#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fildes, int cmd, /* arg */ ...);
The fcntl() function provides for control over open files. The
fildes argument is an open file descriptor.
The fcntl() function can take a third argument, arg,
whose data type, value, and use depend upon the value of cmd. The
cmd argument specifies the operation to be performed by
fcntl().
The values for cmd are defined in <fcntl.h>
and include:
F_DUPFD
Return a new file descriptor which is the lowest numbered
available (that is, not already open) file descriptor greater than or equal to
the third argument,
arg, taken as an integer of type
int. The
new file descriptor refers to the same open file description as the original
file descriptor, and shares any locks. The
FD_CLOEXEC flag associated
with the new file descriptor is cleared to keep the file open across calls to
one of the
exec(2) functions.
F_DUP2FD
Similar to F_DUPFD, but always returns arg.
F_DUP2FD closes arg if it is open and not equal to
fildes. F_DUP2FD is equivalent to dup2(fildes,
arg).
F_DUPFD_CLOEXEC
Similar to F_DUPFD except that instead of clearing
FD_CLOEXEC it is explicitly set on the returned file descriptor.
F_DUP2FD_CLOEXEC
Similar to F_DUP2FD with two exceptions. The
FD_CLOEXEC flag is explicitly set on the returned file descriptor. If
filedes equals arg, the call will fail setting errno to
EINVAL.
F_FREESP
Free storage space associated with a section of the
ordinary file
fildes. The section is specified by a variable of data
type
struct flock pointed to by
arg. The data type
struct
flock is defined in the <
fcntl.h> header (see
fcntl.h(3HEAD)) and is described below. Note that all file systems
might not support all possible variations of
F_FREESP arguments. In
particular, many file systems allow space to be freed only at the end of a
file.
F_FREESP64
Equivalent to F_FREESP, but takes a struct
flock64 argument rather than a struct flock argument.
F_ALLOCSP
Allocate space for a section of the ordinary file
fildes. The section is specified by a variable of data type
struct
flock pointed to by
arg. The data type
struct flock is
defined in the <
fcntl.h> header (see
fcntl.h(3HEAD) and is
described below.
F_ALLOCSP64
Equivalent to F_ALLOCSP, but takes a struct
flock64 argument rather than a struct flock argument.
F_GETFD
Get the file descriptor flags defined in
<fcntl.h> that are associated with the file descriptor
fildes. File descriptor flags are associated with a single file
descriptor and do not affect other file descriptors that refer to the same
file.
F_GETFL
Get the file status flags and file access modes, defined
in <fcntl.h>, for the file descriptor specified by fildes.
The file access modes can be extracted from the return value using the mask
O_ACCMODE, which is defined in <fcntl.h>. File status
flags and file access modes do not affect other file descriptors that refer to
the same file with different open file descriptions.
F_GETOWN
If fildes refers to a socket, get the process or
process group ID specified to receive SIGURG signals when
out-of-band data is available. Positive values indicate a process ID; negative
values, other than −1, indicate a process group ID. If fildes
does not refer to a socket, the results are unspecified.
F_GETXFL
Get the file status flags, file access modes, and file
creation and assignment flags, defined in <fcntl.h>, for the file
descriptor specified by fildes. The file access modes can be extracted
from the return value using the mask O_ACCMODE, which is defined in
<fcntl.h>. File status flags, file access modes, and file
creation and assignment flags do not affect other file descriptors that refer
to the same file with different open file descriptions.
F_SETFD
Set the file descriptor flags defined in
<fcntl.h>, that are associated with fildes, to the third
argument, arg, taken as type int. If the FD_CLOEXEC flag
in the third argument is 0, the file will remain open across the exec()
functions; otherwise the file will be closed upon successful execution of one
of the exec() functions.
F_SETFL
Set the file status flags, defined in
<fcntl.h>, for the file descriptor specified by fildes
from the corresponding bits in the arg argument, taken as type
int. Bits corresponding to the file access mode and file creation and
assignment flags that are set in arg are ignored. If any bits in
arg other than those mentioned here are changed by the application, the
result is unspecified.
F_SETOWN
If fildes refers to a socket, set the process or
process group ID specified to receive SIGURG signals when
out-of-band data is available, using the value of the third argument,
arg, taken as type int. Positive values indicate a process ID;
negative values, other than −1, indicate a process group ID. If
fildes does not refer to a socket, the results are unspecified.
