从sqlite检索数据并将其存储到数组c/c++

Question

I have a column of integers(containing port numbers) in my sqlite data base. Now I want to retrieve list of ports from sqlite in an array form in a function and send data to those port numbers. I tried to make a struct having pointer to Vector but it didn't work out. Making char pointers caused Segmentation error. Here is my code:

struct ConnIDs{
    vector<int*> *ids;
};
static int callback(void *NotUsed, int argc, char **argv, char **szColName)
{
    ConnIDs *first=(ConnIDs *)NotUsed;

    for(int i = 0; i < argc; i++)
    {
        if (strcmp(szColName[i], "ConnID") == 0)
        {
            first->ids->push_back(argv[i]);
        }
        std::cout << szColName[i] << " = " << argv[i] << std::endl;
    }
    std::cout << "\n";
    return 0;
}

int main(){
    int result;
    sqlite3 *db;
    ConnIDs first;

    int count = 0; 
    char *szErrMsg = 0;
    int rc = sqlite3_open("Sqlite_Test.db", &db);
    if(rc)
    {
        std::cout << "Can't open database\n";
    } else
    {
        std::cout << "Open database successfully\n";
    }
    char *pSQL2[1];  
    pSQL2[0] = "Select * from Subscribers";
    rc = sqlite3_exec(db, pSQL2[0], callback, &first, &szErrMsg);
    if(rc != SQLITE_OK)
    {
        std::cout << "SQL Error: " << szErrMsg << std::endl;
        sqlite3_free(szErrMsg);
        break;
    }
    else
    {
        printf("count: %s\n", first.ids);
    }
}

Answer 1

This isn't an error regarding sqlite at all, it's just an uninitialised pointer. Let's look at the relevant code:

struct ConnIDs{
    vector<int*> *ids;
};
...

ConnIDs first;
rc = sqlite3_exec(db, pSQL2[0], callback, &first, &szErrMsg);
...
static int callback(void *NotUsed, int argc, char **argv, char **szColName)
{
    ConnIDs *first=(ConnIDs *)NotUsed;
    for(int i = 0; i < argc; i++)
       if (strcmp(szColName[i], "ConnID") == 0)
           first->ids->push_back(argv[i]);
}

I've omitted unnecessary parts. first is an object of ConnIDs , which is a std::vector<int*>* - it's a pointer to a vector. You pass first into your callback , then immediately try to push into ids ; however, you haven't initialised ids , so it's just pointing to rubbish.

The simplest fix is to just not make it a std::vector<int*>* and instead just make it std::vector<int*> , although I'm not certain int* is the correct type here, it will really depend on the data in your database. I would imagine either just int or std::string may be more applicable, but I'll leave that to you:

With the above change, your code will work as-is but you'll need to change first->ids->push_back to first->ids.push_back

---

Additional notes:

char *pSQL2[1];  
pSQL2[0] = "Select * from Subscribers";
rc = sqlite3_exec(db, pSQL2[0], callback, &first, &szErrMsg);

This is kinda weird. I'd probably either just write rc = sqlite3_exec(db, "Select * from Subscribers", ...) or

auto query = "Select * from Subscribers";
rc = sqlite3_exec(db, query.c_str(), callback, &first, &szErrMsg);