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1 .\" $NetBSD: open.2,v 1.56 2013/07/29 18:37:16 njoly Exp $
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30 .\" @(#)open.2 8.2 (Berkeley) 11/16/93
31 .\"
32 .Dd July 29, 2013
33 .Dt OPEN 2
34 .Os
35 .Sh NAME
36 .Nm open ,
37 .Nm openat
38 .Nd open or create a file for reading or writing
39 .Sh LIBRARY
40 .Lb libc
41 .Sh SYNOPSIS
42 .In fcntl.h
43 .Ft int
44 .Fn open "const char *path" "int flags" "..."
45 .Ft int
46 .Fn openat "int fd" "const char *path" "int flags" "..."
47 .Sh DESCRIPTION
48 The file name specified by
49 .Fa path
50 is opened
51 for reading and/or writing as specified by the
52 argument
53 .Fa flags
54 and the file descriptor returned to the calling process.
55 .Pp
56 .Fn openat
57 works the same way as
58 .Fn open
59 except if
60 .Fa path
61 is relative.
62 In that case, it is looked up from a directory whose file
63 descriptor was passed as
64 .Fa fd .
65 Search permission is required on this directory.
66 .\" (These alternatives await a decision about the semantics of O_SEARCH)
67 .\" Search permission is required on this directory
68 .\" except if
69 .\" .Fa fd
70 .\" was opened with the
71 .\" .Dv O_SEARCH
72 .\" flag.
73 .\" - or -
74 .\" This file descriptor must have been opened with the
75 .\" .Dv O_SEARCH
76 .\" flag.
77 .Fa fd
78 can be set to
79 .Dv AT_FDCWD
80 in order to specify the current directory.
81 .Pp
82 The
83 .Fa flags
84 are specified by
85 .Em or Ns 'ing
86 the values listed below.
87 Applications must specify exactly one of the first three values
88 (file access methods):
89 .Bl -tag -offset indent -width O_DIRECTORY
90 .It Dv O_RDONLY
91 Open for reading only.
92 .It Dv O_WRONLY
93 Open for writing only.
94 .It Dv O_RDWR
95 Open for reading and writing.
96 .El
97 .Pp
98 Any combination of the following may be used:
99 .Bl -tag -offset indent -width O_DIRECTORY
100 .It Dv O_NONBLOCK
101 Do not block on open or for data to become available.
102 .It Dv O_APPEND
103 Append to the file on each write.
104 .It Dv O_CREAT
105 Create the file if it does not exist.
106 The third argument of type
107 .Ft mode_t
108 is used to compute the mode bits of the file as described in
109 .Xr chmod 2
110 and modified by the process' umask value (see
111 .Xr umask 2 ) .
112 .It Dv O_TRUNC
113 Truncate size to 0.
114 .It Dv O_EXCL
115 Error if
116 .Dv O_CREAT
117 and the file already exists.
118 .It Dv O_SHLOCK
119 Atomically obtain a shared lock.
120 .It Dv O_EXLOCK
121 Atomically obtain an exclusive lock.
122 .It Dv O_NOFOLLOW
123 If last path element is a symlink, don't follow it.
124 This option is provided for compatibility with other operating
125 systems, but its security value is questionable.
126 .It Dv O_CLOEXEC
127 Set the
128 .Xr close 2
129 on
130 .Xr exec 3
131 flag.
132 .It Dv O_NOSIGPIPE
133 Return
134 .Er EPIPE
135 instead of raising
136 .Dv SIGPIPE .
137 .It Dv O_DSYNC
138 If set, write operations will be performed according to synchronized
139 I/O data integrity completion:
140 each write will wait for the file data to be committed to stable
141 storage.
142 .It Dv O_SYNC
143 If set, write operations will be performed according to synchronized
144 I/O file integrity completion:
145 each write will wait for both the file data and file status to be
146 committed to stable storage.
147 .It Dv O_RSYNC
148 If set, read operations will complete at the same level of
149 integrity which is in effect for write operations:
150 if specified together with
151 .Dv O_SYNC ,
152 each read will wait for the file status to be committed to stable
153 storage.
154 .Pp
155 Combining
156 .Dv O_RSYNC
157 with
158 .Dv O_DSYNC
159 only, or specifying it without any other synchronized I/O integrity
160 completion flag set, has no further effect.
161 .It Dv O_ALT_IO
162 Alternate I/O semantics will be used for read and write operations
163 on the file descriptor.
164 Alternate semantics are defined by the underlying layers and will not
165 have any alternate effect in most cases.
166 .It Dv O_NOCTTY
167 If the file is a terminal device, the opened device is not
168 made the controlling terminal for the session.
169 This flag has no effect on
170 .Nx ,
171 since the system defaults to the abovementioned behaviour.
172 The flag is present only for standards conformance.
173 .It Dv O_DIRECT
174 If set on a regular file, data I/O operations will not buffer the data
175 being transferred in the kernel's cache, but rather transfer the data
176 directly between user memory and the underlying device driver if possible.
