leveldb::DB::Open(options, "./tmp", &db) ==>IO error: ./tmp: Invalid argument

I use wsl(ubuntu on windows) to build leveldb, however, when I want to test leveldb, and some errors happened.

```
#include
#include
#include
#include

#include

int main()
{
leveldb::DB *db;
leveldb::Options options;
leveldb::Status status;

std::string key1 = "key1";
std::string val1 = "val1", val;

options.create_if_missing = true;
status = leveldb::DB::Open(options, "./tmp", &db);
if (!status.ok())
{
std::cout << status.ToString() << std::endl;
exit(1);
}
}

```
```
IO error: ./tmp: Invalid argument
```