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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_GETOWNX
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 associated with the new file descriptor
77 is set to remain open across
82 Get the close-on-exec flag associated with the file descriptor
86 If the returned value ANDed with
89 the file will remain open across
91 otherwise the file will be closed upon execution of
96 Set the close-on-exec flag associated with
106 Get descriptor status flags, as described below
110 Set descriptor status flags to
113 Get the process ID or process group
118 signals; process groups are returned
123 Set the process or process group
129 process groups are specified by supplying
131 as negative, otherwise
133 is interpreted as a process ID.
140 flags are as follows:
141 .Bl -tag -width O_NONBLOCKX
143 Non-blocking I/O; if no data is available to a
147 operation would block,
148 the read or write call returns -1 with the error
151 Force each write to append at the end of file;
157 Minimize or eliminate the cache effects of reading and writing.
159 will attempt to avoid caching the data you read or write.
161 avoid caching the data, it will minimize the impact the data has on the cache.
162 Use of this flag can drastically reduce performance if not used with care.
166 signal to be sent to the process group
167 when I/O is possible, e.g.,
168 upon availability of data to be read.
171 Several commands are available for doing advisory file locking;
172 they all operate on the following structure:
175 off_t l_start; /* starting offset */
176 off_t l_len; /* len = 0 means until end of file */
177 pid_t l_pid; /* lock owner */
178 short l_type; /* lock type: read/write, etc. */
179 short l_whence; /* type of l_start */
182 The commands available for advisory record locking are as follows:
183 .Bl -tag -width F_SETLKWX
185 Get the first lock that blocks the lock description pointed to by the
188 taken as a pointer to a
191 The information retrieved overwrites the information passed to
196 If no lock is found that would prevent this lock from being created,
197 the structure is left unchanged by this system call except for the
198 lock type which is set to
201 Set or clear a file segment lock according to the lock description
202 pointed to by the third argument,
204 taken as a pointer to a
208 is used to establish shared (or read) locks
210 or exclusive (or write) locks,
212 as well as remove either type of lock
214 If a shared or exclusive lock cannot be set,
216 returns immediately with
219 This command is the same as
221 except that if a shared or exclusive lock is blocked by other locks,
222 the process waits until the request can be satisfied.
223 If a signal that is to be caught is received while
225 is waiting for a region, the
227 will be interrupted if the signal handler has not specified the
233 When a shared lock has been set on a segment of a file,
234 other processes can set shared locks on that segment
236 A shared lock prevents any other process from setting an exclusive
237 lock on any portion of the protected area.
238 A request for a shared lock fails if the file descriptor was not
239 opened with read access.
241 An exclusive lock prevents any other process from setting a shared lock or
242 an exclusive lock on any portion of the protected area.
243 A request for an exclusive lock fails if the file was not
244 opened with write access.
253 to indicate that the relative offset,
255 bytes, will be measured from the start of the file,
256 current position, or end of the file, respectively.
259 is the number of consecutive bytes to be locked.
264 means end edge of the region.
267 field is only used with
269 to return the process ID of the process holding a blocking lock.
272 request, the value of
277 Locks may start and extend beyond the current end of a file,
278 but may not start or extend before the beginning of the file.
279 A lock is set to extend to the largest possible value of the
280 file offset for that file if
287 point to the beginning of the file, and
289 is zero, the entire file is locked.
290 If an application wishes only to do entire file locking, the
292 system call is much more efficient.
294 There is at most one type of lock set for each byte in the file.
295 Before a successful return from an
299 request when the calling process has previously existing locks
300 on bytes in the region specified by the request,
301 the previous lock type for each byte in the specified
302 region is replaced by the new lock type.
303 As specified above under the descriptions
304 of shared locks and exclusive locks, an
308 request fails or blocks respectively when another process has existing
309 locks on bytes in the specified region and the type of any of those
310 locks conflicts with the type specified in the request.
312 This interface follows the completely stupid semantics of System V and
314 that require that all locks associated with a file for a given process are
317 file descriptor for that file is closed by that process.
318 This semantic means that applications must be aware of any files that
319 a subroutine library may access.
320 For example if an application for updating the password file locks the
321 password file database while making the update, and then calls
323 to retrieve a record,
324 the lock will be lost because
326 opens, reads, and closes the password database.
327 The database close will release all locks that the process has
328 associated with the database, even if the library routine never
329 requested a lock on the database.
330 Another minor semantic problem with this interface is that
331 locks are not inherited by a child process created using the
336 interface has much more rational last close semantics and
337 allows locks to be inherited by child processes.
340 system call is recommended for applications that want to ensure the integrity
341 of their locks when using library routines or wish to pass locks
349 locks are compatible.
350 Processes using different locking interfaces can cooperate
351 over the same file safely.
352 However, only one of such interfaces should be used within
354 If a file is locked by a process through
356 any record within the file will be seen as locked
357 from the viewpoint of another process using
366 if the process holding a blocking lock previously locked the
370 All locks associated with a file for a given process are
371 removed when the process terminates.
373 All locks obtained before a call to
375 remain in effect until the new program releases them.
376 If the new program does not know about the locks, they will not be
377 released until the program exits.
379 A potential for deadlock occurs if a process controlling a locked region
380 is put to sleep by attempting to lock the locked region of another process.
381 This implementation detects that sleeping until a locked region is unlocked
382 would cause a deadlock and fails with an
386 Upon successful completion, the value returned depends on
389 .Bl -tag -width F_GETOWNX -offset indent
391 A new file descriptor.
393 Value of flag (only the low-order bit is defined).
397 Value of file descriptor owner.
402 Otherwise, a value of -1 is returned and
404 is set to indicate the error.
408 system call will fail if:
421 and the segment of a file to be locked is already
422 exclusive-locked by another process;
423 or the type is an exclusive lock and some portion of the
424 segment of a file to be locked is already shared-locked or
425 exclusive-locked by another process.
430 is not a valid open file descriptor.
444 is not a valid file descriptor open for reading.
458 is not a valid file descriptor open for writing.
464 and a deadlock condition was detected.
470 and the system call was interrupted by a signal.
479 is negative or greater than the maximum allowable number
481 .Xr getdtablesize 2 ) .
490 and the data to which
498 and the maximum number of file descriptors permitted for the
499 process are already in use,
500 or no file descriptors greater than or equal to
510 and satisfying the lock or unlock request would result in the
511 number of locked regions in the system exceeding a system-imposed limit.
522 refers to a file for which locking is not supported.
533 calculation overflowed.
541 the process ID or process group given as an argument is in a
542 different session than the caller.
550 the process ID given as argument is not in use.
555 refers to a descriptor open on a terminal device (as opposed to a
556 descriptor open on a socket), a
560 can fail for the same reasons as in
566 for the reasons as stated in
572 .Xr getdtablesize 2 ,
581 system call appeared in