177 This flag is advisory; the request may be performed in the normal
178 buffered fashion if certain conditions are not met, e.g. if the request
179 is not sufficiently aligned or if the file is mapped.
180 .Pp
181 To meet the alignment requirements for direct I/O, the file offset,
182 the length of the I/O and the address of the buffer in memory must all
183 be multiples of
184 .Dv DEV_BSIZE
185 (512 bytes).
186 If the I/O request is made
187 using an interface that supports scatter/gather via struct iovec, each
188 element of the request must meet the above alignment constraints.
189 .It Dv O_DIRECTORY
190 Fail if the file is not a directory.
191 .It Dv O_ASYNC
192 Enable the
193 .Dv SIGIO
194 signal to be sent to the process group
195 when I/O is possible, e.g.,
196 upon availability of data to be read.
197 .\" (This block awaits a decision about the semantics of O_SEARCH)
198 .\" .It Dv O_SEARCH
199 .\" If opening a directory, search permission checks will not be performed on
200 .\" subsequent usage of the file descriptor for looking up relative paths by
201 .\" .Xr faccessat 2 ,
202 .\" .Xr fchmodat 2 ,
203 .\" .Xr fchownat 2 ,
204 .\" .Xr fstatat 2 ,
205 .\" .Xr linkat 2 ,
206 .\" .Xr mkdirat 2 ,
207 .\" .Xr mkfifoat 2 ,
208 .\" .Xr mknodat 2 ,
209 .\" .Xr openat 2 ,
210 .\" .Xr readlinkat 2 ,
211 .\" .Xr symlinkat 2 ,
212 .\" .Xr unlinkat 2 ,
213 .\" and
214 .\" .Xr utimensat 2 .
215 .El
216 .Pp
217 Opening a file with
218 .Dv O_APPEND
219 set causes each write on the file
220 to be appended to the end.
221 If
222 .Dv O_TRUNC
223 is specified and the
224 file exists, the file is truncated to zero length.
225 .Pp
226 If
227 .Dv O_EXCL
228 is set with
229 .Dv O_CREAT
230 and the file already
231 exists,
232 .Fn open
233 returns an error.
234 This may be used to implement a simple exclusive access locking mechanism.
235 If
236 .Dv O_EXCL
237 is set and the last component of the pathname is
238 a symbolic link,
239 .Fn open
240 will fail even if the symbolic
241 link points to a non-existent name.
242 .Pp
243 If the
244 .Dv O_NONBLOCK
245 flag is specified, do not wait for the device or file to be ready or
246 available.
247 If the
248 .Fn open
249 call would result
250 in the process being blocked for some reason (e.g., waiting for
251 carrier on a dialup line),
252 .Fn open
253 returns immediately.
254 This flag also has the effect of making all subsequent I/O on the open file non-blocking.
255 .Pp
256 When opening a file, a lock with
257 .Xr flock 2
258 semantics can be obtained by setting
259 .Dv O_SHLOCK
260 for a shared lock, or
261 .Dv O_EXLOCK
262 for an exclusive lock.
263 If creating a file with
264 .Dv O_CREAT ,
265 the request for the lock will never fail
266 (provided that the underlying filesystem supports locking).
267 .Pp
268 If
269 .Fn open
270 is successful, the file pointer used to mark the current position within
271 the file is set to the beginning of the file.
272 .Pp
273 When a new file is created it is given the group of the directory
274 which contains it.
275 .Pp
276 The new descriptor is set to remain open across
277 .Xr execve 2
278 system calls; see
279 .Xr close 2
280 and
281 .Xr fcntl 2 .
282 .Pp
283 The system imposes a limit on the number of file descriptors
284 open simultaneously by one process.
285 Calling
286 .Xr getdtablesize 3
287 returns the current system limit.
288 .Sh RETURN VALUES
289 If successful,
290 .Fn open
291 and
292 .Fn openat
293 returns a non-negative integer, termed a file descriptor.
294 Otherwise, a value of \-1 is returned and
295 .Va errno
296 is set to indicate the error.
297 .Sh ERRORS
298 The named file is opened unless:
299 .Bl -tag -width Er
300 .It Bq Er EACCES
301 Search permission is denied for a component of the path prefix,
302 the required permissions (for reading and/or writing)
303 are denied for the given flags, or
304 .Dv O_CREAT
305 is specified,
306 the file does not exist,
307 and the directory in which it is to be created
308 does not permit writing.
309 .It Bq Er EDQUOT
310 .Dv O_CREAT
311 is specified,
312 the file does not exist,
313 and the directory in which the entry for the new file
314 is being placed cannot be extended because the
315 user's quota of disk blocks on the file system
316 containing the directory has been exhausted; or
317 .Dv O_CREAT
318 is specified,
319 the file does not exist,
320 and the user's quota of inodes on the file system on
321 which the file is being created has been exhausted.
322 .It Bq Er EEXIST
323 .Dv O_CREAT
324 and
325 .Dv O_EXCL
326 were specified and the file exists.
327 .It Bq Er EFAULT
328 .Fa path
329 points outside the process's allocated address space.
