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
argument 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 bitmask 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 specific to particular file types or devices.
If timeout is greater than zero,
it specifies a maximum interval (in milliseconds) 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 subsequent 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), connectx(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.