how to write the result of sqlite3_exec not in stdout

Question

I need to execute sql command "select" and return some data from the result of it. I'm trying to do it with sqlite3_exec, but it's only writting in stdout. What I need to do to write the data in array or something like this?

static int callback(void *NotUsed, int argc, char **argv, char **azColName){
    int i;
    for(i=0; i<argc; i++){
        printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
    }
    printf("\n");
    return 0;
}

void my_exec(char * sql) {
    sqlite3 *db;
    char *zErrMsg = nullptr;
    int rc;
    //char * sql;

/* Open database */
    rc = sqlite3_open("data_base.db", &db);
    if (rc) {
        fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
        exit(0);
    } else {
        fprintf(stdout, "Opened database successfully\n");
    }

/* Create SQL statement */
// sql

/* Execute SQL statement */
    rc = sqlite3_exec(db, sql, callback, nullptr, &zErrMsg);
    if (rc != SQLITE_OK) {
        fprintf(stderr, "SQL error: %s\n", zErrMsg);
        sqlite3_free(zErrMsg);
    } else {
        fprintf(stdout, "Success\n");
    }
    /* Close database */
    sqlite3_close(db);
}

Answer 1

Let's look at the fourth parameter of sqlite3_exec() . This is a pointer that is passed to the callback function. Give sqlite3_exec() a pointer to your data structure and store the results to that pointer in the callback.

You can for example use a vector:

std::vector<std::pair<std::string, std::string>> vec;
rc = sqlite3_exec(db, sql, callback, &vec, &zErrMsg);

The callback:

static int callback(void *dataPtr, int argc, char **argv, char **azColName){
    auto vec = static_cast<std::vector<std::pair<std::string, std::string>>*>(dataPtr);
    int i;
    for(i=0; i<argc; i++){
        vec->push_back({azColName[i], argv[i] ? argv[i] : "NULL"});
    }
    return 0;
}