open - implement O_DIRECTORY
[dragonfly.git] / lib / libc / sys / open.2
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32.\" @(#)open.2 8.2 (Berkeley) 11/16/93
33.\" $FreeBSD: src/lib/libc/sys/open.2,v 1.11.2.9 2001/12/14 18:34:01 ru Exp $
3a0c0554 34.\" $DragonFly: src/lib/libc/sys/open.2,v 1.3 2005/07/29 23:16:04 hsu Exp $
984263bc 35.\"
95c2ffe9 36.Dd July 24, 2009
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37.Dt OPEN 2
38.Os
39.Sh NAME
95c2ffe9 40.Nm open , openat
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41.Nd open or create a file for reading or writing
42.Sh LIBRARY
43.Lb libc
44.Sh SYNOPSIS
45.In fcntl.h
46.Ft int
47.Fn open "const char *path" "int flags" "..."
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48.Ft int
49.Fn openat "int fd" "const char *path" "int flags" "..."
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50.Sh DESCRIPTION
51The file name specified by
52.Fa path
53is opened
54for reading and/or writing as specified by the
55argument
56.Fa flags
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57and the lowest unused file descriptor in the process' file descriptor table
58is returned.
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59The
60.Fa flags
61argument may indicate the file is to be
62created if it does not exist (by specifying the
63.Dv O_CREAT
64flag).
65In this case
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66.Fn open
67and
68.Fn openat
69require a third argument
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70.Fa "mode_t mode" ,
71and the file is created with mode
72.Fa mode
73as described in
74.Xr chmod 2
75and modified by the process' umask value (see
76.Xr umask 2 ) .
77.Pp
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78The
79.Fn openat
80function is equivalent to the
81.Fn open
82function except in the case where the
83.Fa path
84specifies a relative path.
85In this case the file to be opened is determined relative to the directory
86associated with the file descriptor
87.Fa fd
88instead of the current working directory.
89The
90.Fa flag
91parameter and the optional fourth parameter correspond exactly to
92the parameters of
93.Fn open .
94If
95.Fn openat
96is passed the special value
97.Dv AT_FDCWD
98in the
99.Fa fd
100parameter, the current working directory is used
101and the behavior is identical to a call to
102.Fn open .
103.Pp
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104The flags specified are formed by
105.Em or Ns 'ing
106the following values
107.Pp
108.Bd -literal -offset indent -compact
109O_RDONLY open for reading only
110O_WRONLY open for writing only
111O_RDWR open for reading and writing
112O_NONBLOCK do not block on open
113O_APPEND append on each write
114O_CREAT create file if it does not exist
115O_TRUNC truncate size to 0
116O_EXCL error if create and file exists
117O_SHLOCK atomically obtain a shared lock
118O_EXLOCK atomically obtain an exclusive lock
119O_DIRECT eliminate or reduce cache effects
120O_FSYNC synchronous writes
121O_NOFOLLOW do not follow symlinks
122.Ed
123.Pp
124Opening a file with
125.Dv O_APPEND
126set causes each write on the file
127to be appended to the end.
128If
129.Dv O_TRUNC
130is specified and the
131file exists, the file is truncated to zero length.
132If
133.Dv O_EXCL
134is set with
135.Dv O_CREAT
136and the file already
137exists,
138.Fn open
139returns an error.
140This may be used to
141implement a simple exclusive access locking mechanism.
142If
143.Dv O_EXCL
144is set and the last component of the pathname is
145a symbolic link,
146.Fn open
147will fail even if the symbolic
148link points to a non-existent name.
149If the
150.Dv O_NONBLOCK
151flag is specified and the
152.Fn open
153call would result
154in the process being blocked for some reason (e.g., waiting for
155carrier on a dialup line),
156.Fn open
157returns immediately.
158The first time the process attempts to perform I/O on the open
159file it will block (not currently implemented).
160.Pp
161If
162.Dv O_FSYNC
163is used in the mask, all writes will
164immediately be written to disk,
165the kernel will not cache written data
166and all writes on the descriptor will not return until
167the data to be written completes.
168.Pp
169If
170.Dv O_NOFOLLOW
171is used in the mask and the target file passed to
172.Fn open
173is a symbolic link then the
174.Fn open
175will fail.
176.Pp
177When opening a file, a lock with
178.Xr flock 2
179semantics can be obtained by setting
180.Dv O_SHLOCK
181for a shared lock, or
182.Dv O_EXLOCK
183for an exclusive lock.
184If creating a file with
185.Dv O_CREAT ,
186the request for the lock will never fail
187(provided that the underlying filesystem supports locking).
188.Pp
189.Dv O_DIRECT
190may be used to minimize or eliminate the cache effects of reading and writing.
191The system will attempt to avoid caching the data you read or write.
192If it cannot avoid caching the data,
193it will minimize the impact the data has on the cache.
194Use of this flag can drastically reduce performance if not used with care.
195.Pp
196If successful,
197.Fn open
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198and
199.Fn openat
200return a non-negative integer, termed a file descriptor.
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201It returns -1 on failure.
202The file pointer used to mark the current position within the
203file is set to the beginning of the file.
204.Pp
205When a new file is created it is given the group of the directory
206which contains it.
