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28 .\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94
29 .\" $FreeBSD: src/lib/libc/sys/fcntl.2,v 1.16.2.13 2002/07/22 15:15:16 bde Exp $
42 .Fn fcntl "int fd" "int cmd" "..."
46 system call provides for control over descriptors.
49 is a descriptor to be operated on by
52 Depending on the value of
55 can take an additional third argument
57 .Bl -tag -width ".Dv F_DUP2FD_CLOEXEC"
59 Return a new descriptor as follows:
61 .Bl -bullet -compact -offset 4n
63 Lowest numbered available descriptor greater than or equal to
66 Same object references as the original descriptor.
68 New descriptor shares the same file offset if the object
71 Same access mode (read, write or read/write).
73 Same file status flags (i.e., both file descriptors
74 share the same file status flags).
76 The close-on-exec flag
78 associated with the new file descriptor is cleared, so the file descriptor is
83 .It Dv F_DUPFD_CLOEXEC
88 flag associated with the new file descriptor is set, so the file descriptor
93 It is functionally equivalent to
94 .Bd -literal -offset indent
97 .It Dv F_DUP2FD_CLOEXEC
102 flag associated with the new file descriptor is set.
108 constants are not portable, so they should not be used if
109 portability is needed.
115 Get the close-on-exec flag associated with the file descriptor
119 If the returned value ANDed with
122 the file will remain open across
124 otherwise the file will be closed upon execution of
129 Set the close-on-exec flag associated with
139 Get descriptor status flags, as described below
143 Set descriptor status flags to
146 Get the process ID or process group
151 signals; process groups are returned
156 Set the process or process group
162 process groups are specified by supplying
164 as negative, otherwise
166 is interpreted as a process ID.
173 flags are as follows:
174 .Bl -tag -width O_NONBLOCKX
176 Non-blocking I/O; if no data is available to a
180 operation would block,
181 the read or write call returns -1 with the error
184 Force each write to append at the end of file;
190 Minimize or eliminate the cache effects of reading and writing.
192 will attempt to avoid caching the data you read or write.
194 avoid caching the data, it will minimize the impact the data has on the cache.
195 Use of this flag can drastically reduce performance if not used with care.
199 signal to be sent to the process group
200 when I/O is possible, e.g.,
201 upon availability of data to be read.
204 Several commands are available for doing advisory file locking;
205 they all operate on the following structure:
208 off_t l_start; /* starting offset */
209 off_t l_len; /* len = 0 means until end of file */
210 pid_t l_pid; /* lock owner */
211 short l_type; /* lock type: read/write, etc. */
212 short l_whence; /* type of l_start */
215 The commands available for advisory record locking are as follows:
216 .Bl -tag -width F_SETLKWX
218 Get the first lock that blocks the lock description pointed to by the
221 taken as a pointer to a
224 The information retrieved overwrites the information passed to
229 If no lock is found that would prevent this lock from being created,
230 the structure is left unchanged by this system call except for the
231 lock type which is set to
234 Set or clear a file segment lock according to the lock description
235 pointed to by the third argument,
237 taken as a pointer to a
241 is used to establish shared (or read) locks
243 or exclusive (or write) locks,
245 as well as remove either type of lock
247 If a shared or exclusive lock cannot be set,
249 returns immediately with
252 This command is the same as
254 except that if a shared or exclusive lock is blocked by other locks,
255 the process waits until the request can be satisfied.
256 If a signal that is to be caught is received while
258 is waiting for a region, the
260 will be interrupted if the signal handler has not specified the
266 When a shared lock has been set on a segment of a file,
267 other processes can set shared locks on that segment
269 A shared lock prevents any other process from setting an exclusive
270 lock on any portion of the protected area.
271 A request for a shared lock fails if the file descriptor was not
272 opened with read access.
274 An exclusive lock prevents any other process from setting a shared lock or
275 an exclusive lock on any portion of the protected area.
276 A request for an exclusive lock fails if the file was not
277 opened with write access.
286 to indicate that the relative offset,
288 bytes, will be measured from the start of the file,
289 current position, or end of the file, respectively.
292 is the number of consecutive bytes to be locked.
