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poll.2freebsd
Langue: en
Version: 265891 (debian - 07/07/09)
Section: 2 (Appels système)
BSD mandoc
NAME
poll - synchronous I/O multiplexingLIBRARY
Lb libcSYNOPSIS
In poll.h Ft int Fn poll struct pollfd fds[] nfds_t nfds int timeoutDESCRIPTION
The Fn poll system call examines a set of file descriptors to see if some of them are ready for I/O. The Fa fds argument is a pointer to an array of pollfd structures as defined in In poll.h (shown below). The Fa nfds argument determines the size of the Fa fds array.struct pollfd { int fd; /* file descriptor */ short events; /* events to look for */ short revents; /* events returned */ };
The fields of Fa struct pollfd are as follows:
- fd
- File descriptor to poll. If fd is equal to -1 then Fa 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 Fa events and Fa 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 Fa events bitmask.
- POLLHUP
- The device or socket has been disconnected. This flag is always checked, even if not present in the Fa events bitmask. Note that POLLHUP and POLLOUT should never be present in the Fa revents bitmask at the same time.
- POLLNVAL
- The file descriptor is not open. This flag is always checked, even if not present in the Fa events bitmask.
If Fa timeout is neither zero nor INFTIM (-1), it specifies a maximum interval to wait for any file descriptor to become ready, in milliseconds. If Fa timeout is INFTIM (-1), the poll blocks indefinitely. If Fa timeout is zero, then Fn poll will return without blocking.
RETURN VALUES
The Fn 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, Fn poll returns 0. If Fn poll returns with an error, including one due to an interrupted system call, the Fa fds array will be unmodified.COMPATIBILITY
This implementation differs from the historical one in that a given file descriptor may not cause Fn 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)Nsed descriptor), this implementation instead copies the Fa events bitmask to the Fa 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 Fn poll indicates:- Bq Er EFAULT
- The Fa fds argument points outside the process's allocated address space.
- Bq Er EINTR
- A signal was delivered before the time limit expired and before any of the selected events occurred.
- Bq Er EINVAL
- The specified time limit is negative.
SEE ALSO
accept(2), connect(2), kqueue(2), read(2), recv(2), select(2), send(2), write(2)HISTORY
The Fn poll function appeared in AT&T System V . This manual page and the core of the implementation was taken from Nx .BUGS
The distinction between some of the fields in the Fa events and Fa revents bitmasks is really not useful without STREAMS. The fields are defined for compatibility with existing software.Contenus ©2006-2024 Benjamin Poulain
Design ©2006-2024 Maxime Vantorre