SQLite: wrong call of sqlite3_prepare_v2 -> SIGSEGV - c

Question

I've got stuck on my homework with SQLite. I use 2 columns; the first for product, the second for count. A user adds new products, which updates the count. We have to control, that the user doesn't add the same product again, or prevent him from picking more units than are available. We have to use it frequently, so I created functions:
int exists(char *param, sqlite3** ppDb) //0 if product exists
{
int error = 0;
char *a = NULL;
sqlite3_stmt **ppStmt = NULL;
const char **pzTail = NULL;
char *zSQL = sqlite3_mprintf("SELECT 'products' FROM 'table' WHERE 'products' LIKE '%q'", param);
//HERE IT FALS
error = sqlite3_prepare_v2(
*ppDb, /* Database handle */
zSQL, /* SQL statement, UTF-8 encoded */
(sizeof(zSQL)+1), /* Maximum length of zSql in bytes. */
ppStmt, /* OUT: Statement handle */
pzTail /* OUT: Pointer to unused portion of zSql */
);
sqlite3_free(zSQL);
a = (char*) sqlite3_column_text(*ppStmt, 0);
return strcmp(a, param); //0 if same -> product is in db yet
}
//similar one for count
Call
int main(int argc, const char *argv[])
{
sqlite3 *pDb;
int error = 0;
//parsing input
error = sqlite3_open(argv[1], &pDb);
if (error == 0)
{
sqlite3_exec(
pDb, /* An open database */
"CREATE TABLE 'table' ('products', 'quantity')", /* SQL */
0, /* Callback function */
NULL, /* 1st argument to callback */
NULL /* Error msg written here */
);
if (exists(param[1], &pDb) == 0)
{
fprintf (stderr, "ERROR: Product exists yet\n");
}
else
{
char *zSQL = sqlite3_mprintf("INSERT INTO 'table' VALUES ('%q', '0')", param[1]);
error = sqlite3_exec(
pDb, /* An open database */
zSQL, /* SQL to be evaluated */
0, /* Callback function */
NULL, /* 1st argument to callback */
NULL /* Error msg written here */
);
sqlite3_free(zSQL);
if (error == 0) printf("Added\n");
else printf("%i", error);
}
}
else return 1;
return 0;
}
It fails on sqlite3_prepare_v2. I expect there is a problem with the pointer on pDb, but I wasn't able to fix it (I'm not fan of pointers - too strong a tool for beginner). When it fails, the debugger stacked on line 93396 in sqlite3.c (*ppStmt = 0; - it writes somewhere, where it should'nt).
Compiled on linux x64:
gcc -std=c99 -Wall -pedantic -Wextra -Werror -DSQLITE_THREADSAFE=0 -ldl -o sqlite main.c sqlite3.c
Nothing wrong (if I've copied wrongly brackets, ignore it - it's not the problem), SQLite 3.7.14.1
Sorry for my English, I'm from Czech.

Answer 1

sqlite3_prepare_v2 wants to write the statement pointer into your output variable, but you are not giving it a pointer to this pointer variable, you are giving it a NULL pointer.
Use:
sqlite3_stmt *pStmt;
sqlite3_prepare_v2(..., &pStmt, ...);
Also note that identifiers should be quoted with "double quotes" or [brackets] or
`backticks`
but not with 'single quotes', which are used for literal strings.