295 is negative, the result is undefined.
298 field is only used with
300 to return the process ID of the process holding a blocking lock.
303 request, the value of
308 Locks may start and extend beyond the current end of a file,
309 but may not start or extend before the beginning of the file.
310 A lock is set to extend to the largest possible value of the
311 file offset for that file if
318 point to the beginning of the file, and
320 is zero, the entire file is locked.
321 If an application wishes only to do entire file locking, the
323 system call is much more efficient.
325 There is at most one type of lock set for each byte in the file.
326 Before a successful return from an
330 request when the calling process has previously existing locks
331 on bytes in the region specified by the request,
332 the previous lock type for each byte in the specified
333 region is replaced by the new lock type.
334 As specified above under the descriptions
335 of shared locks and exclusive locks, an
339 request fails or blocks respectively when another process has existing
340 locks on bytes in the specified region and the type of any of those
341 locks conflicts with the type specified in the request.
343 This interface follows the completely stupid semantics of System V and
345 that require that all locks associated with a file for a given process are
348 file descriptor for that file is closed by that process.
349 This semantic means that applications must be aware of any files that
350 a subroutine library may access.
351 For example if an application for updating the password file locks the
352 password file database while making the update, and then calls
354 to retrieve a record,
355 the lock will be lost because
357 opens, reads, and closes the password database.
358 The database close will release all locks that the process has
359 associated with the database, even if the library routine never
360 requested a lock on the database.
361 Another minor semantic problem with this interface is that
362 locks are not inherited by a child process created using the
367 interface has much more rational last close semantics and
368 allows locks to be inherited by child processes.
371 system call is recommended for applications that want to ensure the integrity
372 of their locks when using library routines or wish to pass locks
380 locks are compatible.
381 Processes using different locking interfaces can cooperate
382 over the same file safely.
383 However, only one of such interfaces should be used within
385 If a file is locked by a process through
387 any record within the file will be seen as locked
388 from the viewpoint of another process using
397 if the process holding a blocking lock previously locked the
401 All locks associated with a file for a given process are
402 removed when the process terminates.
404 All locks obtained before a call to
406 remain in effect until the new program releases them.
407 If the new program does not know about the locks, they will not be
408 released until the program exits.
410 A potential for deadlock occurs if a process controlling a locked region
411 is put to sleep by attempting to lock the locked region of another process.
412 This implementation detects that sleeping until a locked region is unlocked
413 would cause a deadlock and fails with an
417 Upon successful completion, the value returned depends on
420 .Bl -tag -width F_GETOWNX -offset indent
422 A new file descriptor.
424 A file descriptor equal to
427 Value of flag (only the low-order bit is defined).
431 Value of file descriptor owner.
436 Otherwise, a value of -1 is returned and
438 is set to indicate the error.
442 system call will fail if:
455 and the segment of a file to be locked is already
456 exclusive-locked by another process;
457 or the type is an exclusive lock and some portion of the
458 segment of a file to be locked is already shared-locked or
459 exclusive-locked by another process.
464 is not a valid open file descriptor.
472 is not a valid file descriptor.
486 is not a valid file descriptor open for reading.
500 is not a valid file descriptor open for writing.
506 and a deadlock condition was detected.
512 and the system call was interrupted by a signal.
521 is negative or greater than the maximum allowable number
523 .Xr getdtablesize 2 ) .
532 and the data to which
540 and the maximum number of file descriptors permitted for the
541 process are already in use,
542 or no file descriptors greater than or equal to
552 and satisfying the lock or unlock request would result in the
553 number of locked regions in the system exceeding a system-imposed limit.
561 the process ID or process group given as an argument is in a
562 different session than the caller.
570 the process ID given as argument is not in use.
575 refers to a descriptor open on a terminal device (as opposed to a
576 descriptor open on a socket), a
580 can fail for the same reasons as in
586 for the reasons as stated in
593 .Xr getdtablesize 2 ,
602 constant is non portable.
603 It is provided for compatibility with AIX and Solaris.
607 system call appeared in
611 .Dv F_DUPFD_CLOEXEC ,
615 constants first appeared in