The following commands are available for POSIX advisory or
mandatory record locking. POSIX record locking is supported for regular
files, and may be supported for other files. See the FILE LOCKING section of
this manual page for information about the types of file locks available and
their interaction.
F_GETLK
Get the first lock which blocks the POSIX lock
description pointed to by the third argument, arg, taken as a pointer
to type struct flock, defined in <fcntl.h>. The
information retrieved overwrites the information passed to fcntl() in
the structure flock. If no lock is found that would prevent this lock
from being created, then the structure will be left unchanged except for the
lock type which will be set to F_UNLCK. If multiple locks exist that
would prevent this lock from being created, which one is returned is
unspecified. If the blocking lock is an OFD-style lock, −1 will
be returned for the lock's pid value.
F_GETLK64
Equivalent to
F_GETLK, but takes a
struct
flock64 argument rather than a
struct flock argument. See
lf64(7).
F_SETLK
Set or clear a POSIX record lock according to the lock
description pointed to by the third argument, arg, taken as a pointer
to type struct flock, defined in <fcntl.h>.
F_SETLK is used to establish shared (or read) locks (F_RDLCK) or
exclusive (or write) locks (F_WRLCK), as well as to remove either type
of lock (F_UNLCK). F_RDLCK, F_WRLCK and F_UNLCK
are defined in <fcntl.h>. If a shared or exclusive lock cannot be
set, fcntl() will return immediately with a return value of
−1.
F_SETLK64
Equivalent to
F_SETLK, but takes a
struct
flock64 argument rather than a
struct flock argument. See
lf64(7).
F_SETLKW
This command is the same as F_SETLK except that if
a shared or exclusive lock is blocked by other locks, the process will wait
until the request can be satisfied. If a signal that is to be caught is
received while fcntl() is waiting for a region, fcntl() will be
interrupted. Upon return from the process' signal handler, fcntl() will
return −1 with errno set to EINTR, and the lock
operation will not be done.
F_SETLKW64
Equivalent to
F_SETLKW, but takes a
struct
flock64 argument rather than a
struct flock argument. See
lf64(7).
The following commands are available for OFD (open file
description) advisory record locking. OFD record locking is supported for
regular files, and may be supported for other files. See the FILE LOCKING
section of this manual page for information about the types of file locks
available and their interaction. OFD-style record locks are currently
limited to spanning the entire file and these locks are currently not
supported over remote file systems (e.g. nfs(5)) which use the
Network Lock Manager.
F_OFD_GETLK
Get the first lock which blocks the OFD lock description
pointed to by the third argument, arg, taken as a pointer to type
struct flock, defined in <fcntl.h>. The information
retrieved overwrites the information passed to fcntl() in the structure
flock. If no lock is found that would prevent this lock from being
created, then the structure will be left unchanged except for the lock type
which will be set to F_UNLCK. If multiple locks exist that would
prevent this lock from being created, which one is returned is unspecified. If
the blocking lock is an OFD-style lock, −1 will be returned for
the lock's pid value.
F_OFD_GETLK64
Equivalent to
F_OFD_GETLK, but takes a
struct
flock64 argument rather than a
struct flock argument. See
lf64(7). This command exists solely to allow the use of OFD locks with
the transitional 64-bit file interfaces.
F_OFD_SETLK
Set or clear a OFD record lock according to the lock
description pointed to by the third argument, arg, taken as a pointer
to type struct flock, defined in <fcntl.h>.
F_OFD_SETLK is used to establish shared (or read) locks
(F_RDLCK) or exclusive (or write) locks (F_WRLCK), as well as to
remove either type of lock (F_UNLCK). F_RDLCK, F_WRLCK
and F_UNLCK are defined in <fcntl.h>. If a shared or
exclusive lock cannot be set, fcntl() will return immediately with a
return value of −1.
F_OFD_SETLK64
Equivalent to
F_OFD_SETLK, but takes a
struct
flock64 argument rather than a
struct flock argument. See
lf64(7). This command exists solely to allow the use of OFD locks with
the transitional 64-bit file interfaces.
F_OFD_SETLKW
This command is the same as F_OFD_SETLK except
that if a shared or exclusive lock is blocked by other locks, the process will
wait until the request can be satisfied. If a signal that is to be caught is
received while fcntl() is waiting for a region, fcntl() will be
interrupted. Upon return from the process' signal handler, fcntl() will
return −1 with errno set to EINTR, and the lock
operation will not be done.
