1.\" $NetBSD: write.2,v 1.26 2003/04/16 13:34:57 wiz Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)write.2 8.5 (Berkeley) 4/2/94 35.\" 36.Dd December 30, 2002 37.Dt WRITE 2 38.Os 39.Sh NAME 40.Nm write , 41.Nm writev , 42.Nm pwrite , 43.Nm pwritev 44.Nd write output 45.Sh LIBRARY 46.Lb libc 47.Sh SYNOPSIS 48.In unistd.h 49.Ft ssize_t 50.Fn write "int d" "const void *buf" "size_t nbytes" 51.Ft ssize_t 52.Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset" 53.In sys/uio.h 54.Ft ssize_t 55.Fn writev "int d" "const struct iovec *iov" "int iovcnt" 56.Ft ssize_t 57.Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset" 58.Sh DESCRIPTION 59.Fn write 60attempts to write 61.Fa nbytes 62of data to the object referenced by the descriptor 63.Fa d 64from the buffer pointed to by 65.Fa buf . 66.Fn writev 67performs the same action, but gathers the output data 68from the 69.Fa iovcnt 70buffers specified by the members of the 71.Fa iov 72array: iov[0], iov[1], ..., iov[iovcnt\|-\|1]. 73.Fn pwrite 74and 75.Fn pwritev 76perform the same functions, but write to the specified position in 77the file without modifying the file pointer. 78.Pp 79For 80.Fn writev 81and 82.Fn pwritev , 83the 84.Fa iovec 85structure is defined as: 86.Pp 87.Bd -literal -offset indent -compact 88struct iovec { 89 void *iov_base; 90 size_t iov_len; 91}; 92.Ed 93.Pp 94Each 95.Fa iovec 96entry specifies the base address and length of an area 97in memory from which data should be written. 98.Fn writev 99and 100.Fn pwritev 101will always write a complete area before proceeding 102to the next. 103.Pp 104On objects capable of seeking, the 105.Fn write 106starts at a position 107given by the pointer associated with 108.Fa d 109(see 110.Xr lseek 2 ) . 111Upon return from 112.Fn write , 113the pointer is incremented by the number of bytes which were written. 114.Pp 115Objects that are not capable of seeking always write from the current 116position. 117The value of the pointer associated with such an object 118is undefined. 119.Pp 120If the real user is not the super-user, then 121.Fn write 122clears the set-user-id bit on a file. 123This prevents penetration of system security 124by a user who 125.Dq captures 126a writable set-user-id file 127owned by the super-user. 128.Pp 129If 130.Fn write 131succeeds it will update the st_ctime and st_mtime fields of the file's 132meta-data (see 133.Xr stat 2 ) . 134.Pp 135When using non-blocking I/O on objects such as sockets that are subject 136to flow control, 137.Fn write 138and 139.Fn writev 140may write fewer bytes than requested; 141the return value must be noted, 142and the remainder of the operation should be retried when possible. 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 writev , 152.Fn pwrite , 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 EFAULT 171Part of 172.Fa iov 173or data to be written to the file 174points outside the process's allocated address space. 175.It Bq Er EINVAL 176The pointer associated with 177.Fa d 178was negative. 179.It Bq Er EINVAL 180The total length of the I/O is more than can be expressed by the ssize_t 181return value. 182.It Bq Er ENOSPC 183There is no free space remaining on the file system 184containing the file. 185.It Bq Er EDQUOT 186The user's quota of disk blocks on the file system 187containing the file has been exhausted. 188.It Bq Er EIO 189An I/O error occurred while reading from or writing to the file system. 190.It Bq Er EINTR 191A signal was received before any data could be written to a slow 192device. 193See 194.Xr sigaction 2 195for more information on the interaction between signals and system 196calls. 197.It Bq Er EAGAIN 198The file was marked for non-blocking I/O, 199and no data could be written immediately. 200.El 201.Pp 202In addition, 203.Fn writev 204and 205.Fn pwritev 206may return one of the following errors: 207.Bl -tag -width Er 208.It Bq Er EINVAL 209.Fa iovcnt 210was less than or equal to 0, or greater than 211.Dv {IOV_MAX} . 212.It Bq Er EINVAL 213One of the 214.Fa iov_len 215values in the 216.Fa iov 217array was negative. 218.It Bq Er EINVAL 219The sum of the 220.Fa iov_len 221values in the 222.Fa iov 223array overflowed a 32-bit integer. 224.El 225.Pp 226.The 227.Fn pwrite 228and 229.Fn pwritev 230calls may also return the following errors: 231.Bl -tag -width Er 232.It Bq Er EINVAL 233The specified file offset is invalid. 234.It Bq Er ESPIPE 235The file descriptor is associated with a pipe, socket, or FIFO. 236.El 237.Sh SEE ALSO 238.Xr fcntl 2 , 239.Xr lseek 2 , 240.Xr open 2 , 241.Xr pipe 2 , 242.Xr poll 2 , 243.Xr select 2 , 244.Xr sigaction 2 245.Sh STANDARDS 246The 247.Fn write 248function is expected to conform to 249.St -p1003.1-88 . 250The 251.Fn writev 252and 253.Fn pwrite 254functions conform to 255.St -xpg4.2 . 256.Sh HISTORY 257The 258.Fn pwritev 259function call 260appeared in 261.Nx 1.4 . 262The 263.Fn pwrite 264function call 265appeared in 266.At V.4 . 267The 268.Fn writev 269function call 270appeared in 271.Bx 4.2 . 272The 273.Fn write 274function call appeared in 275.At v6 . 276.Sh CAVEATS 277Error checks should explicitly test for \-1. 278Code such as 279.Bd -literal 280 while ((nr = write(fd, buf, sizeof(buf))) > 0) 281.Ed 282.Pp 283is not maximally portable, as some platforms allow for 284.Va nbytes 285to range between 286.Dv SSIZE_MAX 287and 288.Dv SIZE_MAX 289\- 2, in which case the return value of an error-free 290.Fn write 291may appear as a negative number distinct from \-1. 292Proper loops should use 293.Bd -literal 294 while ((nr = write(fd, buf, sizeof(buf))) != -1 && nr != 0) 295.Ed 296