libneo4j-client: Can't connect, can't diagnose

Question

I want to achieve the basic task of connection to a neo4j database using C++.
For this, I got libneo4j-client and copied the code from https://neo4j-client.net/. Compilation works fine.

Now I executed the following code:

neo4j_client_init();

neo4j_connection_t* connection = neo4j_connect("neo4j://user:pass@localhost:7687", NULL, NEO4J_INSECURE);
assert(connection);

This results in the assert stopping the execution as the connection is NULL.
Note that of course I entered an actual user name and password.

I made sure that the server actually exists. Opening localhost:7474 in the browser opens the browser interface that is to expect, accepts the user name and password and mentions that 7687 is a valid port.
Likewise, accessing the database using cypher-shell also works fine.

As far as I see it, neo4j://localhost:7687 should be the correct address.

I tried out several variations of this, like omitting the neo4j:// protocol identifier, trying to replace it with bolt, http and https identifiers, trying out 7474 as port, all combinations of this.

Then I tried to get some diagnostic data, using the logging capabilities of the library:

FILE* log_file = fopen("log.dat" , "w");
neo4j_logger_provider* logger = neo4j_std_logger_provider(log_file, 1, 0);
neo4j_config_t* config = neo4j_new_config();
neo4j_config_set_username(config, "username");
neo4j_config_set_password(config, "password");
neo4j_config_set_logger_provider(config, logger);

neo4j_connection_t* connection = neo4j_connect("neo4j://localhost:7687", config, NEO4J_INSECURE);
assert(connection);

As a result of calling fopen, log.dat is created, but that's it, file is empty afterwards, assertion fails.

At this point, I don't know what else to try or how to do any sort of diagnosis. Again, my goal is to connect with the database in general, I am not bound to libneo4j-client in particular. However, I want to write a program that can work on a new system without further installations, thus I can't use system() to call something like cyper-shell that would have to be installed. Plus later, there will be a lot of data flow, so I want the connection to be as direct as possible. Anybody wiser than me?

One thing that I am not sure are version compatibilities of libneo4j-client and the version of the database.
I also found the four year old question Using Neo4j database from C++ and I will start reading into the cyper API, but I hope somewhat that it will work out with libneo4j-client.
Edit: According to https://neo4j.com/developer/c/, libneo4j-client is in fact the default meant to be used by C++, and there are not any alternatives listed.

Edit: neo4j-client itself seems also not to work:

$ neo4j-client -u neo4j localhost
error: A secure connection could not be esablished (try --insecure)
$ neo4j-client -u neo4j localhost --insecure
error: Could not agree on a protocol version
$ neo4j-client -u neo4j neo4j://localhost:7687
error: A secure connection could not be esablished (try --insecure)
$ neo4j-client -u neo4j neo4j://localhost:7687 --insecure
error: Could not agree on a protocol version

Does this signify an incompatibility? The protocol that neo4j-client using being not that the server uses?

In Connection with neo4j-client failed on OS X yosemite, this was solved by changing the port, however, so not sure about this either.

Answer 1

The problem was solved by downgrading the version of Neo4J.

Version 4.0.1 uses a higher version of the bolt protocol, which isn't documented so far, and thus not used by neo4j-client.
Version 3.5 uses the old protocol (bolt version 1) and is thus compatible.