illumos: manual page: fcntl.2

FCNTL(2) System Calls FCNTL(2)

NAME

fcntl - file control

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>

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

DESCRIPTION

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.

FILE LOCKING

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.

SHARE RESERVATIONS

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.

RETURN VALUES

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.

ERRORS

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.

EOPNOTSUPP
The cmd argument is F_OFD_GETLK, F_OFD_GETLK64, F_OFD_SETLK,
F_OFD_SETLK64, F_OFD_SETLKW, or F_OFD_SETLKW64 and the locking
of files of the type indicated by the fildes argument is not
supported.

ATTRIBUTES

NOTES

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.

February 16, 2015 FCNTL(2)