LOCKF(S) UNIX System V LOCKF(S)
Name
lockf - 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 are used
depending on the mode bits of the file [see chmod(S)]).
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(S) 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 a
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_ULOCK0/* Unlock a previously locked section */
#define F_LOCK1/* Lock a section for exclusive use */
#define F_TLOCK2/* Test and lock a section for exclusive use */
#define F_TEST3/* 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 section to be locked 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. 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] 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(S) 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
is 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] 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(S), close(S), creat(S), fcntl(S), intro(S), open(S),
read(S), write(S)
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.
Standards Conformance
lockf is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 6/20/89)