Provided by: manpages-dev_3.24-1ubuntu1_all bug

NAME

       fcntl - manipulate file descriptor

SYNOPSIS

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

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

DESCRIPTION

       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  long,  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 (long)
              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 (long; 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 closed.

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

       F_SETFD (long)
              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)
              Read the file status flags; arg is ignored.

       F_SETFL (long)
              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 only  change  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 file.

       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 2.5.49.

       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 execve(2).

       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 EAGAIN.

       To make use of mandatory locks, mandatory locking must be enabled  both
       on the file system that contains the file to be locked, and on the file
       itself.  Mandatory locking is enabled on a file system  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 (long)
              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 onwards, 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 interpreted:

              F_OWNER_TID
                     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.

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

              F_OWNER_PGRP
                     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 (long)
              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
              above.

              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).

   Leases
       F_SETLEASE and F_GETLEASE (Linux 2.4 onwards) 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 descriptor.

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

              F_RDLCK
                     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 only be placed
                     on a file descriptor that is opened read-only.

              F_WRLCK
                     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
                     file.

              F_UNLCK
                     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 only be taken out on regular files.  An unprivileged process
       may  only take out a lease on a file whose UID (owner) matches the file
       system 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 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 (long)
              (Linux 2.4 onwards)  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.)

              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  file  system  events.
              See inotify(7).

RETURN VALUE

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

       F_DUPFD  The new descriptor.

       F_GETFD  Value of flags.

       F_GETFL  Value of flags.

       F_GETLEASE
                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.

       All other commands
                Zero.

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

ERRORS

       EACCES or EAGAIN
              Operation is prohibited by locks held by other processes.

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

       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.

       EDEADLK
              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 set.

CONFORMING TO

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

       F_DUPFD_CLOEXEC is specified in POSIX.1-2008.

       F_GETOWN_EX, F_SETOWN_EX, F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and
       F_SETLEASE are Linux-specific.  (Define the _GNU_SOURCE macro to obtain
       these definitions.)

NOTES

       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 fcntl().

       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.

BUGS

       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_SETOWN_EX  and
       F_GETOWN_EX operations avoid this problem.

       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 owner.

       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.

SEE ALSO

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

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

COLOPHON

       This  page  is  part of release 3.24 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.