F_OFD_SETLKW64
Equivalent to
F_OFD_SETLKW, but takes a
struct
flock64 argument rather than a
struct flock argument. See
lf64(7). This command exists solely to allow the use of OFD locks with
the transitional 64-bit file interfaces.
The following values for cmd are used for file share
reservations. A share reservation is placed on an entire file to allow
cooperating processes to control access to the file. See the SHARE
RESERVATIONS section of this manual page below for additional
information.
F_SHARE
Sets a share reservation on a file with the specified
access mode and designates which types of access to deny.
F_UNSHARE
Remove an existing share reservation.
Two types of file locks are supported: POSIX-style and OFD-style. OFD-style
locks are associated with the open file description (not descriptor) instead
of with a process. Either type is advisory by default, but POSIX-style locks
can be mandatory if, and only if, mandatory locking has been enabled on the
file being locked. Each type of lock may be created through two different
interfaces. POSIX-style locks are created via the F_SETLK,
F_SETLK64, F_SETLKW, or F_SETLKW64 commands to this
system call or by use of the lockf(3C) routine. There is no difference
between locks created via one mechanism or the other. Likewise, OFD-style
locks are created via the F_OFD_SETLK, F_OFD_SETLK64,
F_OFD_SETLKW, or F_OFD_SETLKW64 commands to this system call or
by use of the Linux/BSD-compatible flock(3C) routine. Note that this
system call supports the creation of range-specified OFD-style file locks,
while flock(3C) does not. However, the current implementation of
OFD-style locking is limited to locking the entire file. This limitation might
be removed in the future.
The essential distinction between POSIX-style locks and OFD-style
locks lie in how ownership of a lock is scoped. POSIX locks are scoped to a
process. All POSIX locks associated with a file for a given process are
removed when any file descriptor for that file is closed by that process or
the process holding that file descriptor terminates. POSIX-style locks are
not inherited by a child process created using fork(2). An OFD-style
lock is scoped to the file description for a file, not the process or open
file descriptor. Thus all file descriptors referring to the same description
(i.e. those created via the F_DUPFD, F_DUP2FD,
F_DUPFD_CLOEXEC, or F_DUP2FD_CLOEXEC commands to the
fcntl(2) system call, or those created via the dup(2) system
call, or those inherited by a child process created via fork(2))
reference the same lock, but a file descriptor obtained via a separate
open(2) call on the same file will reference a different lock. A lock
is removed only on the last close(2) of the description, or when the
lock is explicitly unlocked.
Locks of both styles are compatible. A file that has been locked
with one style of lock will be regarded as locked when creation of a lock of
either style is attempted, and information about the lock will be provided
via any of the F_GETLK, F_GETLK64, F_OFD_GETLK, or
F_OFD_GETLK64 commands to this system call if that lock would
conflict with an attempt to create the specified lock regardless of whether
the specified lock is of the same style as the conflicting extant lock.
Because ownership of OFD-style locks is scoped to the open description
rather than the calling process, the l_pid field of a lock descriptor
for any such lock will always be set to −1.
When a shared lock is set on a segment of a file, other callers
(regardless of whether in the same or different process and of whether
referenced via the same open file) will be able to set shared locks on that
segment or a portion of it. A POSIX-style shared lock prevents any other
process from setting an exclusive lock on any portion of the protected area.
A OFD-style shared lock prevents any caller (even callers in the same
process) from setting an exclusive lock on any portion of the protected
area, unless the caller makes the request against a file descriptor
referencing the same open file against which the shared lock was created, in
which case the lock will be downgraded to a shared lock with respect to the
specified region. A request for a shared lock of either style will fail if
the file descriptor was not opened with read access.
A POSIX-style exclusive lock will prevent any other process from
setting a shared lock or an exclusive lock (of either style) on any portion
of the protected area. A request for an exclusive lock will fail if the file
descriptor was not opened with write access.
