add openat(2) system call
[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 $
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35.\"
36.Dd November 16, 1993
37.Dt OPEN 2
38.Os
39.Sh NAME
40.Nm open
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" "..."
48.Sh DESCRIPTION
49The file name specified by
50.Fa path
51is opened
52for reading and/or writing as specified by the
53argument
54.Fa flags
3a0c0554
JH
55and the lowest unused file descriptor in the process' file descriptor table
56is returned.
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57The
58.Fa flags
59argument may indicate the file is to be
60created if it does not exist (by specifying the
61.Dv O_CREAT
62flag).
63In this case
64.Nm
65requires a third argument
66.Fa "mode_t mode" ,
67and the file is created with mode
68.Fa mode
69as described in
70.Xr chmod 2
71and modified by the process' umask value (see
72.Xr umask 2 ) .
73.Pp
74The flags specified are formed by
75.Em or Ns 'ing
76the following values
77.Pp
78.Bd -literal -offset indent -compact
79O_RDONLY open for reading only
80O_WRONLY open for writing only
81O_RDWR open for reading and writing
82O_NONBLOCK do not block on open
83O_APPEND append on each write
84O_CREAT create file if it does not exist
85O_TRUNC truncate size to 0
86O_EXCL error if create and file exists
87O_SHLOCK atomically obtain a shared lock
88O_EXLOCK atomically obtain an exclusive lock
89O_DIRECT eliminate or reduce cache effects
90O_FSYNC synchronous writes
91O_NOFOLLOW do not follow symlinks
92.Ed
93.Pp
94Opening a file with
95.Dv O_APPEND
96set causes each write on the file
97to be appended to the end.
98If
99.Dv O_TRUNC
100is specified and the
101file exists, the file is truncated to zero length.
102If
103.Dv O_EXCL
104is set with
105.Dv O_CREAT
106and the file already
107exists,
108.Fn open
109returns an error.
110This may be used to
111implement a simple exclusive access locking mechanism.
112If
113.Dv O_EXCL
114is set and the last component of the pathname is
115a symbolic link,
116.Fn open
117will fail even if the symbolic
118link points to a non-existent name.
119If the
120.Dv O_NONBLOCK
121flag is specified and the
122.Fn open
123call would result
124in the process being blocked for some reason (e.g., waiting for
125carrier on a dialup line),
126.Fn open
127returns immediately.
128The first time the process attempts to perform I/O on the open
129file it will block (not currently implemented).
130.Pp
131If
132.Dv O_FSYNC
133is used in the mask, all writes will
134immediately be written to disk,
135the kernel will not cache written data
136and all writes on the descriptor will not return until
137the data to be written completes.
138.Pp
139If
140.Dv O_NOFOLLOW
141is used in the mask and the target file passed to
142.Fn open
143is a symbolic link then the
144.Fn open
145will fail.
146.Pp
147When opening a file, a lock with
148.Xr flock 2
149semantics can be obtained by setting
150.Dv O_SHLOCK
151for a shared lock, or
152.Dv O_EXLOCK
153for an exclusive lock.
154If creating a file with
155.Dv O_CREAT ,
156the request for the lock will never fail
157(provided that the underlying filesystem supports locking).
158.Pp
159.Dv O_DIRECT
160may be used to minimize or eliminate the cache effects of reading and writing.
161The system will attempt to avoid caching the data you read or write.
162If it cannot avoid caching the data,
163it will minimize the impact the data has on the cache.
164Use of this flag can drastically reduce performance if not used with care.
165.Pp
166If successful,
167.Fn open
168returns a non-negative integer, termed a file descriptor.
169It returns -1 on failure.
170The file pointer used to mark the current position within the
171file is set to the beginning of the file.
172.Pp
173When a new file is created it is given the group of the directory
174which contains it.
175.Pp
176The new descriptor is set to remain open across
177.Xr execve 2
178system calls; see
179.Xr close 2
180and
181.Xr fcntl 2 .
182.Pp
183The system imposes a limit on the number of file descriptors
184open simultaneously by one process.
185.Xr Getdtablesize 2
186returns the current system limit.
187.Sh RETURN VALUES
188If successful,
189.Fn open
190returns a non-negative integer, termed a file descriptor.
