需要帮助在C中打开sqlite3数据库

Question

So, I'm not a programmer by trade, however, I've been tasked with a program that will poll an indicator using the modbus protocol and then log the information into a database. I was able to get it working, however, today, in an attempt to refine my code and get the main parts of the program (open modbus connection, write to database, open/create database) to be functions to be called during the main cycle, I've encountered trouble with the opening of the database. When I call the function, I'm getting nulls when I print value of the rc, both in the opening of the database and the creation of the table portions of the code.

This is the way I had this before I tried to clean up the code and do it all by calling a function:

int main(int argc, char*argv[]){
    sqlite3 *db;
//open database
int rc = sqlite3_open(db_name, &db);
if (rc != SQLITE_OK){
    fprintf(stderr, "Cannot open database: %s\n",
    sqlite3_errmsg(db));
    sqlite3_close(db);
    return 1;
}
char sql_query[1024];
// create sqlite table
snprintf(sql_query,sizeof(sql_query), "CREATE TABLE IF NOT EXISTS %s(%s INTEGER PRIMARY KEY,%s TEXT NOT NULL, %s TEXT NOT NULL, %s TEXT NOT NULL,%s REAL NOT NULL, %s REAL NOT NULL, %s REAL NOT NULL);",tbl_id1,tbl_col1,tbl_col2,tbl_col3,tbl_col4,tbl_col5,tbl_col6,tbl_col7);
rc = sqlite3_exec(db, sql_query, callback, 0, NULL);

if (rc != SQLITE_OK ){
    fprintf(stderr, "Failed to select data\n");
    fprintf(stderr, "SQL error: %s\n", NULL);

    sqlite3_free(NULL);
    sqlite3_close(db);

    return 1;
}

I've no problem running it like that, and the function I'm trying to use is that same code enclosed in

void open_Database(db){//sqlite code}

Invoking the function:

open_Database(db) (same db declared in the main routine)

Answer 1

Maybe you missed the fact, that sqlite3_open() needs a pointer to a variable pointing to a sqlite3 object. In your original code you declared a pointer to such an object and gave the address of that pointer to the open-function.

When you declare a sub-function doing that job, you should also give it a pointer-pointer:

void open_database (sqlite3 **db)
{
   int rc;
   rc = sqlite3_open(db_name, db);
   // ... now do error checking
}

And remember to not add that de-referencing ampersand before the db parameter in the call to sqlite3_open() !