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POLL(2) OpenBSD Programmer's 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. If fd is -1, the pollfd structure is considered unused, and revents will be cleared. The events and revents members are bitmasks of conditions to monitor and conditions 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 un- til 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 other than high-priority data may be read without block- ing. POLLRDNORM Normal data may be read without blocking. POLLRDBAND Priority data may be read without blocking. POLLNORM Same as POLLRDNORM. This flag is provided for source code compatibility with older programs and should not be used in new code. POLLPRI High-priority data may be read without blocking. POLLOUT Normal data may be written without blocking. POLLWRNORM Same as POLLOUT. POLLWRBAND Priority data may be written. POLLERR An error has occurred on the device or socket. This flag is only valid in the revents bitmask; it is ignored in the events member. POLLHUP The device or socket has been disconnected. This event and POLLOUT are mutually-exclusive; a descriptor can never be writable if a hangup has occurred. However, this event and POLLIN, POLLRDNORM, POLLRDBAND, or POLLPRI are not mutually- exclusive. This flag is only valid in the revents bitmask; it is ignored in the events member. POLLNVAL The corresponding file descriptor is invalid. This flag is only valid in the revents bitmask; it is ignored in the events member. 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. RETURN VALUES Upon error, poll() returns a -1 and sets the global variable errno to in- dicate 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] 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 getrlimit(2), read(2), select(2), write(2) HISTORY A poll() system call appeared in AT&T System V.3 UNIX. BUGS The POLLHUP and POLLERR flags are currently unimplemented in OpenBSD. Because OpenBSD does not implement STREAMS, there is no distinction be- tween some of the fields in the events and revents bitmasks. As a re- sult, the POLLIN, POLLNORM, POLLPRI, POLLRDNORM and POLLRDBAND flags are all equivalent. Also, the POLLWRBAND flag is equivalent to POLLWRNORM. OpenBSD 3.4 December 13, 1994 2
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY | BUGS
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