.\" $OpenBSD: write.2,v 1.9 1999/02/27 21:57:02 deraadt Exp $ .\" $NetBSD: write.2,v 1.6 1995/02/27 12:39:43 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. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. 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 July 28, 1998 .Dt WRITE 2 .Os BSD 4 .Sh NAME .Nm write , .Nm writev .Nd write output .Sh SYNOPSIS .Fd #include .Fd #include .Fd #include .Ft ssize_t .Fn write "int d" "const void *buf" "size_t nbytes" .Ft ssize_t .Fn writev "int d" "const struct iovec *iov" "int iovcnt" .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]. .Pp For .Fn writev , 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 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 a 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. .Pp Note that .Fn writev will fail if the value of .Fa iovcnt exceedes the constant .Dv IOV_MAX . .Sh RETURN VALUES Upon successful completion the number of bytes which were written is returned. Otherwise a -1 is returned and the global variable .Va errno is set to indicate the error. .Sh ERRORS .Fn write and .Fn writev will fail and the file pointer will remain unchanged if: .Bl -tag -width Er .It Bq Er EBADF .Fa d is not a valid descriptor open for writing. .It Bq Er EPIPE An attempt is made to write to a pipe that is not open for reading by any process. .It Bq Er EPIPE An attempt is made to write to a socket of type .DV SOCK_STREAM that is not connected to a peer socket. .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 EFAULT Part of .Fa iov or data to be written to the file points outside the process's allocated address space. .It Bq Er EINVAL The pointer associated with .Fa d was negative. .It Bq Er ENOSPC There is no free space remaining on the file system containing the file. .It Bq Er EDQUOT The user's quota of disk blocks on the file system containing the file has been exhausted. .It Bq Er EIO An I/O error occurred while reading from or writing to the file system. .It Bq Er EAGAIN The file was marked for non-blocking I/O, and no data could be written immediately. .El .Pp In addition, .Fn write may return following error: .Bl -tag -width Er .It Bq Er EINVAL .Fa nbytes was larger than .Ev SSIZE_MAX . .El .Pp Also, .Fn writev 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 .Dv IOV_MAX . .It Bq Er EINVAL The sum of the .Fa iov_len values in the .Fa iov array overflowed an .Em ssize_t . .El .Sh SEE ALSO .Xr fcntl 2 , .Xr lseek 2 , .Xr open 2 , .Xr pipe 2 , .Xr poll 2 , .Xr select 2 .Sh STANDARDS The .Fn write function is expected to conform to .St -p1003.1-88 . .Sh HISTORY The .Fn writev function call appeared in .Bx 4.2 . The .Fn write function call appeared in .At v6 .