| 1 | .\" Copyright (c) 1980, 1991, 1993 |
| 2 | .\" The Regents of the University of California. All rights reserved. |
| 3 | .\" |
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| 5 | .\" modification, are permitted provided that the following conditions |
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| 8 | .\" notice, this list of conditions and the following disclaimer. |
| 9 | .\" 2. Redistributions in binary form must reproduce the above copyright |
| 10 | .\" notice, this list of conditions and the following disclaimer in the |
| 11 | .\" documentation and/or other materials provided with the distribution. |
| 12 | .\" 3. All advertising materials mentioning features or use of this software |
| 13 | .\" must display the following acknowledgement: |
| 14 | .\" This product includes software developed by the University of |
| 15 | .\" California, Berkeley and its contributors. |
| 16 | .\" 4. Neither the name of the University nor the names of its contributors |
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| 20 | .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND |
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| 22 | .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE |
| 23 | .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE |
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| 25 | .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS |
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| 30 | .\" SUCH DAMAGE. |
| 31 | .\" |
| 32 | .\" @(#)write.2 8.5 (Berkeley) 4/2/94 |
| 33 | .\" $FreeBSD: src/lib/libc/sys/write.2,v 1.12.2.7 2001/12/14 18:34:02 ru Exp $ |
| 34 | .\" |
| 35 | .Dd March 18, 2015 |
| 36 | .Dt WRITE 2 |
| 37 | .Os |
| 38 | .Sh NAME |
| 39 | .Nm pwrite , |
| 40 | .Nm write , |
| 41 | .Nm writev |
| 42 | .Nd write output |
| 43 | .Sh LIBRARY |
| 44 | .Lb libc |
| 45 | .Sh SYNOPSIS |
| 46 | .In unistd.h |
| 47 | .Ft ssize_t |
| 48 | .Fo pwrite |
| 49 | .Fa "int fildes" |
| 50 | .Fa "const void *buf" |
| 51 | .Fa "size_t nbyte" |
| 52 | .Fa "off_t offset" |
| 53 | .Fc |
| 54 | .Ft ssize_t |
| 55 | .Fo write |
| 56 | .Fa "int fildes" |
| 57 | .Fa "const void *buf" |
| 58 | .Fa "size_t nbyte" |
| 59 | .Fc |
| 60 | .In sys/uio.h |
| 61 | .Ft ssize_t |
| 62 | .Fo writev |
| 63 | .Fa "int fildes" |
| 64 | .Fa "const struct iovec *iov" |
| 65 | .Fa "int iovcnt" |
| 66 | .Fc |
| 67 | .Sh DESCRIPTION |
| 68 | .Fn write |
| 69 | attempts to write |
| 70 | .Fa nbyte |
| 71 | of data to the object referenced by the descriptor |
| 72 | .Fa fildes |
| 73 | from the buffer pointed to by |
| 74 | .Fa buf . |
| 75 | .Fn writev |
| 76 | performs the same action, but gathers the output data |
| 77 | from the |
| 78 | .Fa iovcnt |
| 79 | buffers specified by the members of the |
| 80 | .Fa iov |
| 81 | array: iov[0], iov[1], ..., iov[iovcnt\|-\|1]. |
| 82 | .Fn pwrite |
| 83 | performs the same function, but writes to the specified position in |
| 84 | the file without modifying the file pointer. |
| 85 | .Pp |
| 86 | For |
| 87 | .Fn writev , |
| 88 | the |
| 89 | .Fa iovec |
| 90 | structure is defined as: |
| 91 | .Pp |
| 92 | .Bd -literal -offset indent -compact |
| 93 | struct iovec { |
| 94 | char *iov_base; /* Base address. */ |
| 95 | size_t iov_len; /* Length. */ |
| 96 | }; |
| 97 | .Ed |
| 98 | .Pp |
| 99 | Each |
| 100 | .Fa iovec |
| 101 | entry specifies the base address and length of an area |
| 102 | in memory from which data should be written. |
| 103 | .Fn writev |
| 104 | will always write a complete area before proceeding |
| 105 | to the next. |
| 106 | .Pp |
| 107 | On objects capable of seeking, the |
| 108 | .Fn write |
| 109 | starts at a position |
