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

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

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 
}