bsd/man/man2/fcntl.2

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  57 .\"     @(#)fcntl.2     8.2 (Berkeley) 1/12/94
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  59 .Dd October 2, 2008
  60 .Dt FCNTL 2
  61 .Os BSD 4.2
  62 .Sh NAME
  63 .Nm fcntl
  64 .Nd file control
  65 .Sh SYNOPSIS
  66 .Fd #include <fcntl.h>
  67 .Ft int
  68 .Fo fcntl
  69 .Fa "int fildes"
  70 .Fa "int cmd"
  71 .Fa "..."
  72 .Fc
  73 .Sh DESCRIPTION
  74 .Fn Fcntl
  75 provides for control over descriptors.
  76 The argument
  77 .Fa fildes
  78 is a descriptor to be operated on by
  79 .Fa cmd
  80 as follows:
  81 .Bl -tag -width F_WRITEBOOTSTRAPX
  82 .It Dv F_DUPFD
  83 Return a new descriptor as follows:
  84 .Pp
  85 .Bl -bullet -compact -offset 4n
  86 .It
  87 Lowest numbered available descriptor greater than or equal to
  88 .Fa arg .
  89 .It
  90 Same object references as the original descriptor.
  91 .It
  92 New descriptor shares the same file offset if the object
  93 was a file.
  94 .It
  95 Same access mode (read, write or read/write).
  96 .It
  97 Same file status flags (i.e., both file descriptors
  98 share the same file status flags).
  99 .It
 100 The close-on-exec flag associated with the new file descriptor
 101 is set to remain open across
 102 .Xr execv 2
 103 system calls.
 104 .El
 105 .It Dv F_GETFD
 106 Get the close-on-exec flag associated with the file descriptor
 107 .Fa fildes .
 108 If the low-order bit of the returned value is 0,
 109 the file will remain open across
 110 .Fn exec ,
 111 otherwise the file will be closed upon execution of
 112 .Fn exec
 113 .Fa ( arg
 114 is ignored).
 115 .It Dv F_SETFD
 116 Set the close-on-exec flag associated with
 117 .Fa fildes
 118 to the low order bit of
 119 .Fa arg
 120 (0 or 1 as above).
 121 .It Dv F_GETFL
 122 Get descriptor status flags, as described below
 123 .Fa ( arg
 124 is ignored).
 125 .It Dv F_SETFL
 126 Set descriptor status flags to
 127 .Fa arg .
 128 .It Dv F_GETOWN
 129 Get the process ID or process group
 130 currently receiving
 131 .Dv SIGIO
 132 and
 133 .Dv SIGURG
 134 signals; process groups are returned
 135 as negative values
 136 .Fa ( arg
 137 is ignored).
 138 .It Dv F_SETOWN
 139 Set the process or process group
 140 to receive
 141 .Dv SIGIO
 142 and
 143 .Dv SIGURG
 144 signals;
 145 process groups are specified by supplying
 146 .Fa arg
 147 as negative, otherwise
 148 .Fa arg
 149 is interpreted as a process ID.
 150 .It Dv F_GETPATH
 151 Get the path of the file descriptor
 152 .Fa Fildes .
 153 The argument must be a buffer of size
 154 .Sy MAXPATHLEN
 155 or greater.
 156 .It Dv F_PREALLOCATE
 157 Preallocate file storage space. Note: upon success,
 158 the space that is allocated can be the same size or
 159 larger than the space requested.
 160 .It Dv F_SETSIZE
 161 Truncate a file without zeroing space.
 162 The calling process must have root privileges.
 163 .It Dv F_RDADVISE
 164 Issue an advisory read async with no copy to user.
 165 .It Dv F_RDAHEAD
 166 Turn read ahead off/on.
 167 A zero value in
 168 .Fa arg
 169 disables read ahead.
 170 A non-zero value in
 171 .Fa arg
 172 turns read ahead on.
 173 .It Dv F_READBOOTSTRAP
 174 Read bootstrap from disk.
 175 .It Dv F_WRITEBOOTSTRAP
 176 Write bootstrap on disk.
 177 The calling process must have root privileges.
 178 .It Dv F_NOCACHE
 179 Turns data caching off/on. A non-zero value in
 180 .Fa arg
 181 turns data caching off.
 182 A value of zero in
 183 .Fa arg
 184 turns data caching on.
