1.\" $OpenBSD: write.2,v 1.28 2004/04/15 19:52:18 jmc Exp $ 2.\" $NetBSD: write.2,v 1.6 1995/02/27 12:39:43 cgd Exp $ 3.\" 4.\" Copyright (c) 1980, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)write.2 8.5 (Berkeley) 4/2/94 32.\" 33.Dd July 28, 1998 34.Dt WRITE 2 35.Os 36.Sh NAME 37.Nm write , 38.Nm writev , 39.Nm pwrite , 40.Nm pwritev 41.Nd write output 42.Sh SYNOPSIS 43.Fd #include <sys/types.h> 44.Fd #include <unistd.h> 45.Ft ssize_t 46.Fn write "int d" "const void *buf" "size_t nbytes" 47.Ft ssize_t 48.Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset" 49.Pp 50.Fd #include <sys/types.h> 51.Fd #include <sys/uio.h> 52.Fd #include <unistd.h> 53.Ft ssize_t 54.Fn writev "int d" "const struct iovec *iov" "int iovcnt" 55.Ft ssize_t 56.Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset" 57.Sh DESCRIPTION 58.Fn write 59attempts to write 60.Fa nbytes 61of data to the object referenced by the descriptor 62.Fa d 63from the buffer pointed to by 64.Fa buf . 65.Fn writev 66performs the same action, but gathers the output data from the 67.Fa iovcnt 68buffers specified by the members of the 69.Fa iov 70array: iov[0], iov[1], ..., iov[iovcnt\|-\|1]. 71.Fn pwrite 72and 73.Fn pwritev 74perform the same functions, but write to the specified position in 75the file without modifying the file pointer. 76.Pp 77For 78.Fn writev 79and 80.Fn pwritev , 81the 82.Fa iovec 83structure is defined as: 84.Bd -literal -offset indent 85struct iovec { 86 void *iov_base; 87 size_t iov_len; 88}; 89.Ed 90.Pp 91Each 92.Fa iovec 93entry specifies the base address and length of an area 94in memory from which data should be written. 95.Fn writev 96and 97.Fn pwritev 98will always write a complete area before proceeding to the next. 99.Pp 100On objects capable of seeking, the 101.Fn write 102starts at a position given by the pointer associated with 103.Fa d 104(see 105.Xr lseek 2 ) . 106Upon return from 107.Fn write , 108the pointer is incremented by the number of bytes which were written. 109.Pp 110Objects that are not capable of seeking always write from the current 111position. 112The value of the pointer associated with such an object is undefined. 113.Pp 114If the real user is not the superuser, then 115.Fn write 116clears the set-user-ID bit on a file. 117This prevents penetration of system security by a user who 118.Dq captures 119a writable set-user-ID file owned by the superuser. 120.Pp 121If 122.Fn write 123succeeds it will update the st_ctime and st_mtime fields of the file's 124meta-data (see 125.Xr stat 2 ) . 126.Pp 127When using non-blocking I/O on objects such as sockets that are subject 128to flow control, 129.Fn write 130and 131.Fn writev 132may write fewer bytes than requested; the return value must be noted, 133and the remainder of the operation should be retried when possible. 134.Pp 135Note that 136.Fn writev 137and 138.Fn pwritev 139will fail if the value of 140.Fa iovcnt 141exceeds the constant 142.Dv IOV_MAX . 143.Sh RETURN VALUES 144Upon successful completion the number of bytes which were written 145is returned. 146Otherwise, a \-1 is returned and the global variable 147.Va errno 148is set to indicate the error. 149.Sh ERRORS 150.Fn write , 151.Fn pwrite , 152.Fn writev , 153and 154.Fn pwritev 155will fail and the file pointer will remain unchanged if: 156.Bl -tag -width Er 157.It Bq Er EBADF 158.Fa d 159is not a valid descriptor open for writing. 160.It Bq Er EPIPE 161An attempt is made to write to a pipe that is not open 162for reading by any process. 163.It Bq Er EPIPE 164An attempt is made to write to a socket of type 165.Dv SOCK_STREAM 166that is not connected to a peer socket. 167.It Bq Er EFBIG 168An attempt was made to write a file that exceeds the process's 169file size limit or the maximum file size. 170.It Bq Er EINVAL 171The pointer associated with 172.Fa d 173was negative. 174.It Bq Er ENOSPC 175There is no free space remaining on the file system containing the file. 176.It Bq Er EDQUOT 177The user's quota of disk blocks on the file system containing the file 178has been exhausted. 179.It Bq Er EIO 180An I/O error occurred while reading from or writing to the file system. 181.It Bq Er EAGAIN 182The file was marked for non-blocking I/O, and no data could be 183written immediately. 184.El 185.Pp 186In addition, 187.Fn write 188and 189.Fn pwrite 190may return the following error: 191.Bl -tag -width Er 192.It Bq Er EFAULT 193Part of 194.Fa iov 195or data to be written to the file points outside the process's 196allocated address space. 197.It Bq Er EINVAL 198.Fa nbytes 199was larger than 200.Ev SSIZE_MAX . 201.El 202.Pp 203Also, 204.Fn writev 205and 206.Fn pwritev 207may return one of the following errors: 208.Bl -tag -width Er 209.It Bq Er EDESTADDRREQ 210The destination is no longer available when writing to a 211.Ux 212domain datagram socket on which 213.Xr connect 2 214had been used to set a destination address. 215.It Bq Er EINVAL 216.Fa iovcnt 217was less than or equal to 0, or greater than 218.Dv IOV_MAX . 219.It Bq Er EINVAL 220The sum of the 221.Fa iov_len 222values in the 223.Fa iov 224array overflowed an 225.Em ssize_t . 226.El 227.Sh SEE ALSO 228.Xr fcntl 2 , 229.Xr lseek 2 , 230.Xr open 2 , 231.Xr pipe 2 , 232.Xr poll 2 , 233.Xr select 2 234.Sh STANDARDS 235The 236.Fn write 237function conforms to 238.St -p1003.1-90 . 239The 240.Fn writev 241and 242.Fn pwrite 243functions conform to 244.St -xpg4.2 . 245.Sh HISTORY 246The 247.Fn pwritev 248function call appeared in 249.Ox 2.7 . 250The 251.Fn pwrite 252function call appeared in 253.At V.4 . 254The 255.Fn writev 256function call appeared in 257.Bx 4.2 . 258The 259.Fn write 260function call appeared in 261.At v2 . 262.Sh CAVEATS 263Error checks should explicitly test for \-1. 264Code such as 265.Bd -literal -offset indent 266while ((nr = write(fd, buf, sizeof(buf))) > 0) 267.Ed 268.Pp 269is not maximally portable, as some platforms allow for 270.Va nbytes 271to range between 272.Dv SSIZE_MAX 273and 274.Dv SIZE_MAX 275\- 2, in which case the return value of an error-free 276.Fn write 277may appear as a negative number distinct from \-1. 278Proper loops should use 279.Bd -literal -offset indent 280while ((nr = write(fd, buf, sizeof(buf))) != -1 && nr != 0) 281.Ed 282