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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)write.2 8.5 (Berkeley) 4/2/94 29.\" $FreeBSD: src/lib/libc/sys/write.2,v 1.12.2.8 2002/10/13 17:42:14 schweikh Exp $ 30.\" $DragonFly: src/lib/libc/sys/write.2,v 1.5 2006/04/27 16:01:33 swildner Exp $ 31.\" 32.Dd April 27, 2006 33.Dt WRITE 2 34.Os 35.Sh NAME 36.Nm write , 37.Nm writev , 38.Nm pwrite , 39.Nm pwritev 40.Nd write output 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In sys/types.h 45.In sys/uio.h 46.In unistd.h 47.Ft ssize_t 48.Fn write "int d" "const void *buf" "size_t nbytes" 49.Ft ssize_t 50.Fn writev "int d" "const struct iovec *iov" "int iovcnt" 51.Ft ssize_t 52.Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset" 53.Ft ssize_t 54.Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset" 55.Sh DESCRIPTION 56.Fn Write 57attempts to write 58.Fa nbytes 59of data to the object referenced by the descriptor 60.Fa d 61from the buffer pointed to by 62.Fa buf . 63.Fn Writev 64and 65.Fn pwritev 66perform the same action, but gather the output data 67from the 68.Fa iovcnt 69buffers specified by the members of the 70.Fa iov 71array: iov[0], iov[1], ..., iov[iovcnt\|-\|1]. 72.Fn Pwrite 73and 74.Fn pwritev 75perform the same function, but write to the specified position in 76the file without modifying the file pointer. 77.Pp 78For 79.Fn writev 80and 81.Fn pwritev , 82the 83.Fa iovec 84structure is defined as: 85.Pp 86.Bd -literal -offset indent -compact 87struct iovec { 88 char *iov_base; /* Base address. */ 89 size_t iov_len; /* Length. */ 90}; 91.Ed 92.Pp 93Each 94.Fa iovec 95entry specifies the base address and length of an area 96in memory from which data should be written. 97.Fn Writev 98and 99.Fn pwritev 100will always write a complete area before proceeding 101to the next. 102.Pp 103On objects capable of seeking, the 104.Fn write 105starts at a position 106given by the pointer associated with 107.Fa d , 108see 109.Xr lseek 2 . 110Upon return from 111.Fn write , 112the pointer is incremented by the number of bytes which were written. 113.Pp 114Objects that are not capable of seeking always write from the current 115position. The value of the pointer associated with such an object 116is undefined. 117.Pp 118If the real user is not the super-user, then 119.Fn write 120clears the set-user-id bit on a file. 121This prevents penetration of system security 122by a user who 123.Dq captures 124a writable set-user-id file 125owned by the super-user. 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; 133the return value must be noted, 134and the remainder of the operation should be retried when possible. 135.Sh RETURN VALUES 136Upon successful completion the number of bytes which were written 137is returned. Otherwise a -1 is returned and the global variable 138.Va errno 139is set to indicate the error. 140.Sh ERRORS 141.Fn Write , 142.Fn writev , 143.Fn pwrite , 144and 145.Fn pwritev 146will fail and the file pointer will remain unchanged if: 147.Bl -tag -width Er 148.It Bq Er EBADF 149.Fa D 150is not a valid descriptor open for writing. 151.It Bq Er EPIPE 152An attempt is made to write to a pipe that is not open 153for reading by any process. 154.It Bq Er EPIPE 155An attempt is made to write to a socket of type 156.Dv SOCK_STREAM 157that is not connected to a peer socket. 158.It Bq Er EFBIG 159An attempt was made to write a file that exceeds the process's 160file size limit or the maximum file size. 161.It Bq Er EFAULT 162Part of 163.Fa iov 164or data to be written to the file 165points outside the process's allocated address space. 166.It Bq Er EINVAL 167The pointer associated with 168.Fa d 169was negative. 170.It Bq Er ENOSPC 171There is no free space remaining on the file system 172containing the file. 173.It Bq Er EDQUOT 174The user's quota of disk blocks on the file system 175containing the file has been exhausted. 176.It Bq Er EIO 177An I/O error occurred while reading from or writing to the file system. 178.It Bq Er EINTR 179A signal interrupted the write before it could be completed. 180.It Bq Er EAGAIN 181The file was marked for non-blocking I/O, 182and no data could be written immediately. 183.It Bq Er EROFS 184An attempt was made to write over a disk label area at the beginning 185of a disk. 186Use 187.Xr disklabel 8 188.Fl W 189to enable writing on the disk label area. 190.El 191.Pp 192In addition, 193.Fn writev 194and 195.Fn pwritev 196may return one of the following errors: 197.Bl -tag -width Er 198.It Bq Er EDESTADDRREQ 199The destination is no longer available when writing to a 200.Ux 201domain datagram socket on which 202.Xr connect 2 203had been used to set a destination address. 204.It Bq Er EINVAL 205.Fa Iovcnt 206was less than or equal to 0, or greater than 207.Dv UIO_MAXIOV . 208.It Bq Er EINVAL 209One of the 210.Fa iov_len 211values in the 212.Fa iov 213array was negative. 214.It Bq Er EINVAL 215The sum of the 216.Fa iov_len 217values in the 218.Fa iov 219array overflowed a 32-bit integer. 220.It Bq Er ENOBUFS 221The mbuf pool has been completely exhausted when writing to a socket. 222.El 223.Pp 224The 225.Fn pwrite 226and 227.Fn pwritev 228calls may also return the following errors: 229.Bl -tag -width Er 230.It Bq Er EINVAL 231The specified file offset is invalid. 232.It Bq Er ESPIPE 233The file descriptor is associated with a pipe, socket, or FIFO. 234.El 235.Sh SEE ALSO 236.Xr fcntl 2 , 237.Xr lseek 2 , 238.Xr open 2 , 239.Xr pipe 2 , 240.Xr select 2 241.Sh STANDARDS 242The 243.Fn write 244function call is expected to conform to 245.St -p1003.1-90 . 246The 247.Fn writev 248and 249.Fn pwrite 250functions are expected to conform to 251.St -xpg4.2 . 252.Sh HISTORY 253The 254.Fn pwritev 255function call was added in 256.Dx 1.5 . 257The 258.Fn pwrite 259function call 260appeared in 261.At V.4 . 262The 263.Fn writev 264function call 265appeared in 266.Bx 4.2 . 267A 268.Fn write 269function call appeared in 270.At v6 . 271