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       fcntl - manipulate file descriptor


       #include <unistd.h>
       #include <fcntl.h>

       int fcntl(int fd, int cmd, ... /* arg */ );


       fcntl()  performs  one  of  the operations described below on the open file descriptor fd.
       The operation is determined by cmd.

       fcntl() can take an optional third argument.  Whether or not this argument is required  is
       determined  by cmd.  The required argument type is indicated in parentheses after each cmd
       name (in most cases, the required type is int, and we identify the argument using the name
       arg), or void is specified if the argument is not required.

   Duplicating a file descriptor
       F_DUPFD (int)
              Find the lowest numbered available file descriptor greater than or equal to arg and
              make it be a copy of fd.  This is different from dup2(2), which  uses  exactly  the
              descriptor specified.

              On success, the new descriptor is returned.

              See dup(2) for further details.

       F_DUPFD_CLOEXEC (int; since Linux 2.6.24)
              As  for  F_DUPFD,  but  additionally  set  the close-on-exec flag for the duplicate
              descriptor.  Specifying this flag permits a program to avoid an additional  fcntl()
              F_SETFD  operation to set the FD_CLOEXEC flag.  For an explanation of why this flag
              is useful, see the description of O_CLOEXEC in open(2).

   File descriptor flags
       The following commands manipulate the flags associated with a file descriptor.  Currently,
       only  one such flag is defined: FD_CLOEXEC, the close-on-exec flag.  If the FD_CLOEXEC bit
       is 0, the file descriptor will remain open across  an  execve(2),  otherwise  it  will  be

       F_GETFD (void)
              Read the file descriptor flags; arg is ignored.

       F_SETFD (int)
              Set the file descriptor flags to the value specified by arg.

   File status flags
       Each open file description has certain associated status flags, initialized by open(2) and
       possibly  modified  by  fcntl().   Duplicated  file   descriptors   (made   with   dup(2),
       fcntl(F_DUPFD), fork(2), etc.) refer to the same open file description, and thus share the
       same file status flags.

       The file status flags and their semantics are described in open(2).

       F_GETFL (void)
              Get the file access mode and the file status flags; arg is ignored.

       F_SETFL (int)
              Set the file status flags  to  the  value  specified  by  arg.   File  access  mode
              (O_RDONLY,  O_WRONLY,  O_RDWR)  and  file  creation  flags  (i.e., O_CREAT, O_EXCL,
              O_NOCTTY, O_TRUNC) in arg are ignored.  On Linux this command can change  only  the
              O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.

   Advisory locking
       F_GETLK,  F_SETLK and F_SETLKW are used to acquire, release, and test for the existence of
       record locks (also known as file-segment or file-region locks).  The third argument, lock,
       is a pointer to a structure that has at least the following fields (in unspecified order).

           struct flock {
               short l_type;    /* Type of lock: F_RDLCK,
                                   F_WRLCK, F_UNLCK */
               short l_whence;  /* How to interpret l_start:
                                   SEEK_SET, SEEK_CUR, SEEK_END */
               off_t l_start;   /* Starting offset for lock */
               off_t l_len;     /* Number of bytes to lock */
               pid_t l_pid;     /* PID of process blocking our lock
                                   (F_GETLK only) */

       The  l_whence,  l_start,  and l_len fields of this structure specify the range of bytes we
       wish to lock.  Bytes past the end of the file may be locked,  but  not  bytes  before  the
       start of the file.

       l_start  is  the  starting offset for the lock, and is interpreted relative to either: the
       start of the file (if l_whence is SEEK_SET); the  current  file  offset  (if  l_whence  is
       SEEK_CUR);  or  the  end  of  the file (if l_whence is SEEK_END).  In the final two cases,
       l_start can be a negative number provided the offset does not lie before the start of  the

       l_len specifies the number of bytes to be locked.  If l_len is positive, then the range to
       be locked covers bytes l_start up to and  including  l_start+l_len-1.   Specifying  0  for
       l_len  has  the  special  meaning:  lock  all  bytes starting at the location specified by
       l_whence and l_start through to the end of file, no matter how large the file grows.

