lockf.3

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)lockf.3	2.1	3/10/87
.TH lockf 3
.SH Name
lockf \- record locking on files
.SH Syntax
\fB#include <unistd.h>\fP
.PP
\fBlockf\^(\^\fIfildes\fP, \fIfunction\fP, \fIsize\fP\^)
.br
\fBlong \fIsize\fP\^;
.br
\fBint \fIfildes\fP, \fIfunction\fP\^;
.SH Description
.NXR "lockf subroutine"
.NXR "file" "locking region"
.NXA "fcntl system call" "lockf subroutine"
The 
.PN lockf 
subroutine allows sections of a file
to be locked.
These are advisory mode locks.
.\" Mandatory or enforcement mode record locks are
.\" not available.
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 
.MS fcntl 2 .
.PP
The
\fIfildes\fR is an open file descriptor.
The file descriptor must have
.SM O_WRONLY
or
.SM O_RDWR
permission in order to establish lock with this function call.
.PP
The \fIfunction\fR is a control value
which specifies the action to be taken.
The permissible values for \fIfunction\fR
are defined in <unistd.h>
as follows:
.PP
.EX 0
.sp
#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 \(**/
.EE
.PP
All other values of \fIfunction\fR are reserved for future extensions
and result in an error return if not implemented.
.PP
.SM F_TEST
is used to detect if a lock by another process is
present on the specified section.
.SM F_LOCK
and
.SM F_TLOCK
both lock a section of a file if the section is available.
.SM F_UNLOCK
removes locks from a section of the file.
.PP
The
\fIsize\fR 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 \fIsize\fR 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.
.PP
The sections locked with
.SM F_LOCK
or
.SM 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.
.PP
.SM F_LOCK
and
.SM F_TLOCK
requests differ only by the action taken if the resource
is not available.
.SM F_LOCK
causes the calling process to sleep until the resource is available.
.SM F_TLOCK
causes the function to return a \-1 and set
\fIerrno\fR to
.SM [EACCES]
error if the section is already locked by another process.
.PP
.SM 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
.SM [EDEADLK]
error is returned and the requested section is not released.
.PP
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 
.PN lock 
or 
.PN 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.
.PP
Sleeping on a resource is interrupted with any signal.
You can use the
.MS alarm 3 
command to provide a timeout facility
in applications which require this facility.
.PP
File region locking is supported over NFS,
if the NFS locking service has been enabled.
.SH Restrictions
.NXR "lockf subroutine" "restrictions"
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
unexpected buffering.
.SH Return Values
Upon successful completion,
0 is returned.
Otherwise, a \-1 is returned and
the global variable \fIerrno\fR is set to
indicate the error.
.SH Diagnostics
.NXR "lockf subroutine" "diagnostics"
The 
.PN lockf 
subroutine fails if:
.br
.sp
.TP 15
.SM [EBADF]
The \fIfildes\fR is not a valid open descriptor.
.TP 15
.SM [EACCESS]
The \fIcmd\fR is
.SM F_TLOCK
or
.SM 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 enabled.
.TP 15
.SM [EDEADLK]
The \fIcmd\fR is
.SM F_LOCK
or
.SM F_TLOCK
and a deadlock would occur.
Also the \fIcmd\fR is either of the above or
.SM F_ULOCK
and the number of entries in the lock table
would exceed the number allocated on the system.
.TP 15
.SM [EINVAL]
The value given for the 
\fIrequest\fR
argument is invalid.
.SH See Also
close(2), creat(2), fcntl(2), intro(2), open(2), read(2), write(2), lockd(8c)