The poll() function provides
applications with a mechanism for multiplexing input/output over a set of file descriptors. For each member of the array pointed to by fds, poll() examines the given file descriptor for the event(s) specified in events. The number of pollfd structures in the fds array is specified by nfds. The poll() function identifies those file descriptors on which an application can read or write data, or on which certain events have occurred.
The fds argument specifies the file descriptors to be examined and the events of interest for each file descriptor. It is a pointer to an array with one member for each open file descriptor of interest. The array's members are pollfd structures, which
contain the following members:
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
The fd member specifies an open file descriptor and the events and revents members are bitmasks constructed by a logical OR operation of any combination of the following event flags:
- Data other than high priority data may be read without blocking. For STREAMS, this flag is set in revents even if the message is of zero length.
- Normal data (priority band equals 0) may be read without blocking. For STREAMS, this flag is set in revents even if the message is of zero length.
- Data from a non-zero priority band may be read without blocking. For STREAMS, this flag is set in revents even if the message is of zero length.
- High priority data may be received without blocking. For STREAMS, this flag is set in revents even if the message is of zero length.
- Normal data (priority band equals 0) may be written without blocking.
- The same as POLLOUT.
- Priority data (priority band > 0) may be written. This event only examines bands that have been written to at least once.
- An error has occurred on the device or stream. This flag is only valid in the revents bitmask; it is not used in the events member.
- A hangup has occurred on the stream. This event and POLLOUT are mutually exclusive; a stream 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 not used in the events member.
- The specified fd value does not belong to an open file. This flag is only valid in the revents member; it is not used in the events member.
If the value fd is less than 0, events is ignored and revents is set to 0 in that entry on return from poll().
The results of the poll() query are stored in the revents member in the pollfd structure. Bits are set in the revents bitmask to indicate which of the requested events are true. If none are true, none of the specified bits
are set in revents when the poll() call returns. The event flags POLLHUP, POLLERR, and POLLNVAL are always set in revents if the conditions they indicate are true;
this occurs even though these flags were not present in events.
If none of the defined events have occurred on any selected file descriptor, poll() waits at least timeout milliseconds for an event to occur on any of the selected file descriptors. On a computer where millisecond timing accuracy is not available, timeout is rounded up to the nearest legal value available on that system. If the value timeout is 0, poll() returns immediately. If the value of timeout is -1, poll() blocks until a requested event
occurs or until the call is interrupted. The poll() function is not affected by the O_NDELAY and O_NONBLOCK flags.
The poll() function supports regular files, terminal and pseudo-terminal devices, STREAMS-based files, FIFOs and pipes. The behavior of poll() on elements of fds that refer to other types of file is unspecified.
The poll() function supports sockets.
A file descriptor for a socket that is listening for connections will indicate that it is ready for reading, once connections are available. A file descriptor for a socket that is connecting asynchronously will indicate that it is ready for writing, once a connection has been established.
Regular files always poll() TRUE for reading and writing.