       POSIX.1-2001 allows (but does not require) an implementation to support a  negative  l_len
       value;  if l_len is negative, the interval described by lock covers bytes l_start+l_len up
       to and including l_start-1.  This is supported by Linux since kernel versions  2.4.21  and

       The  l_type  field  can  be  used to place a read (F_RDLCK) or a write (F_WRLCK) lock on a
       file.  Any number of processes may hold a read lock (shared lock) on a  file  region,  but
       only  one  process may hold a write lock (exclusive lock).  An exclusive lock excludes all
       other locks, both shared and exclusive.  A single process can hold only one type  of  lock
       on  a file region; if a new lock is applied to an already-locked region, then the existing
       lock is converted to  the  new  lock  type.   (Such  conversions  may  involve  splitting,
       shrinking, or coalescing with an existing lock if the byte range specified by the new lock
       does not precisely coincide with the range of the existing lock.)

       F_SETLK (struct flock *)
              Acquire a lock (when l_type is F_RDLCK or F_WRLCK) or release a lock  (when  l_type
              is  F_UNLCK)  on  the bytes specified by the l_whence, l_start, and l_len fields of
              lock.  If a conflicting lock is held by another process, this call returns  -1  and
              sets errno to EACCES or EAGAIN.

       F_SETLKW (struct flock *)
              As  for  F_SETLK, but if a conflicting lock is held on the file, then wait for that
              lock to be released.  If a signal  is  caught  while  waiting,  then  the  call  is
              interrupted  and  (after the signal handler has returned) returns immediately (with
              return value -1 and errno set to EINTR; see signal(7)).

       F_GETLK (struct flock *)
              On input to this call, lock describes a lock we would like to place  on  the  file.
              If  the  lock  could  be  placed,  fcntl()  does not actually place it, but returns
              F_UNLCK in the l_type field of lock and leaves the other fields  of  the  structure
              unchanged.  If one or more incompatible locks would prevent this lock being placed,
              then fcntl() returns details about one of these  locks  in  the  l_type,  l_whence,
              l_start,  and  l_len  fields  of  lock  and sets l_pid to be the PID of the process
              holding that lock.

       In order to place a read lock, fd must be open for reading.  In order  to  place  a  write
       lock, fd must be open for writing.  To place both types of lock, open a file read-write.

       As  well  as being removed by an explicit F_UNLCK, record locks are automatically released
       when the process terminates or if it closes any file descriptor referring  to  a  file  on
       which  locks  are held.  This is bad: it means that a process can lose the locks on a file
       like /etc/passwd or /etc/mtab when for some reason a library  function  decides  to  open,
       read and close it.

       Record locks are not inherited by a child created via fork(2), but are preserved across an

       Because of the buffering performed by the stdio(3) library, the use of record locking with
       routines in that package should be avoided; use read(2) and write(2) instead.

   Mandatory locking
       (Non-POSIX.)  The above record locks may be either advisory or mandatory, and are advisory
       by default.

       Advisory locks are not enforced and are useful only between cooperating processes.

       Mandatory locks are enforced for  all  processes.   If  a  process  tries  to  perform  an
       incompatible  access (e.g., read(2) or write(2)) on a file region that has an incompatible
       mandatory lock, then the result depends upon whether the O_NONBLOCK flag  is  enabled  for
       its  open  file  description.   If the O_NONBLOCK flag is not enabled, then system call is
       blocked until the lock is removed or converted to a  mode  that  is  compatible  with  the
       access.   If  the  O_NONBLOCK  flag  is enabled, then the system call fails with the error

       To make use of mandatory locks, mandatory locking must be enabled both on  the  filesystem
       that contains the file to be locked, and on the file itself.  Mandatory locking is enabled
       on a filesystem using the "-o mand" option  to  mount(8),  or  the  MS_MANDLOCK  flag  for
       mount(2).  Mandatory locking is enabled on a file by disabling group execute permission on
       the file and enabling the set-group-ID permission bit (see chmod(1) and chmod(2)).

