Добрый день!
Изучаю MYSQL API

...
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <mysql/mysql.h>
...
    MYSQL *my_conn;
    MYSQL_RES *res_prt;
    MYSQL_ROW sqlrow;
    my_conn = mysql_init(NULL);
    if (!my_conn) {
        fprintf (stderr, "mysql_init failed\n");
        return EXIT_FAILURE;
    }
    
    int res;
    
    res = mysql_real_connect (&my_conn, DB_ADDR, DB_USER, DB_PASS, DB_NAME, DB_PORT, NULL, 0);
    if (!my_conn) {
        fprintf (stderr, "mysql_real_connect failed - can not connect\n");
        return EXIT_FAILURE;
    }
    
    res = mysql_query (&my_conn, "SELECT value2 FROM ls_config WHERE param IS max_cs");
    
    
    res_ptr = mysql_store_result (&my_conn);
    
    printf ("Retrived %lu rows\n", (unsigned long) mysql_num_rows (res_ptr));
    
    
    
    mysql_free_result (res_ptr);
    mysql_close (&my_conn);

Компилирую вот так:
gcc -I/usr/include/mysql ls.c -L/usr/lib/mysql -o ls ls.o -lmysqlclient -lz
И получаю:
error: `res_ptr' undeclared (first use in this function)
error: (Each undeclared identifier is reported only once
error: for each function it appears in.)
с указанием на строку: res_ptr = mysql_store_result (&my_conn);