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+.\"	$OpenBSD: read.2,v 1.38 2021/11/21 23:44:55 jan Exp $
+.\"	$NetBSD: read.2,v 1.6 1995/02/27 12:35:47 cgd Exp $
+.\"
+.\" Copyright (c) 1980, 1991, 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. 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.
+.\"
+.\"     @(#)read.2	8.4 (Berkeley) 2/26/94
+.\"
+.Dd $Mdocdate: November 21 2021 $
+.Dt READ 2
+.Os
+.Sh NAME
+.Nm read ,
+.Nm readv ,
+.Nm pread ,
+.Nm preadv
+.Nd read input
+.Sh SYNOPSIS
+.In unistd.h
+.Ft ssize_t
+.Fn read "int d" "void *buf" "size_t nbytes"
+.Ft ssize_t
+.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset"
+.Pp
+.In sys/uio.h
+.Ft ssize_t
+.Fn readv "int d" "const struct iovec *iov" "int iovcnt"
+.In sys/types.h
+.In sys/uio.h
+.Ft ssize_t
+.Fn preadv "int d" "const struct iovec *iov" "int iovcnt" "off_t offset"
+.Sh DESCRIPTION
+.Fn read
+attempts to read
+.Fa nbytes
+of data from the object referenced by the descriptor
+.Fa d
+into the buffer pointed to by
+.Fa buf .
+.Fn readv
+performs the same action, but scatters the input data into the
+.Fa iovcnt
+buffers specified by the members of the
+.Fa iov
+array: iov[0], iov[1], ..., iov[iovcnt-1].
+.Fn pread
+and
+.Fn preadv
+perform the same functions, but read from the specified position
+.Fa offset
+in the file without modifying the file pointer.
+.Pp
+For
+.Fn readv
+and
+.Fn preadv ,
+the
+.Fa iovec
+structure is defined as:
+.Bd -literal -offset indent
+struct iovec {
+	void	*iov_base;
+	size_t	 iov_len;
+};
+.Ed
+.Pp
+Each
+.Fa iovec
+entry specifies the base address and length of an area
+in memory where data should be placed.
+.Fn readv
+and
+.Fn preadv
+will always fill an area completely before proceeding to the next.
+.Pp
+On objects capable of seeking, the
+.Fn read
+starts at a position given by the pointer associated with
+.Fa d
+(see
+.Xr lseek 2 ) .
+Upon return from
+.Fn read ,
+the pointer is incremented by the number of bytes actually read.
+.Pp
+Objects that are not capable of seeking always read from the current
+position.
+The value of the pointer associated with such an object is undefined.
+.Pp
+Upon successful completion,
+.Fn read ,
+.Fn readv ,
+.Fn pread ,
+and
+.Fn preadv
+return the number of bytes actually read and placed in the buffer.
+The system guarantees to read the number of bytes requested if
+the descriptor references a normal file that has that many bytes left
+before the end-of-file, but in no other case.
+.Pp
+Note that
+.Fn readv
+and
+.Fn preadv
+will fail if the value of
+.Fa iovcnt
+exceeds the constant
+.Dv IOV_MAX .
+.Sh RETURN VALUES
+If successful, the
+number of bytes actually read is returned.
+Upon reading end-of-file, zero is returned.
+Otherwise, a \-1 is returned and the global variable
+.Va errno
+is set to indicate the error.
+.Sh ERRORS
+.Fn read ,
+.Fn readv ,
+.Fn pread ,
+and
+.Fn preadv
+will fail if:
+.Bl -tag -width Er
+.It Bq Er EBADF
+.Fa d
+is not a valid file or socket descriptor open for reading.
+.It Bq Er EFAULT
+Part of
+.Fa buf
+points outside the process's allocated address space.
+.It Bq Er EINTR
+A read from a slow device
+(i.e. one that might block for an arbitrary amount of time)
+was interrupted by the delivery of a signal
+before any data arrived.
+.It Bq Er EIO
+An I/O error occurred while reading from the file system.
+.It Bq Er EISDIR
+The underlying file is a directory.
+.El
+.Pp
+In addition,
+.Fn read
+and
+.Fn readv
+may return the following errors:
+.Bl -tag -width Er
+.It Bq Er EAGAIN
+The file was marked for non-blocking I/O,
+and no data were ready to be read.
+.It Bq Er ENOTCONN
+The file is a socket associated with a connection-oriented protocol
+and has not been connected.
+.It Bq Er EIO
+The process is a member of a background process attempting to read
+from its controlling terminal, the process is ignoring or blocking
+the
+.Dv SIGTTIN
+signal or the process group is orphaned.
+.El
+.Pp
+.Fn read
+and
+.Fn pread
+may return the following error:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+.Fa nbytes
+was larger than
+.Dv SSIZE_MAX .
+.El
+.Pp
+.Fn pread
+and
+.Fn preadv
+may return the following errors:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+.Fa offset
+was negative.
+.It Bq Er ESPIPE
+.Fa d
+is associated with a pipe, socket, FIFO, or tty.
+.El
+.Pp
+.Fn readv
+and
+.Fn preadv
+may return the following errors:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+.Fa iovcnt
+was less than or equal to 0, or greater than
+.Dv IOV_MAX .
+.It Bq Er EINVAL
+The sum of the
+.Fa iov_len
+values in the
+.Fa iov
+array overflowed an
+.Vt ssize_t .
+.It Bq Er EFAULT
+Part of
+.Fa iov
+points outside the process's allocated address space.
+.El
+.Sh SEE ALSO
+.Xr dup 2 ,
+.Xr fcntl 2 ,
+.Xr open 2 ,
+.Xr pipe 2 ,
+.Xr poll 2 ,
+.Xr select 2 ,
+.Xr socket 2 ,
+.Xr socketpair 2
+.Sh STANDARDS
+The
+.Fn read ,
+.Fn readv ,
+and
+.Fn pread
+functions conform to
+.St -p1003.1-2008 .
+.Sh HISTORY
+A
+.Fn read
+system call first appeared in
+.At v1 ;
+.Fn readv
+in
+.Bx 4.1c ;
+.Fn pread
+in
+.At V.4 ;
+and
+.Fn preadv
+in
+.Ox 2.7 .
+.Sh CAVEATS
+Error checks should explicitly test for \-1.
+Code such as
+.Bd -literal -offset indent
+while ((nr = read(fd, buf, sizeof(buf))) > 0)
+.Ed
+.Pp
+is not maximally portable, as some platforms allow for
+.Fa nbytes
+to range between
+.Dv SSIZE_MAX
+and
+.Dv SIZE_MAX
+\- 2, in which case the return value of an error-free
+.Fn read
+may appear as a negative number distinct from \-1.
+Proper loops should use
+.Bd -literal -offset indent
+while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)
+.Ed