /* nanoseconds */ }; .fi .in -0.25i If .I timeout 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 non-zero .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 EBADF An invalid file descriptor was given in one of the sets. .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. .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. The .BR poll () library call was introduced in libc 5.4.28 (and provides emulation using select(2) if your kernel does not have a .BR poll () system call). 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. .BR ppoll () is Linux specific. .\" NetBSD 3.0 has a pollts() which is like Linux ppoll(). .SH NOTES Some implementations define the non-standard constant .B INFTIM with the value \-1 for use as a .IR timeout . This constant is not provided in glibc. .SS "Linux Notes" The Linux .BR ppoll () system call modifies its .I timeout 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 timeout argument. .SH BUGS See the discussion of spurious readiness notifications under the BUGS section of .BR select (2). .SH "SEE ALSO" .BR select (2), .BR select_tut (2), .BR feature_test_macros (7)