ppoll (2)
Leading comments
$NetBSD: poll.2,v 1.3 1996/09/07 21:53:08 mycroft Exp $
$FreeBSD: releng/11.0/lib/libc/sys/poll.2 276006 2014-12-21 12:36:36Z brueffer $
Copyright (c) 1996 Charles M. Hannum. All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
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1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributi...
NAME
poll - synchronous I/O multiplexingLIBRARY
Lb libcSYNOPSIS
In poll.h Ft int Fn poll struct pollfd fds[] nfds_t nfds int timeout Ft int Fo ppoll Fa struct pollfd fds[] Fa nfds_t nfds Fa const struct timespec * restrict timeout Fa const sigset_t * restrict newsigmask FcDESCRIPTION
The Fn poll system call examines a set of file descriptors to see if some of them are ready for I/O. The Fa fds argument is a pointer to an array of pollfd structures as defined in In poll.h (shown below). The Fa nfds argument determines the size of the Fa fds array.
struct pollfd {
int fd; /* file descriptor */
short events; /* events to look for */
short revents; /* events returned */
};
The fields of Fa struct pollfd are as follows:
- fd
- File descriptor to poll. If fd is equal to -1 then Fa revents is cleared (set to zero), and that pollfd is not checked.
- events
- Events to poll for. (See below.)
- revents
- Events which may occur. (See below.)
The event bitmasks in Fa events and Fa revents have the following bits:
- POLLIN
- Data other than high priority data may be read without blocking.
- POLLRDNORM
- Normal data may be read without blocking.
- POLLRDBAND
- Data with a non-zero priority may be read without blocking.
- POLLPRI
- High priority data may be read without blocking.
- POLLOUT
- POLLWRNORM
- Normal data may be written without blocking.
- POLLWRBAND
- Data with a non-zero priority may be written without blocking.
- POLLERR
- An exceptional condition has occurred on the device or socket. This flag is always checked, even if not present in the Fa events bitmask.
- POLLHUP
- The device or socket has been disconnected. This flag is always checked, even if not present in the Fa events bitmask. Note that POLLHUP and POLLOUT should never be present in the Fa revents bitmask at the same time.
- POLLNVAL
- The file descriptor is not open. This flag is always checked, even if not present in the Fa events bitmask.
If Fa timeout is neither zero nor INFTIM (-1), it specifies a maximum interval to wait for any file descriptor to become ready, in milliseconds. If Fa timeout is INFTIM (-1), the poll blocks indefinitely. If Fa timeout is zero, then Fn poll will return without blocking.
The Fn ppoll system call, unlike Fn poll , is used to safely wait until either a set of file descriptors becomes ready or until a signal is caught. The Fa fds and Fa nfds arguments are identical to the analogous arguments of Fn poll . The Fa timeout argument in Fn ppoll points to a Vt const struct timespec which is defined in In sys/timespec.h (shown below) rather than the Vt int timeout used by Fn poll . A null pointer may be passed to indicate that Fn ppoll should wait indefinitely. Finally, Fa newsigmask specifies a signal mask which is set while waiting for input. When Fn ppoll returns, the original signal mask is restored.
struct timespec {
time_t tv_sec; /* seconds */
long tv_nsec; /* and nanoseconds */
};
RETURN VALUES
The Fn poll system call returns the number of descriptors that are ready for I/O, or -1 if an error occurred. If the time limit expires, Fn poll returns 0. If Fn poll returns with an error, including one due to an interrupted system call, the Fa fds array will be unmodified.COMPATIBILITY
This implementation differs from the historical one in that a given file descriptor may not cause Fn poll to return with an error. In cases where this would have happened in the historical implementation (e.g. trying to poll a revoke(2)Nsed descriptor), this implementation instead copies the Fa events bitmask to the Fa revents bitmask. Attempting to perform I/O on this descriptor will then return an error. This behaviour is believed to be more useful.ERRORS
An error return from Fn poll indicates:- Bq Er EFAULT
- The Fa fds argument points outside the process's allocated address space.
- Bq Er EINTR
- A signal was delivered before the time limit expired and before any of the selected events occurred.
- Bq Er EINVAL
- The specified time limit is invalid. One of its components is negative or too large.