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+.\" $NetBSD: write.2,v 1.36 2019/09/02 00:13:09 sevan 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.
+.\"
+.\" @(#)write.2 8.5 (Berkeley) 4/2/94
+.\"
+.Dd September 2, 2019
+.Dt WRITE 2
+.Os
+.Sh NAME
+.Nm write ,
+.Nm writev ,
+.Nm pwrite ,
+.Nm pwritev
+.Nd write output
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In unistd.h
+.Ft ssize_t
+.Fn write "int d" "const void *buf" "size_t nbytes"
+.Ft ssize_t
+.Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset"
+.In sys/uio.h
+.Ft ssize_t
+.Fn writev "int d" "const struct iovec *iov" "int iovcnt"
+.Ft ssize_t
+.Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset"
+.Sh DESCRIPTION
+.Fn write
+attempts to write
+.Fa nbytes
+of data to the object referenced by the descriptor
+.Fa d
+from the buffer pointed to by
+.Fa buf .
+.Fn writev
+performs the same action, but gathers the output data
+from the
+.Fa iovcnt
+buffers specified by the members of the
+.Fa iov
+array: iov[0], iov[1], ..., iov[iovcnt\|-\|1].
+.Fn pwrite
+and
+.Fn pwritev
+perform the same functions, but write to the specified position in
+the file without modifying the file pointer.
+.Pp
+For
+.Fn writev
+and
+.Fn pwritev ,
+the
+.Fa iovec
+structure is defined as:
+.Pp
+.Bd -literal -offset indent -compact
+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 from which data should be written.
+.Fn writev
+and
+.Fn pwritev
+will always write a complete area before proceeding
+to the next.
+.Pp
+On objects capable of seeking, the
+.Fn write
+starts at a position
+given by the pointer associated with
+.Fa d
+(see
+.Xr lseek 2 ) .
+Upon return from
+.Fn write ,
+the pointer is incremented by the number of bytes which were written.
+.Pp
+Objects that are not capable of seeking always write from the current
+position.
+The value of the pointer associated with such an object
+is undefined.
+.Pp
+If the real user is not the super-user, then
+.Fn write
+clears the set-user-id bit on the file.
+This prevents penetration of system security
+by a user who
+.Dq captures
+a writable set-user-id file
+owned by the super-user.
+.Pp
+If
+.Fn write
+succeeds it will update the st_ctime and st_mtime fields of the file's
+meta-data (see
+.Xr stat 2 ) .
+.Pp
+When using non-blocking I/O on objects such as sockets that are subject
+to flow control,
+.Fn write
+and
+.Fn writev
+may write fewer bytes than requested;
+the return value must be noted,
+and the remainder of the operation should be retried when possible.
+.Sh RETURN VALUES
+Upon successful completion the number of bytes which were written
+is returned.
+Otherwise \-1 is returned and the global variable
+.Va errno
+is set to indicate the error.
+.Sh ERRORS
+.Fn write ,
+.Fn writev ,
+.Fn pwrite ,
+and
+.Fn pwritev
+will fail and the file pointer will remain unchanged if:
+.Bl -tag -width Er
+.It Bq Er EAGAIN
+The file was marked for non-blocking I/O,
+and no data could be written immediately.
+.It Bq Er EBADF
+.Fa d
+is not a valid descriptor open for writing.
+.It Bq Er EDQUOT
+The user's quota of disk blocks on the file system
+containing the file has been exhausted.
+.It Bq Er EFAULT
+Part of
+.Fa iov
+or data to be written to the file
+points outside the process's allocated address space.
+.It Bq Er EFBIG
+An attempt was made to write a file that exceeds the process's
+file size limit or the maximum file size.
+.It Bq Er EINTR
+A signal was received before any data could be written to a slow
+device.
+See
+.Xr sigaction 2
+for more information on the interaction between signals and system
+calls.
+.It Bq Er EINVAL
+The pointer associated with
+.Fa d
+was negative; or
+the total length of the I/O is more than can be expressed by the ssize_t
+return value.
+.It Bq Er EIO
+An I/O error occurred while reading from or writing to the file system.
+.It Bq Er ENOSPC
+There is no free space remaining on the file system
+containing the file.
+.It Bq Er EPIPE
+An attempt is made to write to a pipe that is not open
+for reading by any process; or
+an attempt is made to write to a socket of type
+.Dv SOCK_STREAM
+that is not connected to a peer socket.
+.El
+.Pp
+In addition,
+.Fn writev
+and
+.Fn pwritev
+may return one of the following errors:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+.Fa iovcnt
+was less than or equal to 0, or greater than
+.Brq Dv IOV_MAX ;
+or one of the
+.Fa iov_len
+values in the
+.Fa iov
+array was negative; or
+the sum of the
+.Fa iov_len
+values in the
+.Fa iov
+array overflowed a 32-bit integer.
+.El
+.Pp
+The
+.Fn pwrite
+and
+.Fn pwritev
+calls may also return the following errors:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+The specified file offset is invalid.
+.It Bq Er ESPIPE
+The file descriptor is associated with a pipe, socket, or FIFO.
+.El
+.Sh SEE ALSO
+.Xr fcntl 2 ,
+.Xr lseek 2 ,
+.Xr open 2 ,
+.Xr pipe 2 ,
+.Xr poll 2 ,
+.Xr select 2 ,
+.Xr sigaction 2
+.Sh STANDARDS
+The
+.Fn write
+function is expected to conform to
+.St -p1003.1-88 .
+The
+.Fn writev
+and
+.Fn pwrite
+functions conform to
+.St -xpg4.2 .
+.Sh HISTORY
+The
+.Fn pwritev
+function call
+appeared in
+.Nx 1.4 .
+The
+.Fn pwrite
+function call
+appeared in
+.At V.4 .
+The
+.Fn writev
+function call
+appeared in
+.Bx 4.2 .
+The
+.Fn write
+function call appeared in
+.At v1 .
+.Sh CAVEATS
+Error checks should explicitly test for \-1.
+Code such as
+.Bd -literal
+ while ((nr = write(fd, buf, sizeof(buf))) > 0)
+.Ed
+.Pp
+is not maximally portable, as some platforms allow for
+.Va nbytes
+to range between
+.Dv SSIZE_MAX
+and
+.Dv SIZE_MAX
+\- 2, in which case the return value of an error-free
+.Fn write
+may appear as a negative number distinct from \-1.
+Proper loops should use
+.Bd -literal
+ while ((nr = write(fd, buf, sizeof(buf))) != -1 && nr != 0)
+.Ed