Problem with handling the result of SELECT query in MYSQL C API - mysql

Question

I'm getting a Internal Server Error with one of my scripts. I'm using MYSQL C API. https://dev.mysql.com/doc/refman/5.6/en/c-api.html
Here is the corresponding part of my script:
MYSQL *con;
MYSQL_RES *result;
MYSQL_ROW robe;
con = mysql_init(NULL);
if (!mysql_real_connect(valid values)) {
printf("Content-type: text/html\n\n");
printf("Could not connect\n");
exit(0); }
char somequery[512];
//userinput is sanitized beforehand
int sog = sprintf(somequery, "SELECT password from testtab WHERE username='%s'", userinput);
if (sog < 0) {
printf("Content-type: text/html\n\n");
printf("Something went wrong with Sprintf\n");
exit(0); }
int bos = mysql_real_query(con, somequery, strlen(somequery));
if (bos != 0) {
printf("Content-type: text/html\n\n");
printf("The query produced no result\n");
exit(0); }
result = mysql_store_result(con);
if (result == NULL) {
printf("Content-type: text/html\n\n");
printf("No Result Set Produced\n");
exit(0); }
robe = mysql_fetch_row(result);
char *passdb = robe[0];
printf("Content-type: text/html\n\n");
printf("And it is: %s", passdb);
A HTML form submits via POST to this script (part of which is seen above). When I submit a username which exists in the database beforehand, I'm receiving no error. Everything works fine.
The problem arises, when I'm submitting a username that doesn't exist in the said table(testtab). Well, I'm getting 500 Internal Server Error. I have looked at Apache Error log as well: "End of Script output before Headers".
I have tried a few things so far, but none of them worked. Any help is appreciated.
Note: Doing mysql_num_fields(result); in both cases gives 1.

Answer 1

First, you should NEVER store passwords in a database, especially one that is reachable through an online service. exit(0) indicates success. It's also short-circuiting your output before it is completed. You can't just call exit(0) in the middle of producing output. Use some kind of "data not available" string instead.

Answer 2

I have found the solution elsewhere, thanks to the help of some good people. It seems, that I had made a silly mistake as well as needed a thorough understanding of the difference between two MYSQL C API functions.
I'm writing the answer here, in hope of it benefiting others.
The mistakes is here:
robe = mysql_fetch_row(result);
Though it is correct in itself. I fail to check its result. What happens is that when the SQL query is performed using a username that did not exist in the DB beforehand, the result is a empty set (and not a error).
The mysql_store_result and mysql_fetch_row have a slight difference here. While the former will not return NULL if the set is empty, the later will.
All I have to do is add a check after the above line with the logic:
if (robe == NULL) {
//error occured
} else { //go on
}