       The Linux implementation of mandatory locking is unreliable.  See BUGS below.

   Managing signals
       F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG and F_SETSIG are used to manage I/O
       availability signals:

       F_GETOWN (void)
              Return (as the function result) the process ID or process group currently receiving
              SIGIO and SIGURG signals for  events  on  file  descriptor  fd.   Process  IDs  are
              returned as positive values; process group IDs are returned as negative values (but
              see BUGS below).  arg is ignored.

       F_SETOWN (int)
              Set the process ID or process group ID that will receive SIGIO and  SIGURG  signals
              for events on file descriptor fd to the ID given in arg.  A process ID is specified
              as a positive value; a process group ID is specified as  a  negative  value.   Most
              commonly,  the  calling  process  specifies  itself  as  the owner (that is, arg is
              specified as getpid(2)).

              If you set the O_ASYNC status flag on  a  file  descriptor  by  using  the  F_SETFL
              command  of  fcntl(),  a  SIGIO  signal  is  sent  whenever input or output becomes
              possible on that file descriptor.  F_SETSIG can be used to  obtain  delivery  of  a
              signal  other  than  SIGIO.   If  this  permission  check fails, then the signal is
              silently discarded.

              Sending a signal to the owner process (group) specified by F_SETOWN is  subject  to
              the same permissions checks as are described for kill(2), where the sending process
              is the one that employs F_SETOWN (but see BUGS below).

              If the file descriptor fd refers to a socket, F_SETOWN also selects  the  recipient
              of  SIGURG signals that are delivered when out-of-band data arrives on that socket.
              (SIGURG is sent in any situation where select(2) would report the socket as  having
              an "exceptional condition".)

              The following was true in 2.6.x kernels up to and including kernel 2.6.11:

                     If  a  nonzero value is given to F_SETSIG in a multithreaded process running
                     with a threading library that supports thread groups (e.g.,  NPTL),  then  a
                     positive value given to F_SETOWN has a different meaning: instead of being a
                     process ID identifying a whole process, it is  a  thread  ID  identifying  a
                     specific thread within a process.  Consequently, it may be necessary to pass
                     F_SETOWN the result of  gettid(2)  instead  of  getpid(2)  to  get  sensible
                     results when F_SETSIG is used.  (In current Linux threading implementations,
                     a main thread's thread ID is the same as its process ID.  This means that  a
                     single-threaded  program  can  equally  use  gettid(2)  or getpid(2) in this
                     scenario.)  Note, however, that the statements  in  this  paragraph  do  not
                     apply  to the SIGURG signal generated for out-of-band data on a socket: this
                     signal is always sent to either a process or a process group,  depending  on
                     the value given to F_SETOWN.

              The above behavior was accidentally dropped in Linux 2.6.12, and won't be restored.
              From Linux 2.6.32 onward, use F_SETOWN_EX to target SIGIO and SIGURG signals  at  a
              particular thread.

       F_GETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
              Return  the  current  file  descriptor  owner  settings  as  defined  by a previous
              F_SETOWN_EX operation.  The information is returned in the structure pointed to  by
              arg, which has the following form:

                  struct f_owner_ex {
                      int   type;
                      pid_t pid;

              The   type  field  will  have  one  of  the  values  F_OWNER_TID,  F_OWNER_PID,  or
              F_OWNER_PGRP.  The pid field is  a  positive  integer  representing  a  thread  ID,
              process ID, or process group ID.  See F_SETOWN_EX for more details.

