178 lines
6.3 KiB
Perl
178 lines
6.3 KiB
Perl
.\" $NetBSD: 2.1.t,v 1.3 2003/08/07 10:30:49 agc Exp $
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.\"
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.\" Copyright (c) 1983, 1993, 1994
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)2.1.t 8.4 (Berkeley) 5/26/94
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.\"
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.Sh 2 "Generic operations
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.PP
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Many system abstractions support the
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.Fn read ,
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.Fn write ,
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and
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.Fn ioctl
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operations.
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We describe the basics of these common primitives here.
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Similarly, the mechanisms whereby normally synchronous operations
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may occur in a non-blocking or asynchronous fashion are
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common to all system-defined abstractions and are described here.
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.Sh 3 "Read and write
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.PP
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The
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.Fn read
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and
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.Fn write
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system calls can be applied to communications channels,
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files, terminals and devices.
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They have the form:
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.DS
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.Fd read 3 "read input
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cc = read(fd, buf, nbytes);
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result ssize_t cc; int fd; result void *buf; size_t nbytes;
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.DE
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.DS
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.Fd write 3 "write output
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cc = write(fd, buf, nbytes);
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result ssize_t cc; int fd; void *buf; size_t nbytes;
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.DE
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The
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.Fn read
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call transfers as much data as possible from the
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object defined by \fIfd\fP to the buffer at address \fIbuf\fP of
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size \fInbytes\fP. The number of bytes transferred is
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returned in \fIcc\fP, which is \-1 if a return occurred before
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any data was transferred because of an error or use of non-blocking
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operations.
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A return value of 0 is used to indicate an end-of-file condition.
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.PP
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The
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.Fn write
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call transfers data from the buffer to the
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object defined by \fIfd\fP. Depending on the type of \fIfd\fP,
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it is possible that the
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.Fn write
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call will accept only a portion of the provided bytes;
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the user should resubmit the other bytes in a later request.
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Error returns because of interrupted or otherwise incomplete operations
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are possible, in which case no data will have been transferred.
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.PP
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Scattering of data on input, or gathering of data for output
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is also possible using an array of input/output vector descriptors.
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The type for the descriptors is defined in \fI<sys/uio.h>\fP as:
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.DS
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.TS
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l s s s
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l l l l.
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struct iovec {
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char *iov_base; /* base of a component */
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size_t iov_len; /* length of a component */
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};
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.TE
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.DE
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.LP
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The \fIiov_base\fP field should be treated as if its type were
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``void *'' as POSIX and other versions of the structure may use
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that type.
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Thus, pointer arithmetic should not use this value without a cast.
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.LP
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The calls using an array of \fIiovec\fP structures are:
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.DS
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.Fd readv 3 "read gathered input
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cc = readv(fd, iov, iovlen);
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result ssize_t cc; int fd; struct iovec *iov; int iovlen;
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.DE
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.DS
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.Fd writev 3 "write scattered output
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cc = writev(fd, iov, iovlen);
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result ssize_t cc; int fd; struct iovec *iov; int iovlen;
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.DE
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Here \fIiovlen\fP is the count of elements in the \fIiov\fP array.
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.Sh 3 "Input/output control
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.LP
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Control operations on an object are performed by the
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.Fn ioctl
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operation:
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.DS
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.Fd ioctl 3 "control device
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ioctl(fd, request, buffer);
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int fd; u_long request; caddr_t buffer;
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.DE
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This operation causes the specified \fIrequest\fP to be performed
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on the object \fIfd\fP. The \fIrequest\fP parameter specifies
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whether the argument buffer is to be read, written, read and written,
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or is not used, and also the size of the buffer, as well as the
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request.
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Different descriptor types and subtypes within descriptor types
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may use distinct
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.Fn ioctl
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requests. For example,
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operations on terminals control flushing of input and output
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queues and setting of terminal parameters; operations on
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disks cause formatting operations to occur; operations on tapes
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control tape positioning.
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The names for basic control operations are defined by \fI<sys/ioctl.h>\fP,
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or more specifically by files it includes.
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.Sh 3 "Non-blocking and asynchronous operations
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.PP
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A process that wishes to do non-blocking operations on one of
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its descriptors sets the descriptor in non-blocking mode as
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described in section
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.Xr 1.5.4 .
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Thereafter the
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.Fn read
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call will return a specific EWOULDBLOCK error indication
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if there is no data to be
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.Fn read .
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The process may
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.Fn select
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the associated descriptor to determine when a read is possible.
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.PP
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Output attempted when a descriptor can accept less than is requested
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will either accept some of the provided data, returning a shorter than normal
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length, or return an error indicating that the operation would block.
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More output can be performed as soon as a
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.Fn select
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call indicates
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the object is writable.
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.PP
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Operations other than data input or output
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may be performed on a descriptor in a non-blocking fashion.
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These operations will return with a characteristic error indicating
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that they are in progress
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if they cannot complete immediately. The descriptor
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may then be
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.Fn select 'ed
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for
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.Fn write
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to find out when the operation has been completed.
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When
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.Fn select
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indicates the descriptor is writable, the operation has completed.
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Depending on the nature of the descriptor and the operation,
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additional activity may be started or the new state may be tested.
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