POLL(2) BSD System Calls Manual POLL(2)NAMEpoll — synchronous I/O multiplexing
SYNOPSIS
#include <poll.h>
int
poll(struct pollfd fds[], nfds_t nfds, int timeout);
DESCRIPTIONPoll() 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 VALUESPoll() 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.
ERRORSPoll() 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 ALSOaccept(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