330 .It Bq Er EFTYPE
331 .Dv O_NOFOLLOW
332 was specified, but the last path component is a symlink.
333 .Em Note :
334 .St -p1003.1-2008
335 specifies returning
336 .Bq Er ELOOP
337 for this case.
338 .It Bq Er EINTR
339 The
340 .Fn open
341 operation was interrupted by a signal.
342 .It Bq Er EIO
343 An I/O error occurred while making the directory entry or
344 allocating the inode for
345 .Dv O_CREAT .
346 .It Bq Er EISDIR
347 The named file is a directory, and the arguments specify
348 it is to be opened for writing.
349 .It Bq Er ELOOP
350 Too many symbolic links were encountered in translating the pathname.
351 .It Bq Er EMFILE
352 The process has already reached its limit for open file descriptors.
353 .It Bq Er ENAMETOOLONG
354 A component of a pathname exceeded
355 .Brq Dv NAME_MAX
356 characters, or an entire path name exceeded
357 .Brq Dv PATH_MAX
358 characters.
359 .It Bq Er ENFILE
360 The system file table is full.
361 .It Bq Er ENOENT
362 .Dv O_CREAT
363 is not set and the named file does not exist, or
364 a component of the path name that must exist does not exist.
365 .It Bq Er ENOSPC
366 .Dv O_CREAT
367 is specified,
368 the file does not exist,
369 and the directory in which the entry for the new file is being placed
370 cannot be extended because there is no space left on the file
371 system containing the directory; or
372 .Dv O_CREAT
373 is specified,
374 the file does not exist,
375 and there are no free inodes on the file system on which the
376 file is being created.
377 .It Bq Er ENOTDIR
378 A component of the path prefix is not a directory; or
379 .Dv O_DIRECTORY
380 is specified and the last path component is not a directory.
381 .It Bq Er ENXIO
382 The named file is a character special or block
383 special file, and the device associated with this special file
384 does not exist, or
385 the named file is a
386 .Tn FIFO ,
387 .Dv O_NONBLOCK
388 and
389 .Dv O_WRONLY
390 is set and no process has the file open for reading.
391 .It Bq Er EOPNOTSUPP
392 .Dv O_SHLOCK
393 or
394 .Dv O_EXLOCK
395 is specified but the underlying filesystem does not support locking; or
396 an attempt was made to open a socket (not currently implemented).
397 .It Bq Er EPERM
398 The file's flags (see
399 .Xr chflags 2 )
400 don't allow the file to be opened.
401 .It Bq Er EROFS
402 The named file resides on a read-only file system,
403 and the file is to be modified.
404 .It Bq Er ETXTBSY
405 The file is a pure procedure (shared text) file that is being
406 executed and the
407 .Fn open
408 call requests write access.
409 .El
410 .Pp
411 In addition,
412 .Fn openat
413 will fail if:
414 .Bl -tag -width Er
415 .It Bq Er EBADF
416 .Fa path
417 does not specify an absolute path and
418 .Fa fd
419 is neither
420 .Dv AT_FDCWD
421 nor a valid file descriptor open for reading or searching.
422 .It Bq Er ENOTDIR
423 .Fa path
424 is not an absolute path and
425 .Fa fd
426 is a file descriptor associated with a non-directory file.
427 .El
428 .Sh SEE ALSO
429 .Xr chmod 2 ,
430 .Xr close 2 ,
431 .Xr dup 2 ,
432 .Xr faccessat 2 ,
433 .Xr fchmodat 2 ,
434 .Xr fchownat 2 ,
435 .Xr fstatat 2 ,
436 .Xr linkat 2 ,
437 .Xr lseek 2 ,
438 .Xr mkdirat 2 ,
439 .Xr mkfifoat 2 ,
440 .Xr mknodat 2 ,
441 .Xr read 2 ,
442 .Xr readlinkat 2 ,
443 .Xr symlinkat 2 ,
444 .Xr umask 2 ,
445 .Xr unlinkat 2 ,
446 .Xr utimensat 2 ,
447 .Xr write 2 ,
448 .Xr getdtablesize 3
449 .Sh STANDARDS
450 The
451 .Fn open
452 function conforms to
453 .St -p1003.1-90 .
454 .Fn openat
455 conforms to
456 .St -p1003.1-2008 .
457 .Pp
458 The
459 .Fa flags
460 values
461 .Dv O_DSYNC ,
462 .Dv O_SYNC
463 and
464 .Dv O_RSYNC
465 are extensions defined in
466 .St -p1003.1b-93 .
467 .\" (This block awaits a decision about the semantics of O_SEARCH)
468 .\" .Dv O_SEARCH
469 .\" is defined in
470 .\" .St -p1003.1-2008 .
471 .Pp
472 The
473 .Dv O_SHLOCK
474 and
475 .Dv O_EXLOCK
476 flags are non-standard extensions and should not be used if portability
477 is of concern.
478 .Sh HISTORY
479 An
480 .Fn open
481 function call appeared in
482 .At v2 .