       F_SETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
              This operation performs a similar task to F_SETOWN.  It allows the caller to direct
              I/O availability signals to a specific thread,  process,  or  process  group.   The
              caller  specifies the target of signals via arg, which is a pointer to a f_owner_ex
              structure.  The type field has one of the following values, which define how pid is

                     Send  the signal to the thread whose thread ID (the value returned by a call
                     to clone(2) or gettid(2)) is specified in pid.

                     Send the signal to the process whose ID is specified in pid.

                     Send the signal to the process group whose ID is specified  in  pid.   (Note
                     that,  unlike  with  F_SETOWN, a process group ID is specified as a positive
                     value here.)

       F_GETSIG (void)
              Return (as the function result) the  signal  sent  when  input  or  output  becomes
              possible.   A value of zero means SIGIO is sent.  Any other value (including SIGIO)
              is the signal sent instead, and in this case additional info is  available  to  the
              signal handler if installed with SA_SIGINFO.  arg is ignored.

       F_SETSIG (int)
              Set  the  signal  sent  when input or output becomes possible to the value given in
              arg.  A value of zero means to send the default  SIGIO  signal.   Any  other  value
              (including  SIGIO)  is the signal to send instead, and in this case additional info
              is available to the signal handler if installed with SA_SIGINFO.

              By using F_SETSIG with a nonzero value,  and  setting  SA_SIGINFO  for  the  signal
              handler  (see  sigaction(2)),  extra  information about I/O events is passed to the
              handler in a siginfo_t structure.  If the si_code field  indicates  the  source  is
              SI_SIGIO,  the  si_fd  field  gives  the file descriptor associated with the event.
              Otherwise, there is no indication which  file  descriptors  are  pending,  and  you
              should  use  the  usual mechanisms (select(2), poll(2), read(2) with O_NONBLOCK set
              etc.) to determine which file descriptors are available for I/O.

              By selecting a real time signal (value >= SIGRTMIN), multiple  I/O  events  may  be
              queued  using the same signal numbers.  (Queuing is dependent on available memory).
              Extra information is available if SA_SIGINFO is set  for  the  signal  handler,  as

              Note  that  Linux  imposes  a  limit on the number of real-time signals that may be
              queued to a process (see getrlimit(2) and signal(7)) and if this limit is  reached,
              then  the  kernel  reverts to delivering SIGIO, and this signal is delivered to the
              entire process rather than to a specific thread.

       Using these mechanisms, a program can  implement  fully  asynchronous  I/O  without  using
       select(2) or poll(2) most of the time.

       The  use  of  O_ASYNC,  F_GETOWN,  F_SETOWN  is  specific  to BSD and Linux.  F_GETOWN_EX,
       F_SETOWN_EX, F_GETSIG, and F_SETSIG are Linux-specific.  POSIX has  asynchronous  I/O  and
       the aio_sigevent structure to achieve similar things; these are also available in Linux as
       part of the GNU C Library (Glibc).

       F_SETLEASE and F_GETLEASE (Linux 2.4 onward) are used (respectively) to  establish  a  new
       lease,  and  retrieve  the  current lease, on the open file description referred to by the
       file descriptor fd.  A file lease provides a mechanism whereby  the  process  holding  the
       lease  (the  "lease  holder")  is  notified (via delivery of a signal) when a process (the
       "lease breaker") tries to open(2) or  truncate(2)  the  file  referred  to  by  that  file

       F_SETLEASE (int)
              Set  or remove a file lease according to which of the following values is specified
              in the integer arg:

                     Take out a read lease.  This will cause the calling process to  be  notified
                     when  the  file  is opened for writing or is truncated.  A read lease can be
                     placed only on a file descriptor that is opened read-only.

                     Take out a write lease.  This will cause the caller to be notified when  the
                     file is opened for reading or writing or is truncated.  A write lease may be
                     placed on a file only if there are no other open file  descriptors  for  the

                     Remove our lease from the file.