 185 .It Dv F_LOG2PHYS
 186 Get disk device information.
 187 Currently this only includes the
 188 disk device address that corresponds
 189 to the current file offset.
 190 .It Dv F_FULLFSYNC
 191 Does the same thing as
 192 .Xr fsync 2
 193 then asks the drive to
 194 flush all buffered data to
 195 the permanent storage device
 196 .Fa ( arg
 197 is ignored).
 198 This is currently implemented on HFS, MS-DOS (FAT),
 199 and Universal Disk Format (UDF) file systems.
 200 The operation may take quite a while to complete.
 201 Certain FireWire drives have also been known
 202 to ignore the request to flush their buffered data.
 203 .El
 204 .Pp
 205 The flags for the
 206 .Dv F_GETFL
 207 and
 208 .Dv F_SETFL
 209 commands are as follows:
 210 .Bl -tag -width O_NONBLOCKX -offset indent
 211 .It Dv O_NONBLOCK
 212 Non-blocking I/O; if no data is available to a
 213 .Xr read
 214 call, or if a
 215 .Xr write
 216 operation would block,
 217 the read or write call returns -1 with the error
 218 .Er EAGAIN .
 219 .It Dv O_APPEND
 220 Force each write to append at the end of file;
 221 corresponds to the
 222 .Dv O_APPEND
 223 flag of
 224 .Xr open 2 .
 225 .It Dv O_ASYNC
 226 Enable the
 227 .Dv SIGIO
 228 signal to be sent to the process group
 229 when I/O is possible, e.g.,
 230 upon availability of data to be read.
 231 .El
 232 .Pp
 233 Several commands are available for doing advisory file locking;
 234 they all operate on the following structure:
 235 .ne 7v
 236 .Bd -literal
 237         struct flock {
 238             off_t       l_start;    /* starting offset */
 239             off_t       l_len;      /* len = 0 means until end of file */
 240             pid_t       l_pid;      /* lock owner */
 241             short       l_type;     /* lock type: read/write, etc. */
 242             short       l_whence;   /* type of l_start */
 243         };
 244 .Ed
 245 .Pp
 246 The commands available for advisory record locking are as follows:
 247 .Bl -tag -width F_SETLKWX
 248 .It Dv F_GETLK
 249 Get the first lock that blocks the lock description pointed to by the
 250 third argument,
 251 .Fa arg ,
 252 taken as a pointer to a
 253 .Fa "struct flock"
 254 (see above).
 255 The information retrieved overwrites the information passed to
 256 .Nm fcntl
 257 in the
 258 .Fa flock
 259 structure.
 260 If no lock is found that would prevent this lock from being created,
 261 the structure is left unchanged by this function call except for the
 262 lock type which is set to
 263 .Dv F_UNLCK .
 264 .It Dv F_SETLK
 265 Set or clear a file segment lock according to the lock description
 266 pointed to by the third argument,
 267 .Fa arg ,
 268 taken as a pointer to a
 269 .Fa "struct flock"
 270 (see above).
 271 .Dv F_SETLK
 272 is used to establish shared (or read) locks
 273 .Dv (F_RDLCK)
 274 or exclusive (or write) locks,
 275 .Dv (F_WRLCK) ,
 276 as well as remove either type of lock
 277 .Dv (F_UNLCK) .
 278 If a shared or exclusive lock cannot be set,
 279 .Nm fcntl
 280 returns immediately with
 281 .Er EAGAIN .
 282 .It Dv F_SETLKW
 283 This command is the same as
 284 .Dv F_SETLK
 285 except that if a shared or exclusive lock is blocked by other locks,
 286 the process waits until the request can be satisfied.
 287 If a signal that is to be caught is received while
 288 .Nm fcntl
 289 is waiting for a region, the
 290 .Nm fcntl
 291 will be interrupted if the signal handler has not specified the
 292 .Dv SA_RESTART
 293 (see
 294 .Xr sigaction 2 ) .
 295 .El
 296 .Pp
 297 When a shared lock has been set on a segment of a file,
 298 other processes can set shared locks on that segment
 299 or a portion of it.
 300 A shared lock prevents any other process from setting an exclusive
 301 lock on any portion of the protected area.
 302 A request for a shared lock fails if the file descriptor was not
 303 opened with read access.
 304 .Pp
 305 An exclusive lock prevents any other process from setting a shared lock or
 306 an exclusive lock on any portion of the protected area.
