POLL(2) BSD System Calls Manual POLL(2)
NAME
poll - synchronous I/O multiplexing
SYNOPSIS
#include <poll.h>
int
poll(struct pollfd *fds, int nfds, int timeout);
DESCRIPTION
poll() provides a mechanism for reporting I/O conditions across a set of
file descriptors.
The arguments are as follows:
fds Points to an array of pollfd structures, which are defined as:
struct pollfd {
int fd;
short events;
short revents;
};
The fd member is an open file descriptor. The events and
revents members are bitmasks of conditions to monitor and condi-
tions found, respectively.
nfds The number of pollfd structures in the array.
timeout Maximum interval to wait for the poll to complete, in millisec-
onds. If this value is 0, then poll() will return immediately.
If this value is INFTIM (-1), poll() will block indefinitely
until a condition is found.
The calling process sets the events bitmask and poll() sets the revents
bitmask. Each call to poll() resets the revents bitmask for accuracy.
The condition flags in the bitmasks are defined as:
POLLIN Data is available on the file descriptor for reading.
POLLNORM Same as POLLIN.
POLLPRI Same as POLLIN.
POLLOUT Data can be written to the file descriptor without blocking.
POLLERR This flag is not used in this implementation and is provided
only for source code compatibility.
POLLHUP The file descriptor was valid before the polling process and
invalid after. Presumably, this means that the file descrip-
tor was closed sometime during the poll.
POLLNVAL The corresponding file descriptor is invalid.
POLLRDNORM Same as POLLIN.
POLLRDBAND Same as POLLIN.
POLLWRNORM Same as POLLOUT.
POLLWRBAND Same as POLLOUT.
POLLMSG This flag is not used in this implementation and is provided
only for source code compatibility.
All flags except POLLIN, POLLOUT, and their synonyms are for use only in
the revents member of the pollfd structure. An attempt to set any of
these flags in the events member will generate an error condition.
In addition to I/O multiplexing, poll() can be used to generate simple
timeouts. This functionality may be achieved by passing a null pointer
for fds.
WARNINGS
The POLLHUP flag is only a close approximation and may not always be
accurate.
RETURN VALUES
Upon error, poll() returns a -1 and sets the global variable errno to
indicate the error. If the timeout interval was reached before any
events occurred, a 0 is returned. Otherwise, poll() returns the number
of file descriptors for which revents is non-zero.
ERRORS
poll() will fail if:
[EINVAL] nfds was either a negative number or greater than the number
of available file descriptors.
[EINVAL] An invalid flags was set in the events member of the pollfd
structure.
[EINVAL] The timeout passed to poll() was too large.
[EAGAIN] Resource allocation failed inside of poll(). Subsequent calls
to poll() may succeed.
[EINTR] poll() caught a signal during the polling process.
SEE ALSO
poll(2), select(2), sysconf(3)
HISTORY
A poll() system call appeared in AT&T System V UNIX.
BSD December 13, 1994 BSD
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