mysql_free_result() free the MYSQL_RES, but it didn't set result to NULL. When we want to check result in other code, we can set result as default NULL where after call mysql_free_result(), and Set result as default NULL before store it.
Example:
MYSQL mysql; MYSQL_RES *res; MYSQL_ROW row; res = NULL; // Set res as default NULL
mysql_init(&mysql); mysql_real_connect(&mysql,"localhost","root","","test",0,NULL,0); mysql_query(&mysql,"SELECT * FROM person WHERE no<\'10\'"); res = mysql_store_result(&mysql); ..... other code here..... ... maybe check res is NULL or not ....... mysql_free_result(res); res = NULL; // Set res as default NULL .... mysql_close(&mysql);
//////////// // Notes: If result is NULL, we call mysql_free_result(res) is ok. If call mysql_free_result(res) more once, we can get an memory access error.
Example:(OK) res = mysql_store_result(&mysql); ...... mysql_free_result(res); // call free result once res = NULL; .... mysql_free_result(res); // call free result more then once is ok, because we set res as default NULL when after call mysql_free_result()
Example:(BAD,ERROR) res = mysql_store_result(&mysql); ...... mysql_free_result(res); // must call free result once .... mysql_free_result(res); // call free result more then once is BAD, because res is unallocated now.
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Example:
MYSQL mysql;
MYSQL_RES *res;
MYSQL_ROW row;
res = NULL; // Set res as default NULL
mysql_init(&mysql);
mysql_real_connect(&mysql,"localhost","root","","test",0,NULL,0);
mysql_query(&mysql,"SELECT * FROM person WHERE no<\'10\'");
res = mysql_store_result(&mysql);
..... other code here.....
... maybe check res is NULL or not
.......
mysql_free_result(res);
res = NULL; // Set res as default NULL
....
mysql_close(&mysql);
////////////
// Notes:
If result is NULL, we call mysql_free_result(res) is ok.
If call mysql_free_result(res) more once, we can get an memory access error.
Example:(OK)
res = mysql_store_result(&mysql);
......
mysql_free_result(res); // call free result once
res = NULL;
....
mysql_free_result(res); // call free result more then once is ok, because we set res as default NULL when after call mysql_free_result()
Example:(BAD,ERROR)
res = mysql_store_result(&mysql);
......
mysql_free_result(res); // must call free result once
....
mysql_free_result(res); // call free result more then once is BAD, because res is unallocated now.