lockf(3)lockf(3)Namelockf - record locking on files
lockf(fildes, function, size)
int fildes, function;
The subroutine allows sections of a file to be locked. These are advi‐
sory mode locks. Locking calls from other processes which attempt to
lock the locked file section return either an error value or are put to
sleep until the resource becomes unlocked. All the locks for a process
are removed when the process terminates. For more information about
record locking, see
The fildes is an open file descriptor. The file descriptor must have
O_WRONLY or O_RDWR permission in order to establish lock with this
The function is a control value which specifies the action to be taken.
The permissible values for function are defined in <unistd.h> as fol‐
#define F_ULOCK 0 /∗ Unlock a previously locked section ∗/
#define F_LOCK 1 /∗ Lock a section for exclusive use ∗/
#define F_TLOCK 2 /∗ Test and lock a section for exclusive use ∗/
#define F_TEST 3 /∗ Test section for other processes locks ∗/
All other values of function are reserved for future extensions and
result in an error return if not implemented.
F_TEST is used to detect if a lock by another process is present on the
specified section. F_LOCK and F_TLOCK both lock a section of a file if
the section is available. F_UNLOCK removes locks from a section of the
The size is the number of contiguous bytes to be locked or unlocked.
The resource to be locked or unlocked starts at the current offset in
the file and extends forward for a positive size and backward for a
negative size. If size is zero, the section from the current offset
through the largest file offset is locked (that is, from the current
offset through the present or any future end-of-file). An area need
not be allocated to the file in order to be locked, as such locks may
exist past the end-of-file.
The sections locked with F_LOCK or F_TLOCK may, in whole or in part,
contain or be contained by a previously locked section for the same
process. When this occurs, or if adjacent sections occur, the sections
are combined into a single section. If the request requires that a new
element be added to the table of active locks and this table is already
full, an error is returned, and the new section is not locked.
F_LOCK and F_TLOCK requests differ only by the action taken if the
resource is not available. F_LOCK causes the calling process to sleep
until the resource is available. F_TLOCK causes the function to return
a -1 and set errno to [EACCES] error if the section is already locked
by another process.
F_ULOCK requests may, in whole or in part, release one or more locked
sections controlled by the process. When sections are not fully
released, the remaining sections are still locked by the process.
Releasing the center section of a locked section requires an additional
element in the table of active locks. If this table is full, an
[EDEADLK] error is returned and the requested section is not released.
A potential for deadlock occurs if a process controlling a locked
resource is put to sleep by accessing another process's locked
resource. Thus calls to or scan for a deadlock prior to sleeping on a
locked resource. An error return is made if sleeping on the locked
resource would cause a deadlock.
Sleeping on a resource is interrupted with any signal. You can use the
command to provide a timeout facility in applications which require
File region locking is supported over NFS, if the NFS locking service
has been enabled.
Unexpected results may occur in processes that do buffering in the user
address space. The process may later read or write data which is or
was locked. The standard I/O package is the most common source of
Upon successful completion, 0 is returned. Otherwise, a -1 is returned
and the global variable errno is set to indicate the error.
The subroutine fails if:
[EBADF] The fildes is not a valid open descriptor.
[EACCESS] The cmd is F_TLOCK or F_TEST and the section is already
locked by another process. Or, the file is remotely
mounted, and the NFS locking service has not been
[EDEADLK] The cmd is F_LOCK or F_TLOCK and a deadlock would occur.
Also the cmd is either of the above or F_ULOCK and the
number of entries in the lock table would exceed the
number allocated on the system.
[EINVAL] The value given for the request argument is invalid.
See Alsoclose(2), creat(2), fcntl(2), intro(2), open(2), read(2), write(2),