How can I verify if database is set - mysql

Question

I called mysql_select_db() and it returned no errors, but when I called mysql_query() I got the following error:
No database selected.
I found a parameter of MYSQL struct. It is some char* calls db. What does it represent?
mysql_select_db() does changes this parameter?
Thanks.
Edit:
My main():
#include t-mysql.h
...
T_MYSQL *db = (T_MYSQL*) malloc(sizeof(T_MYSQL));
strcpy(db->database_name,mysql_database_name);
strcpy(db->password,mysql_password);
strcpy(db->root_password,mysql_root_password);
strcpy(db->server,mysql_server);
strcpy(db->table_name,mysql_table_name);
strcpy(db->user,mysql_user);
strcpy(db->query_create_db,query_create_db);
strcpy(db->query_create_table,query_create_table);
t_mysql_init(db);
t_mysql_connect(db);
t_mysql_createdb(db);
t_mysql_setdb(db);
t_mysql_createtable(db);
...
t-mysql.h:
#include <stdio.h>
#include <mysql.h>
#include <my_global.h>
#ifndef T_MYSQL_H_
#define T_MYSQL_H_
#define MAX_NAME_LENGTH 128
#define MAX_QUERY_LENGTH 1024
typedef struct {
MYSQL *con;
char password[MAX_NAME_LENGTH];
char root_password[MAX_NAME_LENGTH];
char user[MAX_NAME_LENGTH];
char server[MAX_NAME_LENGTH];
char database_name[MAX_NAME_LENGTH];
char table_name[MAX_NAME_LENGTH];
char query_create_table[MAX_QUERY_LENGTH];
char query_create_db[MAX_QUERY_LENGTH];
char query_data[MAX_QUERY_LENGTH];
}T_MYSQL;
void finish_with_error(MYSQL *con);
int t_mysql_init(T_MYSQL *mysql);
int t_mysql_connect(T_MYSQL *mysql);
int t_mysql_disconnect(T_MYSQL *mysql);
int t_mysql_createdb(T_MYSQL *mysql);
int t_mysql_createtable(T_MYSQL *mysql);
int t_mysql_setdb(T_MYSQL *mysql);
int t_mysql_insertdata(T_MYSQL *mysql);
int t_mysql_quickinsert(T_MYSQL *mysql);
#endif /* T_MYSQL_H_ */
t-mysql.c:
#include <stdio.h>
#include <mysql.h>
#include <my_global.h>
#include "t-mysql.h"
void finish_with_error(MYSQL *con){
fprintf(stderr, "%s\n", mysql_error(con));
mysql_close(con); //close connection to database
}
int t_mysql_init(T_MYSQL *mysql){
mysql->con = mysql_init(NULL);
if (mysql->con == NULL){
finish_with_error(mysql->con);
return 0;
}
return 1;
}
int t_mysql_connect(T_MYSQL *mysql){
// connect to MySQL
if (mysql_real_connect(mysql->con, mysql->server, mysql->user, mysql->password,NULL, 0, NULL, 0) == NULL){
finish_with_error(mysql->con);
return 0;
}
return 1;
}
int t_mysql_disconnect(T_MYSQL *mysql){
mysql_close(mysql->con);
if ( mysql->con == NULL){
finish_with_error(mysql->con);
return 0;
}
return 1;
}
int t_mysql_createdb(T_MYSQL *mysql){
t_mysql_disconnect(mysql); //disconnect normal user
//connect as root
t_mysql_init(mysql);
if (mysql_real_connect(mysql->con, mysql->server, "root", mysql->root_password,NULL, 0, NULL, 0) == NULL){
finish_with_error(mysql->con);
return 0;
}
// create DB, if it doesn't exist
if (mysql_select_db(mysql->con, mysql->database_name)){
if (mysql_query(mysql->con, mysql->query_create_db)){
finish_with_error(mysql->con);
return 0;
}
else {
fprintf(stdout,"Database %s created\n",mysql->database_name);
}
}else{
fprintf(stdout,"Database %s exists\n",mysql->database_name);
}
//disconnect root user
t_mysql_disconnect(mysql);
//connect as normal user
t_mysql_init(mysql);
t_mysql_connect(mysql);
return 1;
}
int t_mysql_setdb(T_MYSQL *mysql){
if (mysql_select_db(mysql->con, mysql->database_name) == NULL){
finish_with_error(mysql->con);
return 0;
}
return 1;
}
int t_mysql_createtable(T_MYSQL *mysql){
//create table:
if(mysql_query(mysql->con, mysql->query_create_table)){
finish_with_error(mysql->con);
return 0;
}
return 1;
}
int t_mysql_insertdata(T_MYSQL *mysql){
if (mysql_query(mysql->con, mysql->query_data)) {
finish_with_error(mysql->con);
return 0;
}
return 1;
}

Answer 1

I didn't test your code on my system, but I think, the error is here:
int t_mysql_setdb(T_MYSQL *mysql){
if (mysql_select_db(mysql->con, mysql->database_name) == NULL){//<-?
finish_with_error(mysql->con);
return 0;
}
return 1;
}
You are thinking, that 0 is the returned, when there is an error, while connecting. While MySQL docs say, that the opposite is true:
int mysql_select_db(MYSQL *mysql, const char *db)
Return Values:
Zero for success. Nonzero if an error occurred.
http://dev.mysql.com/doc/refman/5.5/en/mysql-select-db.html
So, you are checking for a wrong value. It means, that this statement:
I called mysql_select_db() and it returned no errors
Is false, and there is an error in this function, that you don't catch.