Prepared SELECT statement executes but always returns 0 rows - mysql

Question

I have read the documentation many times and i have searched on google and i found a lot of example and the code below is the result of my research.
But now i have a big problem and i don't find the solution to fix it.
The code below executes the query but doesn't display the result, it returns always 0 row while it have to return 2 rows... i don't understand where is the mistake.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <mysql/mysql.h>
#define STRING_SIZE 50
#define SELECT_SAMPLE "select p.id_paz from pazienti p where p.id_doc = ?"
int main(void)
{
MYSQL *conn;
MYSQL_STMT *stmt;
MYSQL_BIND pbind[1],result[1]; /* results */
unsigned long length;
int row_count;
char login[STRING_SIZE];
my_bool is_null;
unsigned long plength;
char *pdata;
my_bool p_is_null;
// Open Database
const char *server = "localhost";
char *user = "root";
char *password = "password"; /* set me first */
char *database = "ProgettoSi";
conn = mysql_init(NULL);
/* Connect to database */
if (!mysql_real_connect(conn, server, user, password, database, 0, NULL, 0)) {
//...
}
// Allocate statement handler
stmt = mysql_stmt_init(conn);
if (mysql_stmt_prepare(stmt, SELECT_SAMPLE, strlen(SELECT_SAMPLE)))
{
//...
}
fprintf(stdout, " prepare, SELECT successful\n");
plength = STRING_SIZE * sizeof(char);
p_is_null = 0;
pdata = (char*)malloc( STRING_SIZE * sizeof(char) );
/* STRING PARAMETER */
pbind[0].buffer_type= MYSQL_TYPE_STRING;
pbind[0].buffer= (char *)pdata;
pbind[0].buffer_length=STRING_SIZE * sizeof(char);
pbind[0].is_null= &p_is_null;
pbind[0].length= &plength;
if( mysql_stmt_bind_param( stmt, pbind ) ) {
//...
}
mysql_real_escape_string( conn, pdata, "123", strlen("123")*sizeof(char) );
plength = strlen( pdata ) + 1;
printf( "Executing query with parameters %s. \n", pdata);
/* Execute the SELECT query */
if (mysql_stmt_execute(stmt))
{
//...
}
/* Bind the result buffers for all 4 columns before fetching them */
result[0].buffer_type= MYSQL_TYPE_STRING;
result[0].buffer= (char *)login;
result[0].buffer_length= STRING_SIZE;
result[0].is_null= &is_null;
result[0].length= &length;
if (mysql_stmt_bind_result(stmt, result))
{
//...
}
if (mysql_stmt_store_result(stmt))
{
//...
}
row_count= 0;
fprintf(stdout, "Fetching results ...\n");
while (!mysql_stmt_fetch(stmt))
{
row_count++;
fprintf(stdout, " row %d\n", row_count);
/* column 2 */
fprintf(stdout, " column1 (string) : ");
if (is_null)
fprintf(stdout, " NULL\n");
else
fprintf(stdout, " %s(%ld)\n", login, length);
}
/* Validate rows fetched */
fprintf(stdout, " total rows fetched: %d\n", row_count);
/* Close the statement */
if (mysql_stmt_close(stmt))
{
//...
}
return 0;
}
It works if i insert manually the value in the query.
#define SELECT_SAMPLE "select p.id_paz from pazienti p where p.id_doc = 123"
if i execute this query and obviously comment the code in more... it works...

Answer 1

mysql_stmt_fetch has four possible return values: 0 (success), 1 (error), MYSQL_NO_DATA and MYSQL_DATA_TRUNCATED. Your loop terminates on anything other than success - but you don't check what actually happened. You might have 0 rows returned, or you might have some error condition best checked through mysql_stmt_errno() and mysql_stmt_error()...