SQLite queries returning "unknown error" - c++

Question

I am developing a multithreaded cpp app that reads data from an sqlite database. I am running into a problem with multithreaded read attempts causing an "unknown error" to be thrown. I assume this has to be something like database locking.
My question is the following - Why does the database lock on read operations? I would understand if it was reading/writing, but this app only executes select statements.
Potential solutions to my problem: Throw some mutexes at the sections, switch from sqlite to some other database..
I realize sqlite is not an incredibly strong database, so am leaning more toward switching to some other database. Any suggestions?
Thanks!
The code:
//generate a db connection//
bool open(string filename){
if(sqlite3_open(filename.c_str(), &database) == SQLITE_OK)
{
return true;
}
return false;
}
//Query database//
vector<vector<string>> query(string query){
sqlite3_stmt *statement;
vector<vector<string>> results;
if(sqlite3_prepare_v2(database, query.c_str(), -1, &statement, 0) == SQLITE_OK)
{
int cols = sqlite3_column_count(statement);
int result = 0;
while(true)
{
result = sqlite3_step(statement);
if(result == SQLITE_ROW)
{
vector<string> values;
for(int col = 0; col < cols; col++)
{
auto temp_val = (char*)sqlite3_column_text(statement, col);
if(!temp_val){
cout << "FAIL" << endl;
}
else{
values.emplace_back(temp_val);
}
}
results.push_back(values);
}
else
{
break;
}
}
sqlite3_finalize(statement);
}
string error = sqlite3_errmsg(database);
if(error != "not an error") cout << query << " " << error << endl;
return results;
}
void close(){
sqlite3_close(database);
}

Answer 1

You say you are using a single connection from multiple threads. This is probably not the best idea, and depending on the options configured in your particular SQLite installation, it may not work at all. For more on this, see here: https://www.sqlite.org/threadsafe.html
As you can see, a full diagnosis of your problem requires looking at how your SQLite package was built...but you can try setting runtime options to see if that helps, as described on the above page.