       Leases  are  associated  with  an  open  file  description (see open(2)).  This means that
       duplicate file descriptors (created by, for example, fork(2) or dup(2)) refer to the  same
       lease,  and  this  lease  may  be  modified  or  released  using any of these descriptors.
       Furthermore, the lease is released by either an explicit F_UNLCK operation on any of these
       duplicate descriptors, or when all such descriptors have been closed.

       Leases  may  be  taken  out only on regular files.  An unprivileged process may take out a
       lease only on a file whose UID (owner) matches the  filesystem  UID  of  the  process.   A
       process with the CAP_LEASE capability may take out leases on arbitrary files.

       F_GETLEASE (void)
              Indicates what type of lease is associated with the file descriptor fd by returning
              either F_RDLCK, F_WRLCK, or F_UNLCK, indicating, respectively, a  read  lease  ,  a
              write lease, or no lease.  arg is ignored.

       When  a  process  (the  "lease breaker") performs an open(2) or truncate(2) that conflicts
       with a lease established via F_SETLEASE, the system call is blocked by the kernel and  the
       kernel  notifies  the  lease  holder by sending it a signal (SIGIO by default).  The lease
       holder should respond to receipt of this signal by doing whatever cleanup is  required  in
       preparation for the file to be accessed by another process (e.g., flushing cached buffers)
       and then either remove or downgrade its lease.   A  lease  is  removed  by  performing  an
       F_SETLEASE command specifying arg as F_UNLCK.  If the lease holder currently holds a write
       lease on the file, and the lease breaker is opening the  file  for  reading,  then  it  is
       sufficient  for  the lease holder to downgrade the lease to a read lease.  This is done by
       performing an F_SETLEASE command specifying arg as F_RDLCK.

       If the lease holder fails to downgrade or remove the lease within the  number  of  seconds
       specified  in /proc/sys/fs/lease-break-time then the kernel forcibly removes or downgrades
       the lease holder's lease.

       Once a lease break has been initiated, F_GETLEASE returns the target  lease  type  (either
       F_RDLCK  or  F_UNLCK,  depending on what would be compatible with the lease breaker) until
       the lease holder voluntarily downgrades or removes the lease or the kernel  forcibly  does
       so after the lease break timer expires.

       Once  the  lease  has been voluntarily or forcibly removed or downgraded, and assuming the
       lease breaker has not unblocked its system call, the kernel permits  the  lease  breaker's
       system call to proceed.

       If  the lease breaker's blocked open(2) or truncate(2) is interrupted by a signal handler,
       then the system call fails with the error EINTR,  but  the  other  steps  still  occur  as
       described  above.   If the lease breaker is killed by a signal while blocked in open(2) or
       truncate(2), then the other steps still occur as described above.  If  the  lease  breaker
       specifies  the  O_NONBLOCK flag when calling open(2), then the call immediately fails with
       the error EWOULDBLOCK, but the other steps still occur as described above.

       The default signal used to notify the lease holder is SIGIO, but this can be changed using
       the  F_SETSIG command to fcntl().  If a F_SETSIG command is performed (even one specifying
       SIGIO), and the signal handler is established using  SA_SIGINFO,  then  the  handler  will
       receive a siginfo_t structure as its second argument, and the si_fd field of this argument
       will hold the descriptor of the leased file that has been  accessed  by  another  process.
       (This is useful if the caller holds leases against multiple files).

   File and directory change notification (dnotify)
       F_NOTIFY (int)
              (Linux 2.4 onward) Provide notification when the directory referred to by fd or any
              of the files that it contains is changed.  The events to be notified are  specified
              in  arg,  which  is  a  bit  mask  specified  by ORing together zero or more of the
              following bits:

              DN_ACCESS   A file was accessed (read, pread, readv)
              DN_MODIFY   A file was modified (write, pwrite, writev, truncate, ftruncate).
              DN_CREATE   A file was created (open, creat, mknod, mkdir, link, symlink, rename).
              DN_DELETE   A file was unlinked (unlink, rename to another directory, rmdir).
              DN_RENAME   A file was renamed within this directory (rename).
              DN_ATTRIB   The attributes of a file were changed (chown, chmod, utime[s]).