 307 A request for an exclusive lock fails if the file was not
 308 opened with write access.
 309 .Pp
 310 The value of
 311 .Fa l_whence
 312 is
 313 .Dv SEEK_SET ,
 314 .Dv SEEK_CUR ,
 315 or
 316 .Dv SEEK_END
 317 to indicate that the relative offset,
 318 .Fa l_start
 319 bytes, will be measured from the start of the file,
 320 current position, or end of the file, respectively.
 321 The value of
 322 .Fa l_len
 323 is the number of consecutive bytes to be locked.
 324 If
 325 .Fa l_len
 326 is negative, the result is undefined.
 327 The
 328 .Fa l_pid
 329 field is only used with
 330 .Dv F_GETLK
 331 to return the process ID of the process holding a blocking lock.
 332 After a successful
 333 .Dv F_GETLK
 334 request, the value of
 335 .Fa l_whence
 336 is
 337 .Dv SEEK_SET .
 338 .Pp
 339 Locks may start and extend beyond the current end of a file,
 340 but may not start or extend before the beginning of the file.
 341 A lock is set to extend to the largest possible value of the
 342 file offset for that file if
 343 .Fa l_len
 344 is set to zero. If
 345 .Fa l_whence
 346 and
 347 .Fa l_start
 348 point to the beginning of the file, and
 349 .Fa l_len
 350 is zero, the entire file is locked.
 351 If an application wishes only to do entire file locking, the
 352 .Xr flock 2
 353 system call is much more efficient.
 354 .Pp
 355 There is at most one type of lock set for each byte in the file.
 356 Before a successful return from an
 357 .Dv F_SETLK
 358 or an
 359 .Dv F_SETLKW
 360 request when the calling process has previously existing locks
 361 on bytes in the region specified by the request,
 362 the previous lock type for each byte in the specified
 363 region is replaced by the new lock type.
 364 As specified above under the descriptions
 365 of shared locks and exclusive locks, an
 366 .Dv F_SETLK
 367 or an
 368 .Dv F_SETLKW
 369 request fails or blocks respectively when another process has existing
 370 locks on bytes in the specified region and the type of any of those
 371 locks conflicts with the type specified in the request.
 372 .Pp
 373 This interface follows the completely stupid semantics of System V and
 374 .St -p1003.1-88
 375 that require that all locks associated with a file for a given process are
 376 removed when \fIany\fP file descriptor for that file is closed by that process.
 377 This semantic means that applications must be aware of any files that
 378 a subroutine library may access.
 379 For example if an application for updating the password file locks the
 380 password file database while making the update, and then calls
 381 .Xr getpwname 3
 382 to retrieve a record,
 383 the lock will be lost because
 384 .Xr getpwname 3
 385 opens, reads, and closes the password database.
 386 The database close will release all locks that the process has
 387 associated with the database, even if the library routine never
 388 requested a lock on the database.
 389 Another minor semantic problem with this interface is that
 390 locks are not inherited by a child process created using the
 391 .Xr fork 2
 392 function.
 393 The
 394 .Xr flock 2
 395 interface has much more rational last close semantics and
 396 allows locks to be inherited by child processes.
 397 .Xr Flock 2
 398 is recommended for applications that want to ensure the integrity
 399 of their locks when using library routines or wish to pass locks
 400 to their children.
 401 Note that
 402 .Xr flock 2
 403 and
 404 .Xr fcntl 2
 405 locks may be safely used concurrently.
 406 .Pp
 407 All locks associated with a file for a given process are
 408 removed when the process terminates.
 409 .Pp
 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
 414 .Er EDEADLK
 415 error.
 416 .Pp
 417 The
 418 .Dv F_PREALLOCATE
 419 command operates on the following structure:
 420 .ne 7v
 421 .Bd -literal
 422         typedef struct fstore {
 423             u_int32_t fst_flags;      /* IN: flags word */
 424             int       fst_posmode;    /* IN: indicates offset field */
 425             off_t     fst_offset;     /* IN: start of the region */
 426             off_t     fst_length;     /* IN: size of the region */
 427             off_t     fst_bytesalloc; /* OUT: number of bytes allocated */
 428         } fstore_t;
 429 .Ed
 430 .Pp
 431 The flags (fst_flags) for the
 432 .Dv F_PREALLOCATE
 433 command are as follows:
 434 .Bl -tag -width F_ALLOCATECONTIGX -offset indent
 435 .It Dv F_ALLOCATECONTIG
 436 Allocate contiguous space.
 437 .It Dv F_ALLOCATEALL
 438 Allocate all requested space or no space at all.
 439 .El
 440 .Pp
 441 The position modes (fst_posmode) for the
 442 .Dv F_PREALLOCATE
 443 command indicate how to use the offset field.
