I've been having a problem with mysql_fetch_row() crashing my Socketmud. I've made sure that the result is not null and that the number of rows in the result is greater than zero. Anything else I should check for that would make mysql_fetch_row() cause a crash?

Do you have more information, e.g. a backtrace or what lines of code are causing trouble?

The function:

SOCK_LIST * nation_list()
{
	SOCK_LIST *nlist;
	char	buf[MAX_BUFFER];
	
	nlist = AllocList();

	sprintf(buf,"SELECT * FROM nations");
	db_query("SELECT * FROM nations");
	result = mysql_store_result(&mysqlconn);
	if(result == NULL) return nlist;
	if(mysql_num_rows(result) < 1) return nlist;
	while ((row = mysql_fetch_row (result)) != NULL) /* This line is where the problem is */
	{
        	NATION_DATA *n = nation_create();
		nation_load(n,atoi( row[0] ));
        	AttachToList(n,nlist);
        	n = NULL;
	}
	db_free();
	return nlist;
}

The backtrace is:

#0  0x00b66ddd in mysql_fetch_row () from /usr/lib/mysql/libmysqlclient.so.15
#1  0x0804f153 in nation_list () at nation.c:75
#2  0x0804f182 in cmd_nation_list (ch=0x83892e0, argument=0x8395323 "") at nation.c:92
#3  0x0804c804 in handle_cmd_input (dsock=0x8391310, arg=0x8395323 "") at interp.c:29
#4  0x08051089 in GameLoop (control=3) at socket.c:181
#5  0x08050e0c in main (argc=1, argv=0xbfac3e64) at socket.c:87

Where is result coming from? I don't see it declared.

What about the connection; are you sure it's valid? What is db_query doing – is it leaving the connection in a valid state?

Zeno has a point there. Make sure you have pointers to those structs declared either somewhere locally in the function or within the scope of the file.
Also, where is the declaration for row?
Try adding something like this to the the function:

MYSQL_ROW row;
MYSQL_RES *result;

You don't see the declaration but you see the assignment, and that's all that really matters here. (If it weren't declared, it wouldn't compile. The assignment voids whatever good or bad value it had previously.) What's more worrisome is when you don't see how globals like the connection are managed.

All db_query does is mysql_real_query(). If mysql_real_query() returns an error, it logs the error with mysql_error(). I use db_query in many places throughout the code, and it works fine. It just crashes in this particular function.

I don't see a check to see if the resault set is empty. Try putting a check tomake sure mysql_numrows returns > 0

Does it die on the first row fetch, or somewhere after the first?

I didn't think to look for that. It does the first one, but then dies. There are four other rows in the DB.

What about this from the manual:


You don't seem to be checking for those, although you are checking for the null.


Is this the first DB read in your entire program? What if you set it to be the first? Perhaps you are not cleaning up the DB correctly in some prior read.

Are the values read in for the first row useful, or garbage?

Well, I'll be damned. It something entirely stupid on my part as I expected. You see the call to nation_load() in the while loop? That function uses the same global result pointer and then clears it when it's done. I really should use a local result set in the function rather than have one declared globally. How I've missed these dumb oversights since I've starting coding again.

Thanks for the help. May the Source be with Y'all.

This is a pretty classic example of why globals are bad. :smile: Glad you found the problem! These things usually have to do with connections being left in invalid states one way or another.