191It returns -1 on failure, and sets
192.Va errno
193to indicate the error.
194.Sh ERRORS
195The named file is opened unless:
196.Bl -tag -width Er
197.It Bq Er ENOTDIR
198A component of the path prefix is not a directory.
199.It Bq Er ENAMETOOLONG
200A component of a pathname exceeded 255 characters,
201or an entire path name exceeded 1023 characters.
202.It Bq Er ENOENT
203.Dv O_CREAT
204is not set and the named file does not exist.
205.It Bq Er ENOENT
206A component of the path name that must exist does not exist.
207.It Bq Er EACCES
208Search permission is denied for a component of the path prefix.
209.It Bq Er EACCES
210The required permissions (for reading and/or writing)
211are denied for the given flags.
212.It Bq Er EACCES
213.Dv O_CREAT
214is specified,
215the file does not exist,
216and the directory in which it is to be created
217does not permit writing.
218.It Bq Er ELOOP
219Too many symbolic links were encountered in translating the pathname.
220.It Bq Er EISDIR
221The named file is a directory, and the arguments specify
222it is to be opened for writing.
223.It Bq Er EROFS
224The named file resides on a read-only file system,
225and the file is to be modified.
226.It Bq Er EMFILE
227The process has already reached its limit for open file descriptors.
228.It Bq Er ENFILE
229The system file table is full.
230.It Bq Er EMLINK
231.Dv O_NOFOLLOW
232was specified and the target is a symbolic link.
233.It Bq Er ENXIO
234The named file is a character special or block
235special file, and the device associated with this special file
236does not exist.
237.It Bq Er ENXIO
238The named file is a fifo, no process has
239it open for reading, and the arguments specify it is
240to be opened for writing.
241.It Bq Er EINTR
242The
243.Fn open
244operation was interrupted by a signal.
245.It Bq Er EOPNOTSUPP
246.Dv O_SHLOCK
247or
248.Dv O_EXLOCK
249is specified but the underlying filesystem does not support locking.
250.It Bq Er EWOULDBLOCK
251.Dv O_NONBLOCK
252and one of
253.Dv O_SHLOCK
254or
255.Dv O_EXLOCK
256is specified and the file is locked.
257.It Bq Er ENOSPC
258.Dv O_CREAT
259is specified,
260the file does not exist,
261and the directory in which the entry for the new file is being placed
262cannot be extended because there is no space left on the file
263system containing the directory.
264.It Bq Er ENOSPC
265.Dv O_CREAT
266is specified,
267the file does not exist,
268and there are no free inodes on the file system on which the
269file is being created.
270.It Bq Er EDQUOT
271.Dv O_CREAT
272is specified,
273the file does not exist,
274and the directory in which the entry for the new file
275is being placed cannot be extended because the
276user's quota of disk blocks on the file system
277containing the directory has been exhausted.
278.It Bq Er EDQUOT
279.Dv O_CREAT
280is specified,
281the file does not exist,
282and the user's quota of inodes on the file system on
283which the file is being created has been exhausted.
284.It Bq Er EIO
285An I/O error occurred while making the directory entry or
286allocating the inode for
287.Dv O_CREAT .
288.It Bq Er ETXTBSY
289The file is a pure procedure (shared text) file that is being
290executed and the
291.Fn open
292call requests write access.
293.It Bq Er EFAULT
294.Fa Path
295points outside the process's allocated address space.
296.It Bq Er EEXIST
297.Dv O_CREAT
298and
299.Dv O_EXCL
300were specified and the file exists.
301.It Bq Er EOPNOTSUPP
302An attempt was made to open a socket (not currently implemented).
303.It Bq Er EINVAL
304An attempt was made to open a descriptor with an illegal combination
305of
306.Dv O_RDONLY ,
307.Dv O_WRONLY ,
308and
309.Dv O_RDWR .
310.El
311.Sh SEE ALSO
312.Xr chmod 2 ,
313.Xr close 2 ,
314.Xr dup 2 ,
315.Xr getdtablesize 2 ,
316.Xr lseek 2 ,
317.Xr read 2 ,
318.Xr umask 2 ,
319.Xr write 2
320.Sh HISTORY
321An
322.Fn open
323function call appeared in
324.At v6 .