              (In order to obtain these definitions, the _GNU_SOURCE feature test macro  must  be
              defined before including any header files.)

              Directory   notifications   are  normally  "one-shot",  and  the  application  must
              reregister to receive further notifications.   Alternatively,  if  DN_MULTISHOT  is
              included in arg, then notification will remain in effect until explicitly removed.

              A  series of F_NOTIFY requests is cumulative, with the events in arg being added to
              the set already monitored.  To disable notification of all events, make an F_NOTIFY
              call specifying arg as 0.

              Notification  occurs  via  delivery  of a signal.  The default signal is SIGIO, but
              this can be changed using the F_SETSIG command to fcntl().  In the latter case, the
              signal  handler  receives  a  siginfo_t  structure  as  its second argument (if the
              handler was established using SA_SIGINFO) and the si_fd  field  of  this  structure
              contains  the  file  descriptor  which  generated  the  notification  (useful  when
              establishing notification on multiple directories).

              Especially when  using  DN_MULTISHOT,  a  real  time  signal  should  be  used  for
              notification, so that multiple notifications can be queued.

              NOTE:  New  applications  should  use the inotify interface (available since kernel
              2.6.13), which provides a much superior interface for  obtaining  notifications  of
              filesystem events.  See inotify(7).

   Changing the capacity of a pipe
       F_SETPIPE_SZ (int; since Linux 2.6.35)
              Change  the  capacity  of  the pipe referred to by fd to be at least arg bytes.  An
              unprivileged process can adjust the pipe capacity to any value between  the  system
              page  size  and  the  limit  defined  in  /proc/sys/fs/pipe-max-size (see proc(5)).
              Attempts to set the pipe capacity below the page size are silently  rounded  up  to
              the  page size.  Attempts by an unprivileged process to set the pipe capacity above
              the limit in /proc/sys/fs/pipe-max-size yield the error EPERM; a privileged process
              (CAP_SYS_RESOURCE)  can  override  the  limit.   When allocating the buffer for the
              pipe, the kernel may use a capacity larger than arg, if that is convenient for  the
              implementation.    The   F_GETPIPE_SZ  operation  returns  the  actual  size  used.
              Attempting to set the pipe  capacity  smaller  than  the  amount  of  buffer  space
              currently used to store data produces the error EBUSY.

       F_GETPIPE_SZ (void; since Linux 2.6.35)
              Return (as the function result) the capacity of the pipe referred to by fd.


       For a successful call, the return value depends on the operation:

       F_DUPFD  The new descriptor.

       F_GETFD  Value of file descriptor flags.

       F_GETFL  Value of file status flags.

                Type of lease held on file descriptor.

       F_GETOWN Value of descriptor owner.

       F_GETSIG Value of signal sent when read or write becomes possible, or zero for traditional
                SIGIO behavior.

                The pipe capacity.

       All other commands

       On error, -1 is returned, and errno is set appropriately.


              Operation is prohibited by locks held by other processes.

       EAGAIN The operation is prohibited because the file  has  been  memory-mapped  by  another

       EBADF  fd  is  not an open file descriptor, or the command was F_SETLK or F_SETLKW and the
              file descriptor open mode doesn't match with the type of lock requested.

              It was detected that the specified F_SETLKW command would cause a deadlock.

       EFAULT lock is outside your accessible address space.

       EINTR  For F_SETLKW, the command was interrupted by a signal; see signal(7).  For  F_GETLK
              and F_SETLK, the command was interrupted by a signal before the lock was checked or
              acquired.  Most likely when locking a remote file (e.g., locking over NFS), but can
              sometimes happen locally.

       EINVAL For  F_DUPFD,  arg is negative or is greater than the maximum allowable value.  For
              F_SETSIG, arg is not an allowable signal number.

       EMFILE For F_DUPFD, the process already has the maximum number of file descriptors open.

