kdump(1)/truss(1): Fix occasional quickworld breakage.
[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.\"
f3b0543f 36.Dd July 31, 2012
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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
f3b0543f 122O_DIRECTORY error if file is not a directory
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123.Ed
124.Pp
125Opening a file with
126.Dv O_APPEND
127set causes each write on the file
128to be appended to the end.
129If
130.Dv O_TRUNC
131is specified and the
132file exists, the file is truncated to zero length.
133If
134.Dv O_EXCL
135is set with
136.Dv O_CREAT
137and the file already
138exists,
139.Fn open
140returns an error.
141This may be used to
142implement a simple exclusive access locking mechanism.
143If
144.Dv O_EXCL
145is set and the last component of the pathname is
146a symbolic link,
147.Fn open
148will fail even if the symbolic
149link points to a non-existent name.
150If the
151.Dv O_NONBLOCK
152flag is specified and the
153.Fn open
154call would result
155in the process being blocked for some reason (e.g., waiting for
156carrier on a dialup line),
157.Fn open
158returns immediately.
159The first time the process attempts to perform I/O on the open
160file it will block (not currently implemented).
161.Pp
162If
163.Dv O_FSYNC
164is used in the mask, all writes will
165immediately be written to disk,
166the kernel will not cache written data
167and all writes on the descriptor will not return until
168the data to be written completes.
169.Pp
170If
171.Dv O_NOFOLLOW
172is used in the mask and the target file passed to
173.Fn open
174is a symbolic link then the
175.Fn open
176will fail.
177.Pp
178When opening a file, a lock with
179.Xr flock 2
180semantics can be obtained by setting
181.Dv O_SHLOCK
182for a shared lock, or
183.Dv O_EXLOCK
184for an exclusive lock.
185If creating a file with
186.Dv O_CREAT ,
187the request for the lock will never fail
188(provided that the underlying filesystem supports locking).
189.Pp
190.Dv O_DIRECT
191may be used to minimize or eliminate the cache effects of reading and writing.
192The system will attempt to avoid caching the data you read or write.
193If it cannot avoid caching the data,
194it will minimize the impact the data has on the cache.
195Use of this flag can drastically reduce performance if not used with care.
196.Pp
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197.Dv O_DIRECTORY
198may be used to ensure the resulting file descriptor refers to a directory.
199This flag can be used to prevent applications with elevated privileges
200from opening files which are even unsafe to open with
201.Dv O_RDONLY ,
202such as device nodes.
203.Pp
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204If successful,
205.Fn open
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206and
207.Fn openat
208return a non-negative integer, termed a file descriptor.
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209It returns -1 on failure.
210The file pointer used to mark the current position within the
211file is set to the beginning of the file.
212.Pp
213When a new file is created it is given the group of the directory
214which contains it.
215.Pp
216The new descriptor is set to remain open across
217.Xr execve 2
218system calls; see
219.Xr close 2
220and
221.Xr fcntl 2 .
222.Pp
223The system imposes a limit on the number of file descriptors
224open simultaneously by one process.
225.Xr Getdtablesize 2
226returns the current system limit.
227.Sh RETURN VALUES
228If successful,
229.Fn open
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230and
231.Fn openat
232return a non-negative integer, termed a file descriptor.
233They return -1 on failure, and set
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234.Va errno
235to indicate the error.
236.Sh ERRORS
237The named file is opened unless:
238.Bl -tag -width Er
239.It Bq Er ENOTDIR
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240A component of the path prefix is not a directory or the
241.Fa path
242argument is not an absolute path and the
243.Fa fd
5212f0c1 244argument is neither
95c2ffe9 245.Dv AT_FDCWD
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246nor a file descriptor associated with a directory or
247.Dv O_DIRECTORY
248is specified and the file is not a directory.
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249.It Bq Er ENAMETOOLONG
250A component of a pathname exceeded 255 characters,
251or an entire path name exceeded 1023 characters.
252.It Bq Er ENOENT
253.Dv O_CREAT
254is not set and the named file does not exist.
255.It Bq Er ENOENT
256A component of the path name that must exist does not exist.
