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Man Pages

Manual Reference Pages  -  POLL (2)


poll - synchronous I/O multiplexing


Return Values
See Also


.Lb libc


.In poll.h int poll struct pollfd fds[] nfds_t nfds int timeout int
.Fo ppoll struct pollfd fds[] nfds_t nfds const struct timespec * restrict timeout const sigset_t * restrict newsigmask


The poll system call examines a set of file descriptors to see if some of them are ready for I/O. The fds argument is a pointer to an array of pollfd structures as defined in
.In poll.h (shown below). The nfds argument determines 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. If fd is equal to -1 then 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 events and 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.
  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 events bitmask.
POLLHUP The device or socket has been disconnected. This flag is always checked, even if not present in the events bitmask. Note that POLLHUP and POLLOUT should never be present in the revents bitmask at the same time.
POLLNVAL The file descriptor is not open. This flag is always checked, even if not present in the events bitmask.

If timeout is neither zero nor INFTIM (-1), it specifies a maximum interval to wait for any file descriptor to become ready, in milliseconds. If timeout is INFTIM (-1), the poll blocks indefinitely. If timeout is zero, then poll will return without blocking.

The ppoll system call, unlike poll, is used to safely wait until either a set of file descriptors becomes ready or until a signal is caught. The fds and nfds arguments are identical to the analogous arguments of poll. The timeout argument in ppoll points to a
.Vt const struct timespec which is defined in
.In sys/timespec.h (shown below) rather than the
.Vt int timeout used by poll. A null pointer may be passed to indicate that ppoll should wait indefinitely. Finally, newsigmask specifies a signal mask which is set while waiting for input. When ppoll returns, the original signal mask is restored.

struct timespec {
        time_t  tv_sec;         /* seconds */
        long    tv_nsec;        /* and nanoseconds */


The 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, poll returns 0. If poll returns with an error, including one due to an interrupted system call, the fds array will be unmodified.


This implementation differs from the historical one in that a given file descriptor may not cause 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 ed descriptor), this implementation instead copies the events bitmask to the revents bitmask. Attempting to perform I/O on this descriptor will then return an error. This behaviour is believed to be more useful.


An error return from poll indicates:
  The fds argument points outside the process’s allocated address space.
  A signal was delivered before the time limit expired and before any of the selected events occurred.
  The specified time limit is invalid. One of its components is negative or too large.


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


The poll function conforms to -p1003.1-2001. The ppoll is not specified by POSIX.


The poll function appeared in AT&T V . This manual page and the core of the implementation was taken from
.Nx . The ppoll function first appeared in
.Fx 11.0


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