lockf(3C)
_________________________________________________________________
lockf Subroutine
record locking on files
_________________________________________________________________
SYNTAX
#include <unistd.h>
int lockf (fildes, function, size)
long size;
int fildes, function;
DESCRIPTION
The lockf command will allow 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
which 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.]
Fildes is an open file descriptor. The file descriptor must have
O_WRONLY or O_RDWR permission in order to establish lock with
this function call.
Function is a control value which specifies the action to be
taken. The permissible values for function are defined in
<unistd.h> as follows:
#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 will 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_ULOCK removes locks
from a section of the file.
Size is the number of contiguous bytes to be locked or unlocked.
The resource to be locked starts at the current offset in the
file and extends forward for a positive size and backward for a
DG/UX 4.00 Page 1
Licensed material--property of copyright holder(s)
lockf(3C)
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 (i.e., 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 will cause the calling
process to sleep until the resource is available. F_TLOCK will
cause 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 lockf or fcntl 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) command may be used to provide a timeout facility in
applications which require this facility.
The lockf utility will fail if one or more of the following are
true:
[EBADF]
Fildes is not a valid open descriptor.
[EACCES]
Cmd is F_TLOCK or F_TEST and the section is already locked
by another process.
[EDEADLK]
DG/UX 4.00 Page 2
Licensed material--property of copyright holder(s)
lockf(3C)
Cmd is F_LOCK and a deadlock would occur. Also the cmd is
either F_LOCK, F_TLOCK, or F_ULOCK and the number of entries
in the lock table would exceed the number allocated on the
system.
[ECOMM]
Fildes is on a remote machine and the link to that machine
is no longer active.
SEE ALSO
chmod(2), close(2), creat(2), fcntl(2), intro(2), open(2),
read(2), write(2).
DIAGNOSTICS
Upon successful completion, a value of 0 is returned. Otherwise,
a value of -1 is returned and errno is set to indicate the error.
WARNINGS
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 standard I/O package is the most common
source of unexpected buffering.
Because in the future the variable errno will be set to EAGAIN
rather than EACCES when a section of a file is already locked by
another process, portable application programs should expect and
test for either value.
DG/UX 4.00 Page 3
Licensed material--property of copyright holder(s)