| 110 | given by the pointer associated with |
| 111 | .Fa fildes , |
| 112 | see |
| 113 | .Xr lseek 2 . |
| 114 | Upon return from |
| 115 | .Fn write , |
| 116 | the pointer is incremented by the number of bytes which were written. |
| 117 | .Pp |
| 118 | Objects that are not capable of seeking always write from the current |
| 119 | position. The value of the pointer associated with such an object |
| 120 | is undefined. |
| 121 | .Pp |
| 122 | If the real user is not the super-user, then |
| 123 | .Fn write |
| 124 | clears the set-user-id bit on a file. |
| 125 | This prevents penetration of system security |
| 126 | by a user who |
| 127 | .Dq captures |
| 128 | a writable set-user-id file |
| 129 | owned by the super-user. |
| 130 | .Pp |
| 131 | When using non-blocking I/O on objects, such as sockets, |
| 132 | that are subject to flow control, |
| 133 | .Fn write |
| 134 | and |
| 135 | .Fn writev |
| 136 | may write fewer bytes than requested; |
| 137 | the return value must be noted, |
| 138 | and the remainder of the operation should be retried when possible. |
| 139 | .Sh RETURN VALUES |
| 140 | Upon successful completion the number of bytes |
| 141 | which were written is returned. |
| 142 | Otherwise, a -1 is returned and the global variable |
| 143 | .Va errno |
| 144 | is set to indicate the error. |
| 145 | .Sh ERRORS |
| 146 | The |
| 147 | .Fn write , |
| 148 | .Fn writev , |
| 149 | and |
| 150 | .Fn pwrite |
| 151 | system calls will fail and the file pointer will remain unchanged if: |
| 152 | .Bl -tag -width Er |
| 153 | .\" =========== |
| 154 | .It Bq Er EDQUOT |
| 155 | The user's quota of disk blocks on the file system |
| 156 | containing the file is exhausted. |
| 157 | .\" =========== |
| 158 | .It Bq Er EFAULT |
| 159 | Part of |
| 160 | .Fa iov |
| 161 | or data to be written to the file |
| 162 | points outside the process's allocated address space. |
| 163 | .\" =========== |
| 164 | .It Bq Er EINVAL |
| 165 | The pointer associated with |
| 166 | .Fa fildes |
| 167 | is negative. |
| 168 | .El |
| 169 | .Pp |
| 170 | The |
| 171 | .Fn write |
| 172 | and |
| 173 | .Fn pwrite |
| 174 | system calls will fail and the file pointer will remain unchanged if: |
| 175 | .Bl -tag -width Er |
| 176 | .\" =========== |
| 177 | .It Bq Er EAGAIN |
| 178 | The file is marked for non-blocking I/O, |
| 179 | and no data could be written immediately. |
| 180 | .\" =========== |
| 181 | .It Bq Er EBADF |
| 182 | .Fa fildes |
| 183 | is not a valid file descriptor open for writing. |
| 184 | .\" =========== |
| 185 | .It Bq Er ECONNRESET |
| 186 | A write is attempted on a socket that is not connected. |
| 187 | .\" =========== |
| 188 | .It Bq Er EFBIG |
| 189 | An attempt is made to write a file that exceeds the process's |
| 190 | file size limit or the maximum file size. |
| 191 | .\" =========== |
| 192 | .It Bq Er EFBIG |
| 193 | The file is a regular file, |
| 194 | .Fa nbyte |
| 195 | is greater than 0, |
| 196 | and the starting position is greater than or equal |
| 197 | to the offset maximum established in the open file description |
| 198 | associated with |
| 199 | .Fa fildes . |
| 200 | .\" =========== |
| 201 | .It Bq Er EINTR |
| 202 | A signal interrupts the write before it could be completed. |
| 203 | .\" =========== |
| 204 | .It Bq Er EIO |
| 205 | An I/O error occurs while reading from or writing to the file system. |
| 206 | .\" =========== |
| 207 | .It Bq Er ENETDOWN |
| 208 | A write is attempted on a socket |
| 209 | and the local network interface used to reach the destination is down. |
| 210 | .\" =========== |
| 211 | .It Bq Er ENETUNREACH |
