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FCNTL(2)                   Linux Programmer's Manual                  FCNTL(2)

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 argu-
       ment  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 pro-
              gram 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, ini-
       tialized 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  rela-
       tive  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 posi-
       tive, 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  sig-
              nal 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 auto-
       matically 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 manda-
       tory, 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  descrip-
       tion.   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  com-
       patible  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).  Manda-
       tory 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 com-
              monly, 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 ker-
              nel 2.6.11:

                     If a nonzero value is  given  to  F_SETSIG  in  a  multi-
                     threaded  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 pro-
                     cess.  Consequently, it may be necessary to pass F_SETOWN
                     the  result of gettid(2) instead of getpid(2) to get sen-
                     sible 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 gener-
                     ated 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  speci-
                     fied 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 han-
              dler 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  struc-
              ture.   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.  (Queu-
              ing 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  sig-
              nals  that may be queued to a process (see getrlimit(2) and sig-
              nal(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  pro-
                     cess  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 modi-
       fied 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 spec-
       ifies the O_NONBLOCK flag when calling open(2), then the  call  immedi-
       ately 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  han-
       dler  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, trun-
                          cate, 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  appli-
              cation must reregister to receive further notifications.  Alter-
              natively, 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  noti-
              fication  of all events, make an F_NOTIFY call specifying arg as
              0.

              Notification occurs via delivery of a signal.  The default  sig-
              nal 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 struc-
              ture  contains the file descriptor which generated the notifica-
              tion (useful when establishing notification on multiple directo-
              ries).

              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 (avail-
              able since kernel 2.6.13), which provides a much superior inter-
              face  for  obtaining  notifications  of file system events.  See
              inotify(7).

   Changing the capacity of a pipe
       F_SETPIPE_SZ (long; 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-size-max  (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-size-max
              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 capac-
              ity 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.

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.

       F_GETPIPE_SZ
                The pipe capacity.

       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 sig-
              nal(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_SETPIPE_SZ, F_GETPIPE_SZ, F_GETSIG,  F_SET-
       SIG,  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_GETOWN_EX opera-
       tion 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  unprivi-
       leged  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 pro-
       cess (group) is one that the caller has permission to send signals  to.
       Despite  this  error return, the file descriptor owner is set, and sig-
       nals 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 there-
       fore inadvisable to rely on mandatory locking.

SEE ALSO
       dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7),  fea-
       ture_test_macros(7)

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

COLOPHON
       This page is part of release 3.25 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/.

Linux                             2010-06-19                          FCNTL(2)
 

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