fbbd9655e5
Renumber cluase 4 to 3, per what everybody else did when BSD granted them permission to remove clause 3. My insistance on keeping the same numbering for legal reasons is too pedantic, so give up on that point. Submitted by: Jan Schaumann <jschauma@stevens.edu> Pull Request: https://github.com/freebsd/freebsd/pull/96
285 lines
6.6 KiB
Groff
285 lines
6.6 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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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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.\" @(#)read.2 8.4 (Berkeley) 2/26/94
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.\" $FreeBSD$
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.\"
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.Dd December 15, 2015
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.Dt READ 2
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.Os
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.Sh NAME
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.Nm read ,
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.Nm readv ,
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.Nm pread ,
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.Nm preadv
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.Nd read input
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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.Ft ssize_t
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.Fn read "int fd" "void *buf" "size_t nbytes"
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.Ft ssize_t
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.Fn pread "int fd" "void *buf" "size_t nbytes" "off_t offset"
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.In sys/uio.h
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.Ft ssize_t
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.Fn readv "int fd" "const struct iovec *iov" "int iovcnt"
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.Ft ssize_t
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.Fn preadv "int fd" "const struct iovec *iov" "int iovcnt" "off_t offset"
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.Sh DESCRIPTION
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The
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.Fn read
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system call
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attempts to read
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.Fa nbytes
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of data from the object referenced by the descriptor
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.Fa fd
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into the buffer pointed to by
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.Fa buf .
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The
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.Fn readv
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system call
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performs the same action, but scatters the input data
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into the
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.Fa iovcnt
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buffers specified by the members of the
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.Fa iov
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array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
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The
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.Fn pread
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and
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.Fn preadv
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system calls
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perform the same functions, but read from the specified position in
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the file without modifying the file pointer.
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.Pp
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For
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.Fn readv
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and
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.Fn preadv ,
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the
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.Fa iovec
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structure is defined as:
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.Pp
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.Bd -literal -offset indent -compact
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struct iovec {
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void *iov_base; /* Base address. */
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size_t iov_len; /* Length. */
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};
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.Ed
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.Pp
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Each
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.Fa iovec
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entry specifies the base address and length of an area
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in memory where data should be placed.
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The
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.Fn readv
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system call
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will always fill an area completely before proceeding
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to the next.
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.Pp
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On objects capable of seeking, the
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.Fn read
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starts at a position
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given by the pointer associated with
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.Fa fd
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(see
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.Xr lseek 2 ) .
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Upon return from
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.Fn read ,
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the pointer is incremented by the number of bytes actually read.
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.Pp
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Objects that are not capable of seeking always read from the current
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position.
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The value of the pointer associated with such an
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object is undefined.
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.Pp
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Upon successful completion,
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.Fn read ,
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.Fn readv ,
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.Fn pread
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and
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.Fn preadv
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return the number of bytes actually read and placed in the buffer.
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The system guarantees to read the number of bytes requested if
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the descriptor references a normal file that has that many bytes left
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before the end-of-file, but in no other case.
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.Sh RETURN VALUES
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If successful, the
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number of bytes actually read is returned.
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Upon reading end-of-file,
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zero is returned.
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Otherwise, a -1 is returned and the global variable
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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The
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.Fn read ,
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.Fn readv ,
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.Fn pread
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and
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.Fn preadv
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system calls
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will succeed unless:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The
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.Fa fd
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argument
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is not a valid file or socket descriptor open for reading.
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.It Bq Er ECONNRESET
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The
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.Fa fd
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argument refers to a socket, and the remote socket end is
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forcibly closed.
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.It Bq Er EFAULT
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The
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.Fa buf
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argument
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points outside the allocated address space.
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.It Bq Er EIO
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An I/O error occurred while reading from the file system.
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.It Bq Er EBUSY
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Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped
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.It Bq Er EINTR
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A read from a slow device
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(i.e.\& one that might block for an arbitrary amount of time)
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was interrupted by the delivery of a signal
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before any data arrived.
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.It Bq Er EINVAL
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The pointer associated with
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.Fa fd
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was negative.
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.It Bq Er EAGAIN
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The file was marked for non-blocking I/O,
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and no data were ready to be read.
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.It Bq Er EISDIR
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The file descriptor is associated with a directory residing
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on a file system that does not allow regular read operations on
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directories (e.g.\& NFS).
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.It Bq Er EOPNOTSUPP
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The file descriptor is associated with a file system and file type that
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do not allow regular read operations on it.
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.It Bq Er EOVERFLOW
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The file descriptor is associated with a regular file,
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.Fa nbytes
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is greater than 0,
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.Fa offset
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is before the end-of-file, and
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.Fa offset
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is greater than or equal to the offset maximum established
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for this file system.
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.It Bq Er EINVAL
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The value
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.Fa nbytes
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is greater than
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.Dv INT_MAX .
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.El
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.Pp
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In addition,
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.Fn readv
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and
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.Fn preadv
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may return one of the following errors:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The
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.Fa iovcnt
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argument
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was less than or equal to 0, or greater than
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.Dv IOV_MAX .
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.It Bq Er EINVAL
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One of the
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.Fa iov_len
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values in the
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.Fa iov
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array was negative.
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.It Bq Er EINVAL
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The sum of the
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.Fa iov_len
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values in the
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.Fa iov
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array overflowed a 32-bit integer.
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.It Bq Er EFAULT
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Part of the
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.Fa iov
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array points outside the process's allocated address space.
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.El
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.Pp
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The
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.Fn pread
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and
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.Fn preadv
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system calls may also return the following errors:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The
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.Fa offset
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value was negative.
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.It Bq Er ESPIPE
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The file descriptor is associated with a pipe, socket, or FIFO.
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.El
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.Sh SEE ALSO
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.Xr dup 2 ,
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.Xr fcntl 2 ,
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.Xr getdirentries 2 ,
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.Xr open 2 ,
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.Xr pipe 2 ,
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.Xr select 2 ,
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.Xr socket 2 ,
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.Xr socketpair 2 ,
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.Xr fread 3 ,
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.Xr readdir 3
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.Sh STANDARDS
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The
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.Fn read
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system call is expected to conform to
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.St -p1003.1-90 .
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The
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.Fn readv
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and
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.Fn pread
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system calls are expected to conform to
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.St -xpg4.2 .
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.Sh HISTORY
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The
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.Fn preadv
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system call appeared in
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.Fx 6.0 .
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The
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.Fn pread
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function appeared in
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.At V.4 .
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The
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.Fn readv
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system call appeared in
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.Bx 4.2 .
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The
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.Fn read
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function appeared in
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.At v6 .
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