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28 .\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94
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 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.
168 Set or clear the read ahead amount for sequential access to the third
171 which is rounded up to the nearest block size.
174 turns off read ahead, a negative value restores the system default.
176 Equivalent to Darwin counterpart which sets read ahead amount of 128KB
177 when the third argument,
182 turns off read ahead.
189 flags are as follows:
190 .Bl -tag -width O_NONBLOCKX
192 Non-blocking I/O; if no data is available to a
196 operation would block,
197 the read or write call returns -1 with the error
200 Force each write to append at the end of file;
206 Minimize or eliminate the cache effects of reading and writing.
208 will attempt to avoid caching the data you read or write.
210 avoid caching the data, it will minimize the impact the data has on the cache.
211 Use of this flag can drastically reduce performance if not used with care.
215 signal to be sent to the process group
216 when I/O is possible, e.g.,
217 upon availability of data to be read.
220 Several commands are available for doing advisory file locking;
221 they all operate on the following structure:
224 off_t l_start; /* starting offset */
225 off_t l_len; /* len = 0 means until end of file */
226 pid_t l_pid; /* lock owner */
227 short l_type; /* lock type: read/write, etc. */
228 short l_whence; /* type of l_start */
229 int l_sysid; /* remote system id or zero for local */
232 The commands available for advisory record locking are as follows:
233 .Bl -tag -width F_SETLKWX
235 Get the first lock that blocks the lock description pointed to by the
238 taken as a pointer to a
241 The information retrieved overwrites the information passed to
246 If no lock is found that would prevent this lock from being created,
247 the structure is left unchanged by this system call except for the
248 lock type which is set to
251 Set or clear a file segment lock according to the lock description
252 pointed to by the third argument,
254 taken as a pointer to a
258 is used to establish shared (or read) locks
260 or exclusive (or write) locks,
262 as well as remove either type of lock
264 If a shared or exclusive lock cannot be set,
266 returns immediately with
269 This command is the same as
271 except that if a shared or exclusive lock is blocked by other locks,
272 the process waits until the request can be satisfied.
273 If a signal that is to be caught is received while
275 is waiting for a region, the
277 will be interrupted if the signal handler has not specified the
283 When a shared lock has been set on a segment of a file,
284 other processes can set shared locks on that segment
286 A shared lock prevents any other process from setting an exclusive
287 lock on any portion of the protected area.
288 A request for a shared lock fails if the file descriptor was not
289 opened with read access.
291 An exclusive lock prevents any other process from setting a shared lock or
292 an exclusive lock on any portion of the protected area.
293 A request for an exclusive lock fails if the file was not
294 opened with write access.
303 to indicate that the relative offset,
305 bytes, will be measured from the start of the file,
306 current position, or end of the file, respectively.
309 is the number of consecutive bytes to be locked.
314 means end edge of the region.
319 fields are only used with
321 to return the process ID of the process holding a blocking lock and
322 the system ID of the system that owns that process.
323 Locks created by the local system will have a system ID of zero.
326 request, the value of
331 Locks may start and extend beyond the current end of a file,
332 but may not start or extend before the beginning of the file.
333 A lock is set to extend to the largest possible value of the
334 file offset for that file if
341 point to the beginning of the file, and
343 is zero, the entire file is locked.
344 If an application wishes only to do entire file locking, the
346 system call is much more efficient.
348 There is at most one type of lock set for each byte in the file.
349 Before a successful return from an
353 request when the calling process has previously existing locks
354 on bytes in the region specified by the request,
355 the previous lock type for each byte in the specified
356 region is replaced by the new lock type.
357 As specified above under the descriptions
358 of shared locks and exclusive locks, an
362 request fails or blocks respectively when another process has existing
363 locks on bytes in the specified region and the type of any of those
364 locks conflicts with the type specified in the request.
366 This interface follows the completely stupid semantics of System V and
368 that require that all locks associated with a file for a given process are
371 file descriptor for that file is closed by that process.
372 This semantic means that applications must be aware of any files that
373 a subroutine library may access.
374 For example if an application for updating the password file locks the
375 password file database while making the update, and then calls
377 to retrieve a record,
378 the lock will be lost because
380 opens, reads, and closes the password database.
381 The database close will release all locks that the process has
382 associated with the database, even if the library routine never
383 requested a lock on the database.
384 Another minor semantic problem with this interface is that
385 locks are not inherited by a child process created using the
390 interface has much more rational last close semantics and
391 allows locks to be inherited by child processes.
394 system call is recommended for applications that want to ensure the integrity
395 of their locks when using library routines or wish to pass locks
403 locks are compatible.
404 Processes using different locking interfaces can cooperate
405 over the same file safely.
406 However, only one of such interfaces should be used within
408 If a file is locked by a process through
410 any record within the file will be seen as locked
411 from the viewpoint of another process using
420 if the process holding a blocking lock previously locked the
424 All locks associated with a file for a given process are
425 removed when the process terminates.
427 All locks obtained before a call to
429 remain in effect until the new program releases them.
430 If the new program does not know about the locks, they will not be
431 released until the program exits.
433 A potential for deadlock occurs if a process controlling a locked region
434 is put to sleep by attempting to lock the locked region of another process.
435 This implementation detects that sleeping until a locked region is unlocked
436 would cause a deadlock and fails with an
440 Upon successful completion, the value returned depends on
443 .Bl -tag -width F_GETOWNX -offset indent
445 A new file descriptor.
447 A file descriptor equal to
450 Value of flag (only the low-order bit is defined).
454 Value of file descriptor owner.
459 Otherwise, a value of -1 is returned and
461 is set to indicate the error.
465 system call will fail if:
478 and the segment of a file to be locked is already
479 exclusive-locked by another process;
480 or the type is an exclusive lock and some portion of the
481 segment of a file to be locked is already shared-locked or
482 exclusive-locked by another process.
487 is not a valid open file descriptor.
495 is not a valid file descriptor.
509 is not a valid file descriptor open for reading.
523 is not a valid file descriptor open for writing.
529 and a deadlock condition was detected.
535 and the system call was interrupted by a signal.
544 is negative or greater than the maximum allowable number
546 .Xr getdtablesize 2 ) .
555 and the data to which
563 and the maximum number of file descriptors permitted for the
564 process are already in use,
565 or no file descriptors greater than or equal to
575 and satisfying the lock or unlock request would result in the
576 number of locked regions in the system exceeding a system-imposed limit.
587 refers to a file for which locking is not supported.
598 calculation overflowed.
606 the process ID or process group given as an argument is in a
607 different session than the caller.
615 the process ID given as argument is not in use.
620 refers to a descriptor open on a terminal device (as opposed to a
621 descriptor open on a socket), a
625 can fail for the same reasons as in
631 for the reasons as stated in
638 .Xr getdtablesize 2 ,
647 constant is non portable.
648 It is provided for compatibility with AIX and Solaris.
652 system call appeared in
657 constant first appeared in