2fe8fb192f
There is important information about booting non-ack images in docs/UPDATING. ack/aout-format images can't be built any more, and booting clang/ELF-format ones is a little different. Updating to the new boot monitor is recommended. Changes in this commit: . drop boot monitor -> allowing dropping ack support . facility to copy ELF boot files to /boot so that old boot monitor can still boot fairly easily, see UPDATING . no more ack-format libraries -> single-case libraries . some cleanup of OBJECT_FMT, COMPILER_TYPE, etc cases . drop several ack toolchain commands, but not all support commands (e.g. aal is gone but acksize is not yet). . a few libc files moved to netbsd libc dir . new /bin/date as minix date used code in libc/ . test compile fix . harmonize includes . /usr/lib is no longer special: without ack, /usr/lib plays no kind of special bootstrapping role any more and bootstrapping is done exclusively through packages, so releases depend even less on the state of the machine making them now. . rename nbsd_lib* to lib* . reduce mtree
263 lines
6.3 KiB
Groff
263 lines
6.3 KiB
Groff
.\" $NetBSD: read.2,v 1.33 2010/04/05 07:53:47 wiz Exp $
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.\"
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.\" 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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.\"
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.Dd April 3, 2010
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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 d" "void *buf" "size_t nbytes"
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.Ft ssize_t
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.Fn pread "int d" "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 d" "const struct iovec *iov" "int iovcnt"
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.Ft ssize_t
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.Fn preadv "int d" "const struct iovec *iov" "int iovcnt" "off_t offset"
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.Sh DESCRIPTION
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.Fn read
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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 d
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into the buffer pointed to by
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.Fa buf .
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.Fn readv
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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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.Fn pread
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and
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.Fn preadv
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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;
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size_t iov_len;
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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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.Fn readv
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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 file pointer associated with
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.Fa d
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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 file 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 file pointer associated with such an 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, 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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.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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will succeed unless:
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.Bl -tag -width Er
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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 EBADF
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.Fa d
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is not a valid file or socket descriptor open for reading.
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.It Bq Er EFAULT
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.Fa buf
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points outside the allocated address space.
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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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See
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.Xr sigaction 2
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for more information on the interaction between signals and system
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calls.
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.It Bq Er EINVAL
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The file pointer associated with
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.Fa d
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was negative; or
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the total length of the I/O is more than can be expressed by the ssize_t
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return value.
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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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.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 EFAULT
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Part of the
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.Fa iov
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points outside the process's allocated address space.
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.It Bq Er EINVAL
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.Fa iovcnt
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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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or 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; or
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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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.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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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 specified file offset is invalid.
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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 open 2 ,
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.Xr pipe 2 ,
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.Xr poll 2 ,
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.Xr select 2 ,
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.Xr sigaction 2 ,
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.Xr socket 2 ,
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.Xr socketpair 2
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.Sh STANDARDS
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The
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.Fn read
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function conforms 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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functions 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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function call
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appeared in
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.Nx 1.4 .
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The
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.Fn pread
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function call
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appeared in
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.At V.4 .
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The
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.Fn readv
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function call
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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 call appeared in
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.At v2 .
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.Sh CAVEATS
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Error checks should explicitly test for \-1.
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Code such as
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.Bd -literal
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while ((nr = read(fd, buf, sizeof(buf))) \*[Gt] 0)
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.Ed
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.Pp
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is not maximally portable, as some platforms allow for
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.Va nbytes
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to range between
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.Dv SSIZE_MAX
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and
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.Dv SIZE_MAX
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\- 2, in which case the return value of an error-free
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.Fn read
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may appear as a negative number distinct from \-1.
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Proper loops should use
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.Bd -literal
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while ((nr = read(fd, buf, sizeof(buf))) != -1 \*[Am]\*[Am] nr != 0)
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.Ed
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