 444 The modes are as follows:
 445 .Bl -tag -width F_PEOFPOSMODEX -offset indent
 446 .It Dv F_PEOFPOSMODE
 447 Allocate from the physical end of file.
 448 .It Dv F_VOLPOSMODE
 449 Allocate from the volume offset.
 450 .El
 451 .Pp
 452 The
 453 .Dv F_RDADVISE
 454 command operates on the following structure
 455 which holds information passed from the
 456 user to the system:
 457 .ne 7v
 458 .Bd -literal
 459         struct radvisory {
 460            off_t   ra_offset;  /* offset into the file */
 461            int     ra_count;   /* size of the read     */
 462         };
 463 .Ed
 464 .Pp
 465 The
 466 .Dv F_READBOOTSTRAP and F_WRITEBOOTSTRAP
 467 commands operate on the following structure.
 468 .ne 7v
 469 .Bd -literal
 470         typedef struct fbootstraptransfer {
 471             off_t fbt_offset;       /* IN: offset to start read/write */
 472             size_t fbt_length;      /* IN: number of bytes to transfer */
 473             void *fbt_buffer;       /* IN: buffer to be read/written */
 474         } fbootstraptransfer_t;
 475 .Ed
 476 .Pp
 477 The
 478 .Dv F_LOG2PHYS
 479 command operates on the following structure.
 480 .ne 7v
 481 .Bd -literal
 482         struct log2phys {
 483             u_int32_t   l2p_flags;              /* unused so far */
 484             off_t       l2p_contigbytes;        /* unused so far */
 485             off_t       l2p_devoffset;      /* bytes into device */
 486         };
 487 .Ed
 488 .Sh RETURN VALUES
 489 Upon successful completion, the value returned depends on
 490 .Fa cmd
 491 as follows:
 492 .Bl -tag -width F_GETOWNX -offset indent
 493 .It Dv F_DUPFD
 494 A new file descriptor.
 495 .It Dv F_GETFD
 496 Value of flag (only the low-order bit is defined).
 497 .It Dv F_GETFL
 498 Value of flags.
 499 .It Dv F_GETOWN
 500 Value of file descriptor owner.
 501 .It other
 502 Value other than -1.
 503 .El
 504 .Pp
 505 Otherwise, a value of -1 is returned and
 506 .Va errno
 507 is set to indicate the error.
 508 .Sh ERRORS
 509 The
 510 .Fn fcntl
 511 system call will fail if:
 512 .Bl -tag -width Er
 513 .\" ==========
 514 .It Bq Er EAGAIN
 515 The argument
 516 .Fa cmd
 517 is
 518 .Dv F_SETLK ,
 519 the type of lock
 520 .Fa (l_type)
 521 is a shared lock
 522 .Dv (F_RDLCK)
 523 or exclusive lock
 524 .Dv (F_WRLCK) ,
 525 and the segment of a file to be locked is already
 526 exclusive-locked by another process;
 527 or the type is an exclusive lock and some portion of the
 528 segment of a file to be locked is already shared-locked or
 529 exclusive-locked by another process.
 530 .It Bq Er EACCESS
 531 The argument
 532 .Fa cmd
 533 is either
 534 .Dv F_SETSIZE
 535 or
 536 .Dv F_WRITEBOOTSTRAP
 537 and the calling process does not have root privileges.
 538 .\" ==========
 539 .It Bq Er EBADF
 540 .Fa Fildes
 541 is not a valid open file descriptor.
 542 .Pp
 543 The argument
 544 .Fa cmd
 545 is
 546 .Dv F_SETLK
 547 or
 548 .Dv F_SETLKW ,
 549 the type of lock
 550 .Fa (l_type)
 551 is a shared lock
 552 .Dv (F_RDLCK) ,
 553 and
 554 .Fa fildes
 555 is not a valid file descriptor open for reading.