The flock structure contains at least the following
elements:
short l_type; /* lock operation type */
short l_whence; /* lock base indicator */
off_t l_start; /* starting offset from base */
off_t l_len; /* lock length; l_len == 0 means
until end of file */
int l_sysid; /* system ID running process holding lock */
pid_t l_pid; /* process ID of process holding lock */
The value of l_whence is SEEK_SET, SEEK_CUR,
or SEEK_END, to indicate that the relative offset l_start
bytes will be measured from the start of the file, current position or end
of the file, respectively. The value of l_len is the number of
consecutive bytes to be locked. The value of l_len may be negative
(where the definition of off_t permits negative values of
l_len). After a successful F_GETLK, F_GETLK64,
F_OFD_GETLK, or F_OFD_GETLK64 request, that is, one in which a
lock was found, the value of l_whence will be SEEK_SET.
The l_pid and l_sysid fields are used only with
F_GETLK or F_GETLK64 to return the process ID of the
process holding a POSIX-style blocking lock and to indicate which system is
running that process, or −1 if it is an OFD-style lock. These
fields must both be initialized to 0 prior to issuing a OFD-style locking
command (F_OFD_GETLK or F_OFD_GETLK64).
If l_len is positive, the area affected starts at
l_start and ends at l_start + l_len − 1. If
l_len is negative, the area affected starts at l_start +
l_len and ends at l_start − 1. Locks may start
and extend beyond the current end of a file, but must not be negative
relative to the beginning of the file. A lock will be set to extend to the
largest possible value of the file offset for that file by setting
l_len to 0. If such a lock also has l_start set to 0 and
l_whence is set to SEEK_SET, the whole file will be
locked.
If a lock exists for which l_len is 0 and which includes
the last byte of the requested segment, and an unlock (F_UNLCK)
request is made in which l_len is non-zero and the offset of the last
byte of the requested segment is the maximum value for an object of type
off_t, then the F_UNLCK request will be treated as a request
to unlock from the start of the requested segment with an l_len equal
to 0. Otherwise, the request will attempt to unlock only the requested
segment.
There will be at most one type of lock set for each byte in the
file. Before a successful return from an F_SETLK, F_SETLK64,
F_SETLKW, or F_SETLKW64 request when the calling process has
previously existing POSIX-style locks on bytes in the region specified by
the request, the previous POSIX-style lock type for each byte in the
specified region will be replaced by the new lock type. As specified above
under the descriptions of shared locks and exclusive locks, an
F_SETLK, F_SETLK64, F_SETLKW, or F_SETLKW64
request will (respectively) fail or block when locks exist on bytes in the
specified region and the type of any of those locks conflicts with the type
specified in the request.
Similarly, before a successful return from an F_OFD_SETLK,
F_OFD_SETLK64, F_OFD_SETLKW, or F_OFD_SETLKW64 request
when previously-created OFD-style locks associated with the open file apply
to bytes in the region specified by the request, the previous OFD-style lock
type for each byte in the specified region will be replaced by the new lock
type. As specified above under the descriptions of shared locks and
exclusive locks, an F_OFD_SETLK, F_OFD_SETLK64,
F_OFD_SETLKW, or F_OFD_SETLKW64 request will (respectively)
fail or block when locks exist on bytes in the specified region and the type
of any of those locks conflicts with the type specified in the request.
A potential for deadlock occurs if a process controlling a locked
region is put to sleep by attempting to lock another process' locked region.
If the system detects that sleeping until a locked region is unlocked would
cause a deadlock, fcntl() will fail with an EDEADLK error.
This deadlock detection and error value apply only to POSIX-style locks. No
deadlock detection is performed when attempting to set an OFD-style
lock.
File share reservations are an advisory form of access control among cooperating
processes, on both local and remote machines. They are most often used by
DOS or Windows emulators and DOS based NFS clients.
However, native UNIX versions of DOS or Windows applications may also
choose to use this form of access control.
A share reservation is described by an fshare structure
defined in <sys/fcntl.h>, which is included in
<fcntl.h> as follows:
typedef struct fshare {
short f_access;
short f_deny;
int f_id;
} fshare_t;
A share reservation specifies the type of access, f_access,
to be requested on the open file descriptor. If access is granted, it
further specifies what type of access to deny other processes,
f_deny. A single process on the same file may hold multiple
non-conflicting reservations by specifying an identifier, f_id,
unique to the process, with each request.
An F_UNSHARE request releases the reservation with the
specified f_id. The f_access and f_deny fields are
ignored.
Valid f_access values are:
F_RDACC
Set a file share reservation for read-only access.
F_WRACC
Set a file share reservation for write-only access.
F_RWACC
Set a file share reservation for read and write
access.
