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POLL(2) FreeBSD System Calls Manual POLL(2)
NAME poll - synchronous I/O multiplexing
LIBRARY Standard C Library (libc, -lc)
SYNOPSIS #include <poll.h>
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 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 <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.
POLLOUT
POLLWRNORM Normal data may be written without blocking.
POLLWRBAND Data with a non-zero priority may be written without blocking.
the revents bitmask at the same time.
POLLRDHUP Remote peer closed connection, or shut down writing. Unlike POLLHUP, POLLRDHUP must be present in the events bitmask to be reported. Applies only to stream sockets.
POLLNVAL The file descriptor is not open, or in capability mode the file descriptor has insufficient rights. 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 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.
COMPATIBILITY 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.
ERRORS An error return from poll() indicates:
[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.
[EINVAL] The number of pollfd structures specified by nfds exceeds the system tunable kern.maxfilesperproc and
The poll() function conforms to IEEE Std 1003.1-2001 ("POSIX.1"). The ppoll() is not specified by POSIX. The POLLRDHUP flag is not specified by POSIX, but is compatible with Linux and illumos.
HISTORY The poll() function appeared in AT&T System V UNIX. This manual page and the core of the implementation was taken from NetBSD. The ppoll() function first appeared in FreeBSD 11.0
BUGS 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.
FreeBSD 14.0-RELEASE-p11 April 27, 2021 FreeBSD 14.0-RELEASE-p11