 556 .Pp
 557 The argument
 558 .Fa cmd
 559 is
 560 .Dv F_SETLK
 561 or
 562 .Dv F_SETLKW ,
 563 the type of lock
 564 .Fa (l_type)
 565 is an exclusive lock
 566 .Dv (F_WRLCK) ,
 567 and
 568 .Fa fildes
 569 is not a valid file descriptor open for writing.
 570 .Pp
 571 The argument
 572 .Fa cmd
 573 is
 574 .Dv F_PREALLOCATE
 575 and the calling process does not have
 576 file write permission.
 577 .Pp
 578 The argument
 579 .Fa cmd
 580 is
 581 .Dv F_LOG2PHYS
 582 and
 583 .Fa fildes
 584 is not a valid file descriptor open for reading.
 585 .\" ==========
 586 .It Bq Er EDEADLK
 587 The argument
 588 .Fa cmd
 589 is
 590 .Dv F_SETLKW ,
 591 and a deadlock condition was detected.
 592 .\" ==========
 593 .It Bq Er EINTR
 594 The argument
 595 .Fa cmd
 596 is
 597 .Dv F_SETLKW ,
 598 and the function was interrupted by a signal.
 599 .\" ==========
 600 .It Bq Er EINVAL
 601 .Fa Cmd
 602 is
 603 .Dv F_DUPFD
 604 and
 605 .Fa arg
 606 is negative or greater than the maximum allowable number
 607 (see
 608 .Xr getdtablesize 2 ) .
 609 .Pp
 610 The argument
 611 .Fa cmd
 612 is
 613 .Dv F_GETLK ,
 614 .Dv F_SETLK ,
 615 or
 616 .Dv F_SETLKW
 617 and the data to which
 618 .Fa arg
 619 points is not valid, or
 620 .Fa fildes
 621 refers to a file that does not support locking.
 622 .Pp
 623 The argument
 624 .Fa cmd
 625 is
 626 .Dv F_PREALLOCATE
 627 and the
 628 .Fa fst_posmode
 629 is not a valid mode,
 630 or when
 631 .Dv F_PEOFPOSMODE
 632 is set and
 633 .Fa fst_offset
 634 is a non-zero value,
 635 or when
 636 .Dv F_VOLPOSMODE
 637 is set and
 638 .Fa fst_offset
 639 is a negative or zero value.
 640 .Pp
 641 The argument
 642 .Fa cmd
 643 is either
 644 .Dv F_READBOOTSTRAP
 645 or
 646 .Dv F_WRITEBOOTSTRAP
 647 and the operation was attempted on a non-HFS disk type.
 648 .\" ==========
 649 .It Bq Er EMFILE
 650 .Fa Cmd
 651 is
 652 .Dv F_DUPFD
 653 and the maximum allowed number of file descriptors are currently
 654 open.
 655 .\" ==========
 656 .It Bq Er EMFILE
 657 The argument
 658 .Fa cmd
 659 is
 660 .Dv F_DUPED
 661 and the maximum number of file descriptors permitted for the
 662 process are already in use,
 663 or no file descriptors greater than or equal to
 664 .Fa arg
 665 are available.
 666 .\" ==========
 667 .It Bq Er ENOLCK
 668 The argument
 669 .Fa cmd
 670 is
 671 .Dv F_SETLK
 672 or
 673 .Dv F_SETLKW ,
 674 and satisfying the lock or unlock request would result in the
 675 number of locked regions in the system exceeding a system-imposed limit.
 676 .\" ==========
 677 .It Bq Er EOVERFLOW
 678 A return value would overflow its representation.
 679 For example,
 680 .Fa cmd
 681 is F_GETLK, F_SETLK, or F_SETLKW
 682 and the smallest (or, if l_len is non-zero, the largest) offset
 683 of a byte in the requested segment
 684 will not fit in an object of type off_t.
 685 .\" ==========
 686 .It Bq Er ESRCH
 687 .Fa Cmd
 688 is
 689 .Dv F_SETOWN
 690 and
 691 the process ID given as argument is not in use.
 692 .El
 693 .Sh SEE ALSO
 694 .Xr close 2 ,
 695 .Xr execve 2 ,
 696 .Xr flock 2 ,
 697 .Xr getdtablesize 2 ,
 698 .Xr open 2 ,
 699 .Xr sigaction 3
 700 .Sh HISTORY
 701 The
 702 .Fn fcntl
 703 function call appeared in
 704 .Bx 4.2 .