poll(2) BSD System Calls Manual poll(2)

NAME

 poll -- synchronous I/O multiplexing

SYNOPSIS

 #include <poll.h>
 int
 poll(struct pollfd fds[], nfds_t nfds, int timeout);

DESCRIPTION

 Poll() examines a set of file descriptors to see if some of them are
 ready for I/O or if certain events have occurred on them. The fds argu-
 ment is a pointer to an array of pollfd structures, as defined in
 <poll.h> (shown below). The nfds argument specifies the size of the fds
 array.
 struct pollfd {
 int fd; /* file descriptor */
 short events; /* events to look for */
 short revents; /* events returned */
 };
 The fields of struct pollfd are as follows:
 fd File descriptor to poll.
 events Events to poll for. (See below.)
 revents Events which may occur or have occurred. (See below.)
 The event bitmasks in events and revents have the following bits:
 POLLERR An exceptional condition has occurred on the device or
 socket. This flag is output only, and ignored if present
 in the input events bitmask.
 POLLHUP The device or socket has been disconnected. This flag is
 output only, and ignored if present in the input events
 bitmask. Note that POLLHUP and POLLOUT are mutually
 exclusive and should never be present in the revents bit-
 mask at the same time.
 POLLIN Data other than high priority data may be read without
 blocking. This is equivalent to ( POLLRDNORM | POLLRDBAND
 ).
 POLLNVAL The file descriptor is not open. This flag is output
 only, and ignored if present in the input events bitmask.
 POLLOUT Normal data may be written without blocking. This is
 equivalent to POLLWRNORM.
 POLLPRI High priority data may be read without blocking.
 POLLRDBAND Priority data may be read without blocking.
 POLLRDNORM Normal data may be read without blocking.
 POLLWRBAND Priority data may be written without blocking.
 POLLWRNORM Normal data may be written without blocking.
 The distinction between normal, priority, and high-priority data is spe-
 cific to particular file types or devices.
 If timeout is greater than zero, it specifies a maximum interval (in mil-
 liseconds) to wait for any file descriptor to become ready. If timeout
 is zero, then poll() will return without blocking. If the value of
 timeout is -1, the poll blocks indefinitely.

RETURN VALUES

 Poll() returns the number of descriptors that are ready for I/O, or -1 if
 an error occurred. If the time limit expires, poll() returns 0. If
 poll() returns with an error, including one due to an interrupted call,
 the fds array will be unmodified and the global variable errno will be
 set to indicate the error.

ERRORS

 Poll() will fail if:
 [EAGAIN] Allocation of internal data structures fails. A sub-
 sequent request may succeed.
 [EFAULT] Fds points outside the process's allocated address
 space.
 [EINTR] A signal is delivered before the time limit expires
 and before any of the selected events occurs.
 [EINVAL] The nfds argument is greater than OPEN_MAX or the
 timeout argument is less than -1.

BUGS

 The poll() system call currently does not support devices.

SEE ALSO

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

HISTORY

 The poll() function call appeared in AT&T System V UNIX.
BSD February 27, 2005 BSD