Re: Question on stability of results returned by C api mysql_fetch_row()

In article <[email protected]>,
Lew Pitcher  <[email protected]> wrote:
> I have written a C program that summarizes the contents of a table. It 
> only "SELECT"s from the table, and retrieves rows using mysql_fetch_row
> (). As the program must interpret the row data, it may need to work with 
> as many as five (5) rows as a time.
> 
> I'm chasing down a memory corruption problem with this program, that 
> seems to overwrite the stored row data, and I want to eliminate my use 
> (or possibly, misuse) of the C api as the source.
> 
> My question is: Once I've retrieved a row using mysql_fetch_row(), does 
> that returned row (the MYSQL row, and the strings it's elements point to) 
> remain stable and unaltered by subsequent mysql_fetch_row() calls? If so, 
> then my problem lies outside of my use of the API, otherwise, I suspect 
> that I've used the API incorrectly, and will have to refactor that 
> portion of the code.

Your example looks ok, so you must be doing something subtly different in
the other program.

If you do mysql_store_result(), it allocates memory for all the rows in the
result set, so you can seek around them and fetch multiple rows independently,
as per your example.

If the expected result set is very large, mysql_store_result() can use a lot
of memory, and mysql_use_result() can be used instead. But in this case,
rows are fetched from the server one at a time, you cannot seek around the
data set, and I would think you can only have one live row at a time.

If that doesn't explain it, then you may need to share the appropriate portion
of the real code that doesn't work.

Cheers
Tony

> Here's a brief, naive example of the sort of processing I'm doing. This 
> is NOT the code I'm debugging; this code doesn't seem to suffer the 
> memory corruption my more complex program does. However, this code /does/ 
> illustrate the mysql_fetch_row() assumption that I use in my bigger 
> project.
> 
>   ##### Table definition #####
>   Field	Type	Null	Key	Default	Extra
>   t1ID	int(10) unsigned	NO	PRI	NULL	auto_increment
>   t1Key	varchar(20)	NO		NULL	
>   t1Valu	varchar(80)	YES		NULL	
>   
>   ##### Table contents #####
>   t1ID	t1Key	t1Valu
>   1	HOME	/home/lpitcher
>   2	PWD	/home/lpitcher
>   3	LOGNAME	lpitcher
>   4	TERM	xterm
>   
>   ##### Program source #####
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <string.h>
>   
>   #include <mysql/my_global.h>
>   #include <mysql/mysql.h>
>   
>   /*
>   ** USER macro definition supplied by compile commandline
>   ** PSWD macro definition supplied by compile commandline
>   */
>   
>   int main(void)
>   {
>     MYSQL *dbm;
>     MYSQL_RES *results;
>     MYSQL_ROW row1,row2,row3;
>   
>     dbm = mysql_init(NULL);
>     mysql_real_connect(dbm,"localhost",USER,PSWD,"lptest",0,NULL,0);
>   
>     mysql_query(dbm,"SELECT t1Key, t1Valu FROM lptest.t1;");
>     results = mysql_store_result(dbm);
>   
>     row1 = mysql_fetch_row(results);
>   
>     /*
>     ** Do either of these calls to mysql_fetch_row() somehow
>     ** alter the results accessable from the row returned by
>     ** the prior call(s) to mysql_fetch_row()?
>     */
>     row2 = mysql_fetch_row(results);
>     row3 = mysql_fetch_row(results);
>   
>     /*
>     ** At this point, can I be certain that row1, row2, and row3
>     ** all access different table rows?
>     */
>   
>     if (strcmp(row1[0],row2[0]) == 0)
>       printf("First and second rows have the same key   [%s]\n",row1[0]);
>     if (strcmp(row1[1],row2[1]) == 0)
>       printf("First and second rows have the same value [%s]\n",row1[1]);
>   
>     if (strcmp(row1[0],row3[0]) == 0)
>       printf("First and third rows have the same key    [%s]\n",row1[0]);
>     if (strcmp(row1[1],row3[1]) == 0)
>       printf("First and third rows have the same value  [%s]\n",row1[1]);
>   
>     if (strcmp(row2[0],row3[0]) == 0)
>       printf("Second and third rows have the same key   [%s]\n",row2[0]);
>     if (strcmp(row2[1],row3[1]) == 0)
>       printf("Second and third rows have the same value [%s]\n",row2[1]);
>   
>     mysql_free_result(results);
>     mysql_close(dbm);
>   
>     return 0;
>   }
>   
>   ##### Program execution #####
>   First and second rows have the same value [/home/lpitcher]
> 
> 
> I appreciate any guidance or advice you can give me.
> Thanks, 
> -- 
> Lew Pitcher
> "In Skills, We Trust"


-- 
Tony Mountifield
Work: [email protected] - http://www.softins.co.uk
Play: [email protected] - http://tony.mountifield.org