Valid f_deny values are:
F_COMPAT
Set a file share reservation to compatibility mode.
F_RDDNY
Set a file share reservation to deny read access to other
processes.
F_WRDNY
Set a file share reservation to deny write access to
other processes.
F_RWDNY
Set a file share reservation to deny read and write
access to other processes.
F_NODNY
Do not deny read or write access to any other
process.
Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD
A new file descriptor.
F_FREESP
Value of 0.
F_GETFD
Value of flags defined in <fcntl.h>. The
return value will not be negative.
F_GETFL
Value of file status flags and access modes. The return
value will not be negative.
F_GETLK
Value other than −1.
F_GETLK64
Value other than −1.
F_GETOWN
Value of the socket owner process or process group; this
will not be −1.
F_GETXFL
Value of file status flags, access modes, and creation
and assignment flags. The return value will not be negative.
F_OFD_GETLK
Value other then −1.
F_OFD_GETLK64
Value other then −1.
F_OFD_SETLK
Value other then −1.
F_OFD_SETLK64
Value other then −1.
F_OFD_SETLKW
Value other then −1.
F_OFD_SETLKW64
Value other then −1.
F_SETFD
Value other than −1.
F_SETFL
Value other than −1.
F_SETLK
Value other than −1.
F_SETLK64
Value other than −1.
F_SETLKW
Value other than −1.
F_SETLKW64
Value other than −1.
F_SETOWN
Value other than −1.
F_SHARE
Value other than −1.
F_UNSHARE
Value other than −1.
Otherwise, −1 is returned and errno is set to
indicate the error.
The fcntl() function will fail if:
EAGAIN
The
cmd argument is
F_SETLK,
F_SETLK64,
F_OFD_SETLK, or
F_OFD_SETLK64, the type of
lock
(l_type) is a shared (
F_RDLCK) or exclusive
(
F_WRLCK) lock, and the segment of a file to be locked is already
exclusive-locked by another process or open file; or the type is an exclusive
lock and some portion of the segment of a file to be locked is already
shared-locked or exclusive-locked by another process or open file.
The cmd argument is F_FREESP, the file exists,
mandatory file/record locking is set, and there are outstanding record locks
on the file; or the cmd argument is F_SETLK, F_SETLK64,
F_SETLKW, or F_SETLKW64, mandatory file/record locking is set,
and the file is currently being mapped to virtual memory using
mmap(2).
The cmd argument is F_SHARE and f_access
conflicts with an existing f_deny share reservation.
EBADF
The
fildes argument is not a valid open file
descriptor; or the
cmd argument is
F_SETLK,
F_SETLK64,
F_SETLKW,
F_SETLKW64,
F_OFD_SETLK,
F_OFD_SETLK64,
F_OFD_SETLKW, or
F_OFD_SETLKW64, the type of lock,
l_type, is a shared lock (
F_RDLCK), and
fildes is not a
valid file descriptor open for reading; or the type of lock
l_type is
an exclusive lock (
F_WRLCK) and
fildes is not a valid file
descriptor open for writing.
The cmd argument is F_FREESP and fildes is
not a valid file descriptor open for writing.
The cmd argument is F_DUP2FD, and arg is
negative or is not less than the current resource limit for
RLIMIT_NOFILE.
The cmd argument is F_SHARE, the f_access
share reservation is for write access, and fildes is not a valid file
descriptor open for writing.
The cmd argument is F_SHARE, the f_access
share reservation is for read access, and fildes is not a valid file
descriptor open for reading.
EFAULT
The cmd argument is F_GETLK,
F_GETLK64, F_SETLK, F_SETLK64, F_SETLKW,
F_SETLKW64, F_OFD_GETLK, F_OFD_GETLK64,
F_OFD_SETLK, F_OFD_SETLK64, F_OFD_SETLKW,
F_OFD_SETLKW64, F_SHARE, F_UNSHARE, or F_FREESP
and the arg argument points to an illegal address.
EINTR
The cmd argument is F_SETLKW,
F_SETLKW64, F_OFD_SETLKW, or F_OFD_SETLKW64, and the
function was interrupted by a signal.
