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NAME: dbsetuserdata
FUNCTION:
Use a DBPROCESS structure to save a pointer to user-allocated
data.
SYNTAX:
void dbsetuserdata(dbproc, ptr)
DBPROCESS *dbproc;
BYTE *ptr;
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COMMENTS:
o This routine saves, in a DBPROCESS structure, a pointer to
user-allocated data. The application can access the data later
with the dbgetuserdata() routine.
o dbsetuserdata() allows the application to associate user data
with a particular DBPROCESS. This avoids the necessity of
using global variables for this purpose. One use for this rou-
tine is to handle deadlock, as shown in the example below.
This routine is particularly useful when the application has
multiple DBPROCESSes.
o The application must allocate the data that ptr points to.
DB-Library never manipulates this data; it merely saves the
pointer to it for later use by the application.
o Here's an example of using this routine to handle deadlock, a
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situation which occurs occasionally in high-volume applica-
tions. See the System Administration Guide for more informa-
tion on deadlock. This program fragment sends updates to the
SQL Server. It reruns the transaction when its message handler
detects deadlock.
...
/*
** Deadlock detection:
** In the DBPROCESS structure, we save a pointer to a DBBOOL variable.
** The message handler sets the variable when deadlock occurs.
** The result processing logic checks the variable and resends the
** transaction in case of deadlock.
*/
/*
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** Allocate the space for the DBBOOL variable and save it in
** the DBPROCESS structure.
*/
dbsetuserdata(dbproc, malloc(sizeof(DBBOOL)));
/* Initialize the variable to FALSE. */
*((DBBOOL *) dbgetuserdata(dbproc)) = FALSE;
...
/* Run queries and check for deadlock. */
deadlock:
/*
** Did we get here via deadlock?
** If so, the server has already aborted the transaction.
** We'll just start it again. In a real application, the
** deadlock handling may need to be somewhat more
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** sophisticated. For instance, you may want to keep a
** counter and retry the transaction just a fixed number
** of times.
*/
if (*((DBBOOL *) dbgetuserdata(dbproc)) == TRUE)
{
/* Reset the variable to FALSE. */
*((DBBOOL *) dbgetuserdata(dbproc)) = FALSE;
}
/* Start the transaction. */
dbcmd(dbproc, "begin transaction ");
/* Run the first UPDATE command. */
dbcmd(dbproc, "update ......");
dbsqlexec(dbproc);
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while (dbresults(dbproc) != NO_MORE_RESULTS)
{
/* application code */
}
/* Did we deadlock? */
if (*((DBBOOL *) dbgetuserdata(dbproc)) == TRUE)
goto deadlock;
/* Run the second UPDATE command. */
dbcmd(dbproc, "update ......");
dbsqlexec(dbproc);
while (dbresults(dbproc) != NO_MORE_RESULTS)
{
/* application code */
}
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/* Did we deadlock? */
if (*((DBBOOL *) dbgetuserdata(dbproc)) == TRUE)
goto deadlock;
/* No deadlock -- Commit the transaction. */
dbcmd(dbproc, "commit transaction");
dbsqlexec(dbproc);
dbresults(dbproc);
...
/*
** SERVERMSGS
** This is the server message handler. Assume that the dbmsghandle()
** routine installed it earlier in the program.
*/
servermsgs(dbproc, msgno, msgstate, serverity, msgtext, srvname, procname, line)
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DBPROCESS *dbproc;
DBINT msgno;
int msgstate;
int severity;
char *msgtext;
char *srvname;
char *procname;
DBUSMALLINT line;
{
/* Is this a deadlock message? */
if (msgno == 1205)
{
/* Set the deadlock indicator. */
*((DBBOOL *) dbsetuserdata(dbproc)) = TRUE;
return (0);
}
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/* Normal message handling code here. */
}
PARAMETERS:
dbproc - A pointer to the DBPROCESS structure that provides the
connection for a particular front-end/SQL Server process. It
contains all the information that DB-Library uses to manage
communications and data between the front end and SQL Server.
ptr - A generic BYTE pointer to the user's private data space.
RETURNS:
None.
SEE ALSO:
dbgetuserdata