257.It Bq Er EACCES
258Search permission is denied for a component of the path prefix.
259.It Bq Er EACCES
260The required permissions (for reading and/or writing)
261are denied for the given flags.
262.It Bq Er EACCES
263.Dv O_CREAT
264is specified,
265the file does not exist,
266and the directory in which it is to be created
267does not permit writing.
268.It Bq Er ELOOP
269Too many symbolic links were encountered in translating the pathname.
270.It Bq Er EISDIR
271The named file is a directory, and the arguments specify
272it is to be opened for writing.
273.It Bq Er EROFS
274The named file resides on a read-only file system,
275and the file is to be modified.
276.It Bq Er EMFILE
277The process has already reached its limit for open file descriptors.
278.It Bq Er ENFILE
279The system file table is full.
280.It Bq Er EMLINK
281.Dv O_NOFOLLOW
282was specified and the target is a symbolic link.
283.It Bq Er ENXIO
284The named file is a character special or block
285special file, and the device associated with this special file
286does not exist.
287.It Bq Er ENXIO
288The named file is a fifo, no process has
289it open for reading, and the arguments specify it is
290to be opened for writing.
291.It Bq Er EINTR
292The
293.Fn open
294operation was interrupted by a signal.
295.It Bq Er EOPNOTSUPP
296.Dv O_SHLOCK
297or
298.Dv O_EXLOCK
299is specified but the underlying filesystem does not support locking.
300.It Bq Er EWOULDBLOCK
301.Dv O_NONBLOCK
302and one of
303.Dv O_SHLOCK
304or
305.Dv O_EXLOCK
306is specified and the file is locked.
307.It Bq Er ENOSPC
308.Dv O_CREAT
309is specified,
310the file does not exist,
311and the directory in which the entry for the new file is being placed
312cannot be extended because there is no space left on the file
313system containing the directory.
314.It Bq Er ENOSPC
315.Dv O_CREAT
316is specified,
317the file does not exist,
318and there are no free inodes on the file system on which the
319file is being created.
320.It Bq Er EDQUOT
321.Dv O_CREAT
322is specified,
323the file does not exist,
324and the directory in which the entry for the new file
325is being placed cannot be extended because the
326user's quota of disk blocks on the file system
327containing the directory has been exhausted.
328.It Bq Er EDQUOT
329.Dv O_CREAT
330is specified,
331the file does not exist,
332and the user's quota of inodes on the file system on
333which the file is being created has been exhausted.
334.It Bq Er EIO
335An I/O error occurred while making the directory entry or
336allocating the inode for
337.Dv O_CREAT .
338.It Bq Er ETXTBSY
339The file is a pure procedure (shared text) file that is being
340executed and the
341.Fn open
342call requests write access.
343.It Bq Er EFAULT
344.Fa Path
345points outside the process's allocated address space.
346.It Bq Er EEXIST
347.Dv O_CREAT
348and
349.Dv O_EXCL
350were specified and the file exists.
351.It Bq Er EOPNOTSUPP
352An attempt was made to open a socket (not currently implemented).
353.It Bq Er EINVAL
354An attempt was made to open a descriptor with an illegal combination
355of
356.Dv O_RDONLY ,
357.Dv O_WRONLY ,
358and
359.Dv O_RDWR .
360.El
361.Sh SEE ALSO
362.Xr chmod 2 ,
363.Xr close 2 ,
364.Xr dup 2 ,
365.Xr getdtablesize 2 ,
366.Xr lseek 2 ,
367.Xr read 2 ,
368.Xr umask 2 ,
369.Xr write 2
370.Sh HISTORY
371An
372.Fn open
373function call appeared in
374.At v6 .
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375An
376.Fn openat
1badc694 377function call appeared first in Solaris and was ported to
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378.Dx 2.3 .
379.Sh BUGS
380The Open Group Extended API Set 2 specification requires that the test
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381for
382.Fa fd Ap s
383searchability is based on whether it is open for searching,
384and not whether the underlying directory currently permits searches.
385The present implementation of
386.Fn openat
95c2ffe9 387checks the current permissions of directory instead.