ug_ch7.htm

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<p><font size="2">d_setlock(<i>SET</i>, <i>type</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Lock data files associated with set
type</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_keylock(<i>FIELD</i>, <i>type</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Lock key file containing key field</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_lock(<i>num</i>, <i>pkt</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Lock group of record and set types</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_recfree(<i>REC</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Free lock on record type</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_setfree(<i>SET</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Free lock on set type</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_keyfree(<i>FIELD</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Free lock on key field type</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_freeall(<i>task</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Free all read locks</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_reclstat(<i>REC</i>, <i>type</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Get record type's lock status</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_setlstat(<i>SET</i>, <i>type</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Get set type's lock status</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_keylstat(<i>FIELD</i>, <i>type</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Get key field type's lock status</font></p>
</td>
</tr>
<tr>
<td width="40%" valign="top">
<p><font size="2">d_timeout(<i>secs, task</i>)</font></p>
</td>
<td width="60%" valign="top">
<p><font size="2">Specify wait time before timeout</font></p>
</td>
</tr>
</table>
<p><font size="2">Note that functions <b>d_trend</b> and
<b>d_trabort</b> are also used with file locking although they are
not listed here. Recall that both functions free locked files. The
remaining pages of this section describe the functions listed in
Table 7-3.</font></p>
<p>Function <b>d_reclock</b> locks the data file containing the
record type, as well as all key files containing key fields defined
in the specified record type. For example, assume the following DDL
statements appear in your schema:</p>
<pre>
<font color=
"#0000FF">data file "tims.d01" contains system, key_word, intersect;
data file "tims.d02" contains author, borrower, info, text;
key  file "tims.k01" contains id_code;
key  file "tims.k02" contains friend, word;
...
record info {
   unique key char id_code[16];   /* dewey dec. code */
   char info_title[80];           /* title of book, article, mag. */
   char publisher[32];            /* name of publisher */
   char pub_date[12];             /* date of publication */
   int info_type;                 /* 0=book, 1=mag, 2=art */
}
...
record key_word {
   unique key char word[32];      /* subject key words */
}
...
set key_to_info {
   order last;
   owner key_word;
   member intersect;
}</font>
</pre>
<p><font size="2">The call <b>d_reclock</b>(INFO, "r", task,
CURR_DB) will cause <b>tims.d02</b> and <b>tims.k01</b> to be
read-locked. The call <b>d_reclock</b>(KEY_WORD,"r", task, CURR_DB)
will cause <b>tims.d01</b> and <b>tims.k02</b> to be read-locked.
Record locks are always necessary when creating a record with keys,
or when modifying key fields in records, because they guarantee
that all of the files related to the record will be
locked.</font></p>
<p>Function <b>d_setlock</b> locks the data files that contain the
owner and member record types of the specified set. The call
<b>d_setlock</b>(KEY_TO_INFO, "r", CURR_DB) will cause only file
<b>tims.d01</b> to be read-locked, because both record types are
contained in the same file. This function does not lock key files
associated with records. Set locks are necessary when connecting or
disconnecting records in sets, or when traversing sets, because
they guarantee that all of the files related to the set will be
locked. Note, however, that if record locks had already been
applied to both the <b>key_word</b> and <b>intersect</b> record
types, the set lock would not be needed (although it would be
accepted). Any time the correct files have been locked, even though
by a different locking call, functions that use the files will
execute without locking errors.</p>
<p>Function <b>d_keylock</b> locks the key file that contains the
specified key field. The call <b>d_keylock</b>(ID_CODE, "r",
CURR_DB) will lock only file <b>tims.k01</b>. By locking only a key
file, you may scan keys, as in the following code:</p>
<pre>
<font color="#0000FF">d_keylock(ID_CODE, "r", task, CURR_DB);
for ( (status=d_keyfrst(ID_CODE, task, CURR_DB));
      status == S_OKAY; 
      (status=d_keynext(ID_CODE, task, CURR_DB)) ) 
{
   d_keyread(id_code, task, CURR_DB);
   printf("id code: %s\n", id_code);
}
d_keyfree (ID_CODE, task, CURR_DB);
</font>
</pre>
<p><font size="2">By locking only the key file, you eliminate the
overhead of obtaining locks on the data file, and possibly other
