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@dylib /poll.md
Last active May 20, 2016

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$ manx poll

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.

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), 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.

BSD March 18, 2015 BSD

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