EINVAL
The
cmd argument is invalid or not supported by
the file system; or the
cmd argument is
F_DUPFD and
arg
is negative or greater than or equal to
OPEN_MAX; or the
cmd
argument is
F_GETLK,
F_GETLK64,
F_SETLK,
F_SETLK64,
F_SETLKW,
F_SETLKW64,
F_OFD_GETLK,
F_OFD_GETLK64,
F_OFD_SETLK,
F_OFD_SETLK64,
F_OFD_SETLKW, or
F_OFD_SETLKW64, and the data pointed to by
arg is not valid; or
fildes refers to a file that does not
support locking.
The cmd argument is F_UNSHARE and a reservation with
this f_id for this process does not exist.
The cmd argument is F_DUP2FD_CLOEXEC and
fildes is equal to arg.
EIO
An I/O error occurred while reading from or writing to
the file system.
EMFILE
The cmd argument is F_DUPFD and either
OPEN_MAX file descriptors are currently open in the calling process, or
no file descriptors greater than or equal to arg are available.
ENOLCK
The cmd argument is F_SETLK,
F_SETLK64, F_SETLKW, F_SETLKW64, F_OFD_SETLK,
F_OFD_SETLK64, F_OFD_SETLKW, or F_OFD_SETLKW64, and
satisfying the lock or unlock request would result in the number of locked
regions in the system exceeding a system-imposed limit.
ENOLINK
Either the fildes argument is on a remote machine
and the link to that machine is no longer active; or the cmd argument
is F_FREESP, the file is on a remote machine, and the link to that
machine is no longer active.
EOVERFLOW
One of the values to be returned cannot be represented
correctly.
The cmd argument is F_GETLK, F_SETLK,
F_SETLKW, F_OFD_GETLK, F_OFD_SETLK, or
F_OFD_SETLKW, and the smallest or, if l_len is non-zero, the
largest, offset of any byte in the requested segment cannot be represented
correctly in an object of type off_t.
The cmd argument is F_GETLK64, F_SETLK64,
F_SETLKW64, F_OFD_GETLK64, F_OFD_SETLK64, or
F_OFD_SETLKW64, and the smallest or, if l_len is non-zero, the
largest, offset of any byte in the requested segment cannot be represented
correctly in an object of type off64_t.
The fcntl() function may fail if:
EAGAIN
The
cmd argument is
F_SETLK,
F_SETLK64,
F_SETLKW,
F_SETLKW64,
F_OFD_SETLK,
F_OFD_SETLK64,
F_OFD_SETLKW, or
F_OFD_SETLKW64 and the
file is currently being mapped to virtual memory using
mmap(2).
EDEADLK
The
cmd argument is
F_SETLKW or
F_SETLKW64, the lock is blocked by some lock from another process and
putting the calling process to sleep, waiting for that lock to become free
would cause a deadlock.
The cmd argument is F_FREESP, mandatory record
locking is enabled, O_NDELAY and O_NONBLOCK are clear and a
deadlock condition was detected.
| ATTRIBUTE
TYPE |
ATTRIBUTE VALUE |
| Interface Stability |
Standard |
| MT-Level |
Async-Signal Safe |
chmod(2), close(2), creat(2), dup(2),
exec(2), fork(2), mmap(2), open(2),
pipe(2), read(2), sigaction(2), write(2),
dup2(3C), flock(3C), lockf(3C), fcntl.h(3HEAD),
attributes(7), lf64(7), standards(7), lockd(8)
Programming Interfaces Guide
In the past, the variable errno was set to EACCES rather than
EAGAIN when a section of a file is already locked by another process.
Therefore, portable application programs should expect and test for either
value.
Advisory locks allow cooperating processes to perform consistent
operations on files, but do not guarantee exclusive access. Files can be
accessed without advisory locks, but inconsistencies may result. The network
share locking protocol does not support the f_deny value of
F_COMPAT. For network file systems, if f_access is
F_RDACC, f_deny is mapped to F_RDDNY. Otherwise, it is
mapped to F_RWDNY.
To prevent possible file corruption, the system may reject
mmap() requests for advisory locked files, or it may reject advisory
locking requests for mapped files. Applications that require a file be both
locked and mapped should lock the entire file (l_start and
l_len both set to 0). If a file is mapped, the system may reject an
unlock request, resulting in a lock that does not cover the entire file.
The process ID returned for locked files on network file systems
might not be meaningful.
If the file server crashes and has to be rebooted, the lock
manager (see lockd(8)) attempts to recover all locks that were
associated with that server. If a lock cannot be reclaimed, the process that
held the lock is issued a SIGLOST signal.