| blob | 0e0a1b6418f75740ade708e000a5194c477c5cde |
1 .\" $NetBSD: write.2,v 1.32 2010/04/05 07:53:47 wiz Exp $
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30 .\" @(#)write.2 8.5 (Berkeley) 4/2/94
31 .\"
32 .Dd April 3, 2010
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 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 .In sys/uio.h
50 .Ft ssize_t
51 .Fn writev "int d" "const struct iovec *iov" "int iovcnt"
52 .Ft ssize_t
53 .Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset"
54 .Sh DESCRIPTION
55 .Fn write
56 attempts to write
57 .Fa nbytes
58 of data to the object referenced by the descriptor
59 .Fa d
60 from the buffer pointed to by
61 .Fa buf .
62 .Fn writev
63 performs the same action, but gathers the output data
64 from the
65 .Fa iovcnt
66 buffers specified by the members of the
67 .Fa iov
68 array: iov[0], iov[1], ..., iov[iovcnt\|-\|1].
69 .Fn pwrite
70 and
71 .Fn pwritev
72 perform the same functions, but write to the specified position in
73 the file without modifying the file pointer.
74 .Pp
75 For
76 .Fn writev
77 and
78 .Fn pwritev ,
79 the
80 .Fa iovec
81 structure is defined as:
82 .Pp
83 .Bd -literal -offset indent -compact
84 struct iovec {
85 void *iov_base;
86 size_t iov_len;
87 };
88 .Ed
89 .Pp
90 Each
91 .Fa iovec
92 entry specifies the base address and length of an area
93 in memory from which data should be written.
94 .Fn writev
95 and
96 .Fn pwritev
97 will always write a complete area before proceeding
98 to the next.
99 .Pp
100 On objects capable of seeking, the
101 .Fn write
102 starts at a position
103 given by the pointer associated with
104 .Fa d
105 (see
106 .Xr lseek 2 ) .
107 Upon return from
108 .Fn write ,
109 the pointer is incremented by the number of bytes which were written.
110 .Pp
111 Objects that are not capable of seeking always write from the current
112 position.
113 The value of the pointer associated with such an object
114 is undefined.
115 .Pp
116 If the real user is not the super-user, then
117 .Fn write
118 clears the set-user-id bit on a file.
119 This prevents penetration of system security
120 by a user who
121 .Dq captures
122 a writable set-user-id file
123 owned by the super-user.
124 .Pp
125 If
126 .Fn write
127 succeeds it will update the st_ctime and st_mtime fields of the file's
128 meta-data (see
129 .Xr stat 2 ) .
130 .Pp
131 When using non-blocking I/O on objects such as sockets that are subject
132 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 which were written
141 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 .Fn write ,
147 .Fn writev ,
148 .Fn pwrite ,
149 and
150 .Fn pwritev
151 will fail and the file pointer will remain unchanged if:
152 .Bl -tag -width Er
153 .It Bq Er EAGAIN
154 The file was marked for non-blocking I/O,
155 and no data could be written immediately.
156 .It Bq Er EBADF
157 .Fa d
158 is not a valid descriptor open for writing.
159 .It Bq Er EDQUOT
160 The user's quota of disk blocks on the file system
161 containing the file has been exhausted.
162 .It Bq Er EFAULT
163 Part of
164 .Fa iov
165 or data to be written to the file
166 points outside the process's allocated address space.
167 .It Bq Er EFBIG
168 An attempt was made to write a file that exceeds the process's
169 file size limit or the maximum file size.
170 .It Bq Er EINTR
171 A signal was received before any data could be written to a slow
172 device.
173 See
174 .Xr sigaction 2
175 for more information on the interaction between signals and system
176 calls.
177 .It Bq Er EINVAL
178 The pointer associated with
179 .Fa d
180 was negative; or
181 the total length of the I/O is more than can be expressed by the ssize_t
182 return value.
183 .It Bq Er EIO
184 An I/O error occurred while reading from or writing to the file system.
185 .It Bq Er ENOSPC
186 There is no free space remaining on the file system
187 containing the file.
188 .It Bq Er EPIPE
189 An attempt is made to write to a pipe that is not open
190 for reading by any process; or
191 an attempt is made to write to a socket of type
192 .Dv SOCK_STREAM
193 that is not connected to a peer socket.
194 .El
195 .Pp
196 In addition,
197 .Fn writev
198 and
199 .Fn pwritev
200 may return one of the following errors:
201 .Bl -tag -width Er
202 .It Bq Er EINVAL
203 .Fa iovcnt
204 was less than or equal to 0, or greater than
205 .Dv {IOV_MAX} ;
206 or one of the
207 .Fa iov_len
208 values in the
209 .Fa iov
210 array was negative; or
211 the sum of the
212 .Fa iov_len
213 values in the
214 .Fa iov
215 array overflowed a 32-bit integer.
216 .El
217 .Pp
218 The
219 .Fn pwrite
220 and
221 .Fn pwritev
222 calls may also return the following errors:
223 .Bl -tag -width Er
224 .It Bq Er EINVAL
225 The specified file offset is invalid.
226 .It Bq Er ESPIPE
227 The file descriptor is associated with a pipe, socket, or FIFO.
228 .El
229 .Sh SEE ALSO
230 .Xr fcntl 2 ,
231 .Xr lseek 2 ,
232 .Xr open 2 ,
233 .Xr pipe 2 ,
234 .Xr poll 2 ,
235 .Xr select 2 ,
236 .Xr sigaction 2
237 .Sh STANDARDS
238 The
239 .Fn write
240 function is expected to conform to
241 .St -p1003.1-88 .
242 The
243 .Fn writev
244 and
245 .Fn pwrite
246 functions conform to
247 .St -xpg4.2 .
248 .Sh HISTORY
249 The
250 .Fn pwritev
251 function call
252 appeared in
253 .Nx 1.4 .
254 The
255 .Fn pwrite
256 function call
257 appeared in
258 .At V.4 .
259 The
260 .Fn writev
261 function call
262 appeared in
263 .Bx 4.2 .
264 The
265 .Fn write
266 function call appeared in
267 .At v2 .
268 .Sh CAVEATS
269 Error checks should explicitly test for \-1.
270 Code such as
271 .Bd -literal
272 while ((nr = write(fd, buf, sizeof(buf))) \*[Gt] 0)
273 .Ed
274 .Pp
275 is not maximally portable, as some platforms allow for
276 .Va nbytes
277 to range between
278 .Dv SSIZE_MAX
279 and
280 .Dv SIZE_MAX
281 \- 2, in which case the return value of an error-free
282 .Fn write
283 may appear as a negative number distinct from \-1.
284 Proper loops should use
285 .Bd -literal
286 while ((nr = write(fd, buf, sizeof(buf))) != -1 \*[Am]\*[Am] nr != 0)
287 .Ed