Mysql, Check table and number of rows from C - mysql

Question

I try to make function to check if table exists and to get total number of rows but I get result 1 instead of 99999.
So, what should I do here to work properly?
int sql_table_length(char* database, char* dtable, char* mysql_user_name, char* mysql_password)
{
int retval = 0;
MYSQL *conn;
conn = mysql_init(NULL);
if (conn)
{
if (mysql_real_connect(conn, "localhost", mysql_user_name, mysql_password, database, 0, NULL, 0)!=0)
{
char chktable[512] = {0};
sprintf(chktable,"%s%s%s", "SHOW TABLES LIKE '", dtable, "'");
mysql_query(conn, chktable);
if (mysql_store_result(conn))
{
MYSQL_RES *result;
char lngtable[512] = {0};
sprintf(lngtable, "%s%s", "SELECT COUNT(*) FROM ", dtable);
if (!(mysql_query(conn, lngtable)))
{
result = mysql_store_result(conn);
retval = mysql_num_rows(result); // here I get 1
mysql_free_result(result);
}
else retval = -4; //no rows
}
else retval = -3; //don't exist
}
else retval = -2; //can't connect
}
else retval = -1; //no connection
mysql_close(conn);
return retval;
}
Thanks!

Answer 1

Your query, SELECT COUNT(*) FROM table returns a single row containing the number of rows in the table. Instead of counting the number of rows in the result, you want to query the number that is returned.
You will want to do something like (untested):
MYSQL_STMT *stmt;
MYSQL_BIND bind;
long rows;
my_bool error; /* Omit at your own risk */
stmt = mysql_stmt_init(mysql)
mysql_stmt_prepare(stmt, lngtable, strlen(lngtable));
mysql_stmt_execute(stmt);
memset(&bind, 0, sizeof(bind));
bind.buffer_type = MYSQL_TYPE_LONG;
bind.buffer = (char *)&rows;
bind.error = &error; /* Omit at your own risk */
mysql_stmt_bind_result(stmt, &bind);
mysql_stmt_fetch(stmt);
printf("Number of rows: %ld\n", rows);
You should add error checking to all the mysql calls. See http://dev.mysql.com/doc/refman/5.0/en/mysql-stmt-fetch.html for more examples about binding parameters.
Alternatively, you can change your query to something like SELECT * FROM table and keep the rest of your code, but this is asking MySQL to do a lot more work, and the result will likely take longer.

Answer 2

After David's suggestion, I found a simpler and safer way to get the result. Maybe someone will need this:
int sql_table_length(char* database, char* dtable, char* mysql_user_name, char* mysql_password)
{
MYSQL *conn;
MYSQL_RES *result;
MYSQL_ROW rowdata;
int retval = 0;
conn = mysql_init(NULL);
if (conn)
{
if (mysql_real_connect(conn, "localhost", mysql_user_name, mysql_password, database, 0, NULL, 0)!=0)
{
char chktable[512] = {0};
sprintf(chktable,"%s%s%s", "SHOW TABLES LIKE '", dtable, "'");
if (!(mysql_query(conn, chktable)))
{
result = mysql_store_result(conn);
if (mysql_num_rows(result))
{
char lngtable[512] = {0};
sprintf(lngtable, "%s%s", "SELECT COUNT(*) FROM ", dtable);
if (!(mysql_query(conn, lngtable)))
{
result = mysql_store_result(conn);
if (result)
{
rowdata = mysql_fetch_row(result);
if (rowdata)
retval = atoi(rowdata[0]);
else retval = -7; // data error
}
else retval = -6; // result error
mysql_free_result(result);
}
else retval = -5; // can't select
}
else retval = -4; // table don't exist
}
else retval = -3; // can't query
}
else retval = -2; // can't connect
}
else retval = -1; // no server connection
mysql_close(conn);
return retval;
}
Any remarks on possible memory leaks (or other) are welcome!