Why does mysql_query() print an error? - mysql

Question

I am just starting with C and the MySQL C API. I am using mysql_query() to create a table. According to the manual, the function usage is int mysql_query(MYSQL *mysql, const char *stmt_str), and the function returns 0 if successful or non-zero on error. However, whether I use if (mysql_query()) or if ! (mysql_query()), the program always prints to stdout, "Error 0:". Also, in either case, the program does create the database and table.
#include <my_global.h>
#include <mysql.h>
int main (int argc, char **argv)
{
MYSQL *connection;
const char DATABASE[] = "test";
const char HOSTNAME[] = "localhost";
const char USERNAME[] = "root";
const char PASSWORD[] = "123456";
connection = mysql_init(NULL);
if (connection == NULL)
{
printf("Error %u: %s\n", mysql_errno(connection), mysql_error(connection));
exit(1);
}
if (mysql_real_connect(connection, HOSTNAME, USERNAME, PASSWORD, NULL, 0, NULL, 0) == NULL)
{
printf("Error %u: %s\n", mysql_errno(connection), mysql_error(connection));
exit(1);
}
if (mysql_query(connection, "DROP DATABASE IF EXISTS test"))
{
printf("Error %u: %s\n", mysql_errno(connection), mysql_error(connection));
exit(1);
}
if (mysql_query(connection, "CREATE DATABASE test"))
{
printf("Error %u: %s\n", mysql_errno(connection), mysql_error(connection));
exit(1);
}
if (mysql_select_db(connection, DATABASE))
{
printf("Error %u: %s\n", mysql_errno(connection), mysql_error(connection));
exit(1);
}
/* The problem that I do not understand is happening in the statement below.
* It seems that a successful query returns zero, so I should not see "Error 0:"
*/
if (mysql_query(connection, "CREATE TABLE jobs (id int(11) NOT NULL AUTO_INCREMENT, name varchar(255) NOT NULL, PRIMARY KEY (id), UNIQUE KEY name_index (name)) ENGINE=InnoDB DEFAULT CHARSET=utf8"));
{
printf("Error %u: %s\n", mysql_errno(connection), mysql_error(connection));
exit(1);
}
mysql_close(connection);
}

Answer 1

You simply have put a semicolon ; at the end of the if statement so the code between the curly braces gets always executed!!!
Take away the ; and everything will work as expected

Answer 2

This is not completely clear from the man page, but my guess would be that the return code of the function is the error code. So you'd have to capture that return value in a variable and then print the value of that variable in case of an error.