poll.2freebsd

Langue: en

Version: 265891 (debian - 07/07/09)

Section: 2 (Appels système)


BSD mandoc

NAME

poll - synchronous I/O multiplexing

LIBRARY

Lb libc

SYNOPSIS

In poll.h Ft int Fn poll struct pollfd fds[] nfds_t nfds int timeout

DESCRIPTION

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.

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 negative.

SEE ALSO

accept(2), connect(2), kqueue(2), read(2), recv(2), select(2), send(2), write(2)

HISTORY

The Fn poll function appeared in AT&T System V . This manual page and the core of the implementation was taken from Nx .

BUGS

The distinction between some of the fields in the Fa events and Fa revents bitmasks is really not useful without STREAMS. The fields are defined for compatibility with existing software.