265 lines
6.2 KiB
Groff
265 lines
6.2 KiB
Groff
.\" $NetBSD: read.2,v 1.26 2003/08/07 16:44:04 agc 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 October 16, 2001
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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 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 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 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 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 EIO
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An I/O error occurred while reading from the file system.
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.It Bq Er EINTR
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A read from a slow device was interrupted by the delivery of a
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signal 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 pointer associated with
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.Fa d
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was negative.
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.It Bq Er EINVAL
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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 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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.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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.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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.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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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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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 v6 .
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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))) > 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 && nr != 0)
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.Ed
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