LOCKF(3C) Standard C Library Functions LOCKF(3C)


lockf - POSIX-style record locking on files


#include <unistd.h>

int lockf(int fildes, int function, off_t size);


The lockf() function allows sections of a file to be locked; advisory or
mandatory write locks depending on the mode bits of the file (see
chmod(2)). Calls to lockf() from other threads that attempt to lock the
locked file section will either return an error value or be put to sleep
until the resource becomes unlocked. All the locks for a process are
removed when the process terminates. See fcntl(2) for more information
about record locking.

The fildes argument is an open file descriptor. The file descriptor must
have O_WRONLY or O_RDWR permission in order to establish locks with this
function call.

The function argument is a control value that specifies the action to be
taken. The permissible values for function are defined in <unistd.h> as

#define F_ULOCK 0 /* unlock previously locked section */
#define F_LOCK 1 /* lock section for exclusive use */
#define F_TLOCK 2 /* test & lock section for exclusive use */
#define F_TEST 3 /* test section for other locks */

All other values of function are reserved for future extensions and will
result in an error if not implemented.

F_TEST is used to detect if a lock by another process or open file handle
is present on the specified section. F_LOCK and F_TLOCK both lock a
section of a file if the section is available. F_ULOCK removes locks from
a section of the file.

The size argument 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 (the preceding bytes up to but not including the
current offset). 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. Locked sections will be unlocked starting at the point of the
offset through size bytes or to the end of file if size is (off_t) 0.
When this situation occurs, or if this situation occurs in adjacent
sections, 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 blocks the calling thread until the
resource is available. F_TLOCK causes the function to return -1 and set
errno to EAGAIN if the section is already locked by another process.

File locks are released on first close by the locking process of any file
descriptor for the file.

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 errno is set to EDEADLK
and the requested section is not released.

An F_ULOCK request in which size is non-zero and the offset of the last
byte of the requested section is the maximum value for an object of type
off_t, when the process has an existing lock in which size is 0 and which
includes the last byte of the requested section, will be treated as a
request to unlock from the start of the requested section with a size
equal to 0. Otherwise, an F_ULOCK request will attempt to unlock only the
requested section.

A potential for deadlock occurs if the threads of a process controlling a
locked resource is put to sleep by requesting another process's locked
resource. Thus calls to lockf() or fcntl(2) 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. The alarm(2)
function may be used to provide a timeout facility in applications that
require this facility.


Upon successful completion, 0 is returned. Otherwise, -1 is returned and
errno is set to indicate the error.


The lockf() function will fail if:

The fildes argument is not a valid open file
descriptor; or function is F_LOCK or F_TLOCK and
fildes is not a valid file descriptor open for

The function argument is F_TLOCK or F_TEST and the
section is already locked by another process.

The function argument is F_LOCK and a deadlock is

A signal was caught during execution of the function.

The fildes argument is on a remote machine and the
link to that machine is no longer active.

The function argument is not one of F_LOCK, F_TLOCK,
F_TEST, or F_ULOCK; or size plus the current file
offset is less than 0.

The offset of the first, or if size is not 0 then the
last, byte in the requested section cannot be
represented correctly in an object of type off_t.

The lockf() function may fail if:

The function argument is F_LOCK or F_TLOCK and
the file is mapped with mmap(2).

The function argument is F_LOCK, F_TLOCK, or
F_ULOCK and the request would cause the number of
locks to exceed a system-imposed limit.

The locking of files of the type indicated by the
fildes argument is not supported.


Record-locking should not be used in combination with the fopen(3C),
fread(3C), fwrite(3C) and other stdio functions. Instead, the more
primitive, non-buffered functions (such as open(2)) should be used.
Unexpected results may occur in processes that do buffering in the user
address space. The process may later read/write data which is/was
locked. The stdio functions are the most common source of unexpected

The alarm(2) function may be used to provide a timeout facility in
applications requiring it.

The lockf() function has a transitional interface for 64-bit file
offsets. See lf64(7).


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Standard |
|MT-Level | MT-Safe |


Intro(2), alarm(2), chmod(2), close(2), creat(2), fcntl(2), mmap(2),
open(2), read(2), write(2), attributes(7), lf64(7), standards(7)

February 16, 2015 LOCKF(3C)