1.\" $NetBSD: write.2,v 1.20 2002/08/18 05:55:24 yamt 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 August 18, 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.Fd #include \*[Lt]unistd.h\*[Gt] 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.Fd #include \*[Lt]sys/uio.h\*[Gt] 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. The value of the pointer associated with such an object 117is undefined. 118.Pp 119If the real user is not the super-user, then 120.Fn write 121clears the set-user-id bit on a file. 122This prevents penetration of system security 123by a user who 124.Dq captures 125a writable set-user-id file 126owned by the super-user. 127.Pp 128If 129.Fn write 130succeeds it will update the st_ctime and st_mtime fields of the file's 131meta-data (see 132.Xr stat 2 ) . 133.Pp 134When using non-blocking I/O on objects such as sockets that are subject 135to flow control, 136.Fn write 137and 138.Fn writev 139may write fewer bytes than requested; 140the return value must be noted, 141and the remainder of the operation should be retried when possible. 142.Sh RETURN VALUES 143Upon successful completion the number of bytes which were written 144is returned. Otherwise a -1 is returned and the global variable 145.Va errno 146is set to indicate the error. 147.Sh ERRORS 148.Fn write , 149.Fn writev , 150.Fn pwrite , 151and 152.Fn pwritev 153will fail and the file pointer will remain unchanged if: 154.Bl -tag -width Er 155.It Bq Er EBADF 156.Fa d 157is not a valid descriptor open for writing. 158.It Bq Er EPIPE 159An attempt is made to write to a pipe that is not open 160for reading by any process. 161.It Bq Er EPIPE 162An attempt is made to write to a socket of type 163.Dv SOCK_STREAM 164that is not connected to a peer socket. 165.It Bq Er EFBIG 166An attempt was made to write a file that exceeds the process's 167file size limit or the maximum file size. 168.It Bq Er EFAULT 169Part of 170.Fa iov 171or data to be written to the file 172points outside the process's allocated address space. 173.It Bq Er EINVAL 174The pointer associated with 175.Fa d 176was negative. 177.It Bq Er EINVAL 178The total length of the I/O is more than can be expressed by the ssize_t 179return value. 180.It Bq Er ENOSPC 181There is no free space remaining on the file system 182containing the file. 183.It Bq Er EDQUOT 184The user's quota of disk blocks on the file system 185containing the file has been exhausted. 186.It Bq Er EIO 187An I/O error occurred while reading from or writing to the file system. 188.It Bq Er EAGAIN 189The file was marked for non-blocking I/O, 190and no data could be written immediately. 191.El 192.Pp 193In addition, 194.Fn writev 195and 196.Fn pwritev 197may return one of the following errors: 198.Bl -tag -width Er 199.It Bq Er EINVAL 200.Fa iovcnt 201was less than or equal to 0, or greater than 202.Dv {IOV_MAX} . 203.It Bq Er EINVAL 204One of the 205.Fa iov_len 206values in the 207.Fa iov 208array was negative. 209.It Bq Er EINVAL 210The sum of the 211.Fa iov_len 212values in the 213.Fa iov 214array overflowed a 32-bit integer. 215.El 216.Pp 217.The 218.Fn pwrite 219and 220.Fn pwritev 221calls may also return the following errors: 222.Bl -tag -width Er 223.It Bq Er EINVAL 224The specified file offset is invalid. 225.It Bq Er ESPIPE 226The file descriptor is associated with a pipe, socket, or FIFO. 227.El 228.Sh SEE ALSO 229.Xr fcntl 2 , 230.Xr lseek 2 , 231.Xr open 2 , 232.Xr pipe 2 , 233.Xr poll 2 , 234.Xr select 2 235.Sh STANDARDS 236The 237.Fn write 238function is expected to conform to 239.St -p1003.1-88 . 240The 241.Fn writev 242and 243.Fn pwrite 244functions conform to 245.St -xpg4.2 . 246.Sh HISTORY 247The 248.Fn pwritev 249function call 250appeared in 251.Nx 1.4 . 252The 253.Fn pwrite 254function call 255appeared in 256.At V.4 . 257The 258.Fn writev 259function call 260appeared in 261.Bx 4.2 . 262The 263.Fn write 264function call appeared in 265.At v6 . 266.Sh CAVEATS 267Error checks should explicitly test for \-1. 268Code such as 269.Bd -literal 270 while ((nr = write(fd, buf, sizeof(buf))) > 0) 271.Ed 272.Pp 273is not maximally portable, as some platforms allow for 274.Va nbytes 275to range between 276.Dv SSIZE_MAX 277and 278.Dv SIZE_MAX 279\- 2, in which case the return value of an error-free 280.Fn write 281may appear as a negative number distinct from \-1. 282Proper loops should use 283.Bd -literal 284 while ((nr = write(fd, buf, sizeof(buf))) != -1 && nr != 0) 285.Ed 286