.TH lock "" "" "/rdb Command"
.PC "Lock a record or field of a file"
\fBlock \fIfilename processid from to indexfrom indexto\fR
.PP
.II "record locking"
The command
.B lock
locks
.I filename
by writing a row into the lock file \fB/tmp/L\fIfilename\fR,
which contains one line for each locked record or field.
Its command line contains the following arguments:
.IP \fIprocessid\fR
The process identifier of the program that locks
.IR filename .
.IP \fIfrom\fR
The beginning point of the bytes to be locked.
.IP \fIto\fR
The end point of the bytes to be locked.
.IP \fIindexbegin\fR
The beginning point of the index entry for the data being locked.
.IP \fIindexend\fR
The end point of the index entry for the data being locked.
.PP
When a process attempts to lock a string of bytes that is already locked,
.B lock
returns an error condition.
You can test for this by using the COHERENT command
.B test
to test the shell variable \fB$?\fR.
.SH Example
Let's use
.B lock
on table
.BR inventory ,
which appears as follows:
.DM
Item    Amount  Cost    Value   Description   
----    ------  ----    -----   --------------
   1         3    50      150   rubber gloves
   2       100     5      500   test tubes
   3         5    80      400   clamps
   4        23    19      437   plates
   5        99    24     2376   cleaning cloth
   6        89   147    13083   bunsen burners
   7         5   175      875   scales
.DE
.PP
First, use
.B seek
to locate the record and return the offset and size, as follows:
.DM
	LOCATION=`echo 5 | seek -mb inventory Item`
	echo $LOCATION
	207 245 0 8
.DE
.PP
This means that the record is offset 207 bytes into file
.BR inventory ,
and that it is 38 bytes long, ending at byte 245.
The 0 and 8 mean that there is no secondary index file.
.PP
Now we can call
.B lock
to lock the record:
.DM
	lock inventory $$ $LOCATION
.DE
.PP
Note that we use the shell variable
.B $LOCATION
to supply the arguments
.I from
and
.IR to .
We also use the built-in shell variable \fB$$\fR to
write the process identifier on the command line.
.PP
The lock
.B /tmp/Linventory
has the following contents:
.DM
	207 245 0 8
.DE
.SH "See Also"
.B
seek,
unlock
.R
.br
COHERENT Lexicon:
.B
test
.R
.SO lock