key files related to the same record type. Also, in a multi-user
environment, it is important not to lock any files that are not
going to be used, because you may be preventing other users from
making progress.</font></p>
<p>Function <b>d_lock</b> is used to lock a group of record and set
types, and is described in detail in section 7.3.6, "Grouped Lock
Requests."</p>
<p>Table 7-4 lists the types of locks that can be applied.</p>
<p align="center"><b>Table 7-4. File Lock Types</b></p>
<table cellspacing="0" border="0" cellpadding="7" width="542">
<tr>
<td width="32%" valign="top">
<p><b><font size="2">Type</font></b></p>
</td>
<td width="68%" valign="top">
<p><b><font size="2">Description</font></b></p>
</td>
</tr>
<tr>
<td width="32%" valign="top">
<p><font size="2">'r'</font></p>
</td>
<td width="68%" valign="top">
<p><font size="2">Read lock</font></p>
</td>
</tr>
<tr>
<td width="32%" valign="top">
<p><font size="2">'w'</font></p>
</td>
<td width="68%" valign="top">
<p><font size="2">Write lock</font></p>
</td>
</tr>
<tr>
<td width="32%" valign="top">
<p><font size="2">'x'</font></p>
</td>
<td width="68%" valign="top">
<p><font size="2">Exclusive lock</font></p>
</td>
</tr>
<tr>
<td width="32%" valign="top">
<p><font size="2">'k'</font></p>
</td>
<td width="68%" valign="top">
<p><font size="2">Keep lock</font></p>
</td>
</tr>
</table>
<p><font size="2">Functions <b>d_recfree</b>, <b>d_setfree</b>, and
<b>d_keyfree</b> free read locks or exclusive locks on their
respective record, set, or key field types for their database.
Function <b>d_freeall</b> will free all read-locked files across
all databases.</font></p>
<p>The current lock status of a record, set, or key type can be
found by calling functions <b>d_reclstat</b>, <b>d_setlstat</b>, or
<b>d_keylstat</b>.</p>
<p>Function <b>d_timeout</b> is used to inform the lock manager of
the number of seconds that lock requests from this process are to
wait on the queue before being denied.</p>
<h3><a name="Read" id="Read"></a>7.3.2 Read Locks</h3>
<p><font size="2">A lock type 'r' passed to a locking function
specifies that the files are to be read-locked. A read lock on a
file prevents other processes from placing a write or exclusive
lock on that file. Other processes are allowed to read-lock the
file. Thus, when a file is read-locked, other processes are allowed
to read the file but are prevented from updating the
file.</font></p>
<p>Functions that only read from the database but cannot be safely
executed concurrently with updates require that the files involved
be read-locked. In addition, the key access functions and set
navigation functions require that the files be read-locked. The
record access functions listed in Table 7-5 can be executed
concurrently with updates and, therefore, do not require that the
data file be locked. If the record being read happens to have been
deleted, status code S_DELETED will be returned by the
function.</p>
<blockquote><b><i>Note:</i></b> Using these functions without a
lock will retrieve data from your local cache without ensuring that
it is the most up-to-date data. To ensure synchronization of cache
pages, use a read lock.</blockquote>
<p align="center"><b><font size="2">Table 7-5. Read Functions that
Do Not Need a Lock</font></b></p>
<table cellspacing="0" border="0" cellpadding="7" width="542">
<tr>
<td width="46%" valign="top">
<p><b><font size="2">Function</font></b></p>
</td>
<td width="54%" valign="top">
<p><b><font size="2">Definition</font></b></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_cmtype(<i>SET</i>, <i>&amp;type, task,
dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Get record type of current member</font></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_cotype(<i>SET</i>, <i>&amp;type</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Get record type of current owner</font></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_crread(<i>FIELD</i>, <i>&amp;val</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Read field from current record</font></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_csmread(<i>SET</i>, <i>FIELD</i>,
<i>&amp;val</i>, <i>task, dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Read field from current member</font></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_csoread(<i>SET</i>, <i>FIELD</i>,
<i>&amp;val</i>, <i>task, dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Read field from current owner</font></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_members(<i>SET</i>, <i>&amp;count</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Get count of set members</font></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_recfrst(<i>REC</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Position to first occurrence of record
type</font></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_reclast(<i>REC</i>, <i>task,
dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Position to last occurrence of record
type</font></p>
</td>
</tr>
<tr>
<td width="46%" valign="top">
<p><font size="2">d_recnext(<i>task, dbn</i>)</font></p>
</td>
<td width="54%" valign="top">
<p><font size="2">Position to next occurrence of record