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+.\" Copyright (c) 1983, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"	@(#)2.1.t	8.1 (Berkeley) 6/8/93
+.\"
+.sh "Generic operations
+.PP
+.PP
+Many system abstractions support the
+operations \fIread\fP, \fIwrite\fP and \fIioctl\fP.  We describe
+the basics of these common primitives here.
+Similarly, the mechanisms whereby normally synchronous operations
+may occur in a non-blocking or asynchronous fashion are
+common to all system-defined abstractions and are described here.
+.NH 3
+Read and write
+.PP
+The \fIread\fP and \fIwrite\fP system calls can be applied
+to communications channels, files, terminals and devices.
+They have the form:
+.DS
+cc = read(fd, buf, nbytes);
+result int cc; int fd; result caddr_t buf; int nbytes;
+
+cc = write(fd, buf, nbytes);
+result int cc; int fd; caddr_t buf; int nbytes;
+.DE
+The \fIread\fP call transfers as much data as possible from the
+object defined by \fIfd\fP to the buffer at address \fIbuf\fP of
+size \fInbytes\fP.  The number of bytes transferred is
+returned in \fIcc\fP, which is \-1 if a return occurred before
+any data was transferred because of an error or use of non-blocking
+operations.
+.PP
+The \fIwrite\fP call transfers data from the buffer to the
+object defined by \fIfd\fP.  Depending on the type of \fIfd\fP,
+it is possible that the \fIwrite\fP call will accept some portion
+of the provided bytes; the user should resubmit the other bytes
+in a later request in this case.
+Error returns because of interrupted or otherwise incomplete operations
+are possible.
+.PP
+Scattering of data on input or gathering of data for output
+is also possible using an array of input/output vector descriptors.
+The type for the descriptors is defined in \fI<sys/uio.h>\fP as:
+.DS
+._f
+struct iovec {
+	caddr_t	iov_msg;	/* base of a component */
+	int	iov_len;	/* length of a component */
+};
+.DE
+The calls using an array of descriptors are:
+.DS
+cc = readv(fd, iov, iovlen);
+result int cc; int fd; struct iovec *iov; int iovlen;
+
+cc = writev(fd, iov, iovlen);
+result int cc; int fd; struct iovec *iov; int iovlen;
+.DE
+Here \fIiovlen\fP is the count of elements in the \fIiov\fP array.
+.NH 3
+Input/output control
+.PP
+Control operations on an object are performed by the \fIioctl\fP
+operation:
+.DS
+ioctl(fd, request, buffer);
+int fd, request; caddr_t buffer;
+.DE
+This operation causes the specified \fIrequest\fP to be performed
+on the object \fIfd\fP.  The \fIrequest\fP parameter specifies
+whether the argument buffer is to be read, written, read and written,
+or is not needed, and also the size of the buffer, as well as the
+request.
+Different descriptor types and subtypes within descriptor types
+may use distinct \fIioctl\fP requests.  For example,
+operations on terminals control flushing of input and output
+queues and setting of terminal parameters; operations on
+disks cause formatting operations to occur; operations on tapes
+control tape positioning.
+.PP
+The names for basic control operations are defined in \fI<sys/ioctl.h>\fP.
+.NH 3
+Non-blocking and asynchronous operations
+.PP
+A process that wishes to do non-blocking operations on one of
+its descriptors sets the descriptor in non-blocking mode as
+described in section 1.5.4.  Thereafter the \fIread\fP call will
+return a specific EWOULDBLOCK error indication if there is no data to be
+\fIread\fP.  The process may
+\fIselect\fP the associated descriptor to determine when a read is
+possible.
+.PP
+Output attempted when a descriptor can accept less than is requested
+will either accept some of the provided data, returning a shorter than normal
+length, or return an error indicating that the operation would block.
+More output can be performed as soon as a \fIselect\fP call indicates
+the object is writeable.
+.PP
+Operations other than data input or output
+may be performed on a descriptor in a non-blocking fashion.
+These operations will return with a characteristic error indicating
+that they are in progress
+if they cannot complete immediately.  The descriptor
+may then be \fIselect\fPed for \fIwrite\fP to find out
+when the operation has been completed.  When \fIselect\fP indicates
+the descriptor is writeable, the operation has completed.
+Depending on the nature of the descriptor and the operation,
+additional activity may be started or the new state may be tested.