207.Pp
208The new descriptor is set to remain open across
209.Xr execve 2
210system calls; see
211.Xr close 2
212and
213.Xr fcntl 2 .
214.Pp
215The system imposes a limit on the number of file descriptors
216open simultaneously by one process.
217.Xr Getdtablesize 2
218returns the current system limit.
219.Sh RETURN VALUES
220If successful,
221.Fn open
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222and
223.Fn openat
224return a non-negative integer, termed a file descriptor.
225They return -1 on failure, and set
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226.Va errno
227to indicate the error.
228.Sh ERRORS
229The named file is opened unless:
230.Bl -tag -width Er
231.It Bq Er ENOTDIR
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232A component of the path prefix is not a directory or the
233.Fa path
234argument is not an absolute path and the
235.Fa fd
5212f0c1 236argument is neither
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237.Dv AT_FDCWD
238nor a file descriptor associated with a directory.
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239.It Bq Er ENAMETOOLONG
240A component of a pathname exceeded 255 characters,
241or an entire path name exceeded 1023 characters.
242.It Bq Er ENOENT
243.Dv O_CREAT
244is not set and the named file does not exist.
245.It Bq Er ENOENT
246A component of the path name that must exist does not exist.
247.It Bq Er EACCES
248Search permission is denied for a component of the path prefix.
249.It Bq Er EACCES
250The required permissions (for reading and/or writing)
251are denied for the given flags.
252.It Bq Er EACCES
253.Dv O_CREAT
254is specified,
255the file does not exist,
256and the directory in which it is to be created
257does not permit writing.
258.It Bq Er ELOOP
259Too many symbolic links were encountered in translating the pathname.
260.It Bq Er EISDIR
261The named file is a directory, and the arguments specify
262it is to be opened for writing.
263.It Bq Er EROFS
264The named file resides on a read-only file system,
265and the file is to be modified.
266.It Bq Er EMFILE
267The process has already reached its limit for open file descriptors.
268.It Bq Er ENFILE
269The system file table is full.
270.It Bq Er EMLINK
271.Dv O_NOFOLLOW
272was specified and the target is a symbolic link.
273.It Bq Er ENXIO
274The named file is a character special or block
275special file, and the device associated with this special file
276does not exist.
277.It Bq Er ENXIO
278The named file is a fifo, no process has
279it open for reading, and the arguments specify it is
280to be opened for writing.
281.It Bq Er EINTR
282The
283.Fn open
284operation was interrupted by a signal.
285.It Bq Er EOPNOTSUPP
286.Dv O_SHLOCK
287or
288.Dv O_EXLOCK
289is specified but the underlying filesystem does not support locking.
290.It Bq Er EWOULDBLOCK
291.Dv O_NONBLOCK
292and one of
293.Dv O_SHLOCK
294or
295.Dv O_EXLOCK
296is specified and the file is locked.
297.It Bq Er ENOSPC
298.Dv O_CREAT
299is specified,
300the file does not exist,
301and the directory in which the entry for the new file is being placed
302cannot be extended because there is no space left on the file
303system containing the directory.
304.It Bq Er ENOSPC
305.Dv O_CREAT
306is specified,
307the file does not exist,
308and there are no free inodes on the file system on which the
309file is being created.
310.It Bq Er EDQUOT
311.Dv O_CREAT
312is specified,
313the file does not exist,
314and the directory in which the entry for the new file
315is being placed cannot be extended because the
316user's quota of disk blocks on the file system
317containing the directory has been exhausted.
318.It Bq Er EDQUOT
319.Dv O_CREAT
320is specified,
321the file does not exist,
322and the user's quota of inodes on the file system on
323which the file is being created has been exhausted.
324.It Bq Er EIO
325An I/O error occurred while making the directory entry or
326allocating the inode for
327.Dv O_CREAT .
328.It Bq Er ETXTBSY
329The file is a pure procedure (shared text) file that is being
330executed and the
331.Fn open
332call requests write access.
333.It Bq Er EFAULT
334.Fa Path
335points outside the process's allocated address space.
336.It Bq Er EEXIST
337.Dv O_CREAT
338and
339.Dv O_EXCL
340were specified and the file exists.
341.It Bq Er EOPNOTSUPP
342An attempt was made to open a socket (not currently implemented).
343.It Bq Er EINVAL
344An attempt was made to open a descriptor with an illegal combination
345of
346.Dv O_RDONLY ,
347.Dv O_WRONLY ,
348and
349.Dv O_RDWR .
350.El
351.Sh SEE ALSO
352.Xr chmod 2 ,
353.Xr close 2 ,
354.Xr dup 2 ,
355.Xr getdtablesize 2 ,
356.Xr lseek 2 ,
357.Xr read 2 ,
358.Xr umask 2 ,
359.Xr write 2
360.Sh HISTORY
361An
362.Fn open
363function call appeared in
364.At v6 .
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365An
366.Fn openat
1badc694 367function call appeared first in Solaris and was ported to
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368.Dx 2.3 .
369.Sh BUGS
370The Open Group Extended API Set 2 specification requires that the test
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371for
372.Fa fd Ap s
373searchability is based on whether it is open for searching,
374and not whether the underlying directory currently permits searches.
375The present implementation of
376.Fn openat
95c2ffe9 377checks the current permissions of directory instead.