       ENOLCK Too many segment locks open, lock table is  full,  or  a  remote  locking  protocol
              failed (e.g., locking over NFS).

       EPERM  Attempted  to  clear the O_APPEND flag on a file that has the append-only attribute


       SVr4, 4.3BSD, POSIX.1-2001.  Only  the  operations  F_DUPFD,  F_GETFD,  F_SETFD,  F_GETFL,
       F_SETFL, F_GETLK, F_SETLK and F_SETLKW, are specified in POSIX.1-2001.

       F_GETOWN  and  F_SETOWN  are specified in POSIX.1-2001.  (To get their definitions, define
       BSD_SOURCE, or _XOPEN_SOURCE with the value 500 or greater, or define _POSIX_C_SOURCE with
       the value 200809L or greater.)

       F_DUPFD_CLOEXEC   is   specified   in  POSIX.1-2008.   (To  get  this  definition,  define
       _POSIX_C_SOURCE with the value 200809L or greater, or _XOPEN_SOURCE with the value 700  or

       F_GETLEASE, and F_SETLEASE are Linux-specific.  (Define the _GNU_SOURCE  macro  to  obtain
       these definitions.)


       The  original  Linux fcntl() system call was not designed to handle large file offsets (in
       the flock structure).  Consequently, an fcntl64() system call was added in Linux 2.4.  The
       newer   system  call  employs  a  different  structure  for  file  locking,  flock64,  and
       corresponding commands, F_GETLK64, F_SETLK64, and F_SETLKW64.  However, these details  can
       be  ignored  by  applications  using  glibc,  whose fcntl() wrapper function transparently
       employs the more recent system call where it is available.

       The errors returned by dup2(2) are different from those returned by F_DUPFD.

       Since kernel 2.0, there is no interaction between the types of lock placed by flock(2) and

       Several  systems have more fields in struct flock such as, for example, l_sysid.  Clearly,
       l_pid alone is not going to be very useful if the process holding the lock may live  on  a
       different machine.


       A  limitation  of  the  Linux system call conventions on some architectures (notably i386)
       means that if a (negative) process group ID to be returned by F_GETOWN falls in the  range
       -1  to  -4095,  then  the  return value is wrongly interpreted by glibc as an error in the
       system call; that is, the return value of fcntl() will be -1, and errno will  contain  the
       (positive)  process  group  ID.   The  Linux-specific  F_GETOWN_EX  operation  avoids this
       problem.  Since glibc version 2.11, glibc makes the kernel F_GETOWN problem  invisible  by
       implementing F_GETOWN using F_GETOWN_EX.

       In  Linux  2.4  and earlier, there is bug that can occur when an unprivileged process uses
       F_SETOWN to specify the owner of a socket file descriptor as a process (group) other  than
       the  caller.   In  this case, fcntl() can return -1 with errno set to EPERM, even when the
       owner process (group) is one that the caller has permission to send signals  to.   Despite
       this  error  return,  the  file  descriptor  owner is set, and signals will be sent to the

       The implementation of mandatory locking in all known versions of Linux is subject to  race
       conditions  which  render  it  unreliable:  a  write(2) call that overlaps with a lock may
       modify data after the mandatory lock is acquired; a read(2) call that overlaps with a lock
       may  detect  changes to data that were made only after a write lock was acquired.  Similar
       races exist between mandatory locks and mmap(2).  It is therefore inadvisable to  rely  on
       mandatory locking.


       dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), feature_test_macros(7)

       locks.txt,  mandatory-locking.txt,  and  dnotify.txt  in the Linux kernel source directory
       Documentation/filesystems/  (on  older  kernels,  these  files  are  directly  under   the
       Documentation/ directory, and mandatory-locking.txt is called mandatory.txt)


       This  page  is  part of release 3.54 of the Linux man-pages project.  A description of the
       project,    and    information    about    reporting    bugs,    can    be    found     at