lockf - record locking on files
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)). Locking calls from other
processes 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 follows:
#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 exten-
sions and will result in an error 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_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 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 adja-
cent sections, the sections are combined into a single sec-
tion. 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 will cause the cal-
ling process to sleep until the resource is available.
F_TLOCK will cause the function to return a -1 and set errno
to EAGAIN if the section is already locked by another pro-
File locks are released on first close by the locking pro-
cess 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 sec-
tions 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 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:
EBADF The fildes argument is not a valid open file descrip-
tor; or function is F_LOCK or F_TLOCK and fildes is
not a valid file descriptor open for writing.
EACCES or EAGAIN
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
EINTR A signal was caught during execution of the function.
ECOMM 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).
EDEADLK or ENOLCK
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.
EOPNOTSUPP or EINVAL
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(5).
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| 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(5), lf64(5)
Man(1) output converted with