| 212 | A write is attempted on a socket and no route to the network is present. |
| 213 | .\" =========== |
| 214 | .It Bq Er ENOSPC |
| 215 | There is no free space remaining on the file system containing the file. |
| 216 | .\" =========== |
| 217 | .It Bq Er ENXIO |
| 218 | A request is made of a nonexistent device, |
| 219 | or the request is outside the capabilities of the device. |
| 220 | .\" =========== |
| 221 | .It Bq Er EPIPE |
| 222 | An attempt is made to write to a pipe that is not open |
| 223 | for reading by any process. |
| 224 | .\" =========== |
| 225 | .It Bq Er EPIPE |
| 226 | An attempt is made to write to a socket of type |
| 227 | .Dv SOCK_STREAM |
| 228 | that is not connected to a peer socket. |
| 229 | .El |
| 230 | .Pp |
| 231 | The |
| 232 | .Fn write |
| 233 | and |
| 234 | .Fn writev |
| 235 | calls may also return the following errors: |
| 236 | .Bl -tag -width Er |
| 237 | .\" =========== |
| 238 | .It Bq Er EAGAIN |
| 239 | See EWOULDBLOCK, below. |
| 240 | .\" =========== |
| 241 | .It Bq Er EWOULDBLOCK |
| 242 | The file descriptor is for a socket, is marked O_NONBLOCK, |
| 243 | and write would block. |
| 244 | The exact error code depends on the protocol, |
| 245 | but EWOULDBLOCK is more common. |
| 246 | .El |
| 247 | .Pp |
| 248 | In addition, |
| 249 | .Fn writev |
| 250 | may return one of the following errors: |
| 251 | .Bl -tag -width Er |
| 252 | .\" =========== |
| 253 | .It Bq Er EDESTADDRREQ |
| 254 | The destination is no longer available when writing to a |
| 255 | .Ux |
| 256 | domain datagram socket on which |
| 257 | .Xr connect 2 |
| 258 | or |
| 259 | .Xr connectx 2 |
| 260 | had been used to set a destination address. |
| 261 | .\" =========== |
| 262 | .It Bq Er EINVAL |
| 263 | .Fa Iovcnt |
| 264 | is less than or equal to 0, or greater than |
| 265 | .Dv UIO_MAXIOV . |
| 266 | .\" =========== |
| 267 | .It Bq Er EINVAL |
| 268 | One of the |
| 269 | .Fa iov_len |
| 270 | values in the |
| 271 | .Fa iov |
| 272 | array is negative. |
| 273 | .\" =========== |
| 274 | .It Bq Er EINVAL |
| 275 | The sum of the |
| 276 | .Fa iov_len |
| 277 | values in the |
| 278 | .Fa iov |
| 279 | array overflows a 32-bit integer. |
| 280 | .\" =========== |
| 281 | .It Bq Er ENOBUFS |
| 282 | The mbuf pool has been completely exhausted when writing to a socket. |
| 283 | .El |
| 284 | .Pp |
| 285 | The |
| 286 | .Fn pwrite |
| 287 | call may also return the following errors: |
| 288 | .Bl -tag -width Er |
| 289 | .\" =========== |
| 290 | .It Bq Er EINVAL |
| 291 | The specified file offset is invalid. |
| 292 | .\" =========== |
| 293 | .It Bq Er ESPIPE |
| 294 | The file descriptor is associated with a pipe, socket, or FIFO. |
| 295 | .El |
| 296 | .Sh LEGACY SYNOPSIS |
| 297 | .Fd #include <sys/types.h> |
| 298 | .Fd #include <sys/uio.h> |
| 299 | .Fd #include <unistd.h> |
| 300 | .Pp |
| 301 | These include files are needed for all three functions. |
| 302 | .Sh SEE ALSO |
| 303 | .Xr fcntl 2 , |
| 304 | .Xr lseek 2 , |
| 305 | .Xr open 2 , |
| 306 | .Xr pipe 2 , |
| 307 | .Xr select 2 , |
| 308 | .Xr compat 5 |
| 309 | .Sh STANDARDS |
| 310 | The |
| 311 | .Fn write |
| 312 | function call is expected to conform to |
| 313 | .St -p1003.1-90 . |
| 314 | The |
| 315 | .Fn writev |
| 316 | and |
| 317 | .Fn pwrite |
| 318 | functions are expected to conform to |
| 319 | .St -xpg4.2 . |
| 320 | .Sh HISTORY |
| 321 | The |
| 322 | .Fn pwrite |
| 323 | function call |
| 324 | appeared in |
| 325 | .At V.4 . |
| 326 | The |
| 327 | .Fn writev |
| 328 | function call |
| 329 | appeared in |
| 330 | .Bx 4.2 . |
| 331 | A |
| 332 | .Fn write |
| 333 | function call appeared in |
| 334 | .At v6 . |