NAME¶

LIBRARY¶

SYNOPSIS¶

int
poll(struct pollfd fds[], nfds_t nfds, int timeout);

int
ppoll(struct pollfd fds[], nfds_t nfds, const struct timespec * restrict timeout, const sigset_t * restrict newsigmask);

DESCRIPTION¶

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.

POLLOUT

 

POLLWRNORM

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 const struct timespec which is defined in <sys/timespec.h> (shown below) rather than the 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 */
};

RETURN VALUES¶

COMPATIBILITY¶

ERRORS¶

[EFAULT]

The fds argument points outside the process's allocated address space.

[EINTR]

A signal was delivered before the time limit expired and before any of the selected events occurred.

[EINVAL]

The specified time limit is invalid. One of its components is negative or too large.

SEE ALSO¶

STANDARDS¶

HISTORY¶

BUGS¶