L4Re simple client server example

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We want to understand more deeply the following simple setup. A simple server and a client, which are just sending numbers between each other and print them. While writing it, we focus on the purpose of each method and object and its role in L4Re.

The whole source code can be found on github.com and you definitly have to look there for the complete files and overview.


Our two tasks are called simple-client and simple-server. The simple-client sends a number to the simple-server, which prints the received number and sends the number multiplied by two back to the client, which prints the received number also.

To do this we have to go through different steps:

  • the number has to be put into the UTCB
  • we need to say, that this number should be send to the server
  • the server should be ready to receive the number
  • the number should than be send by the client
  • the number should be processed by the server
  • the doubled number should be send back
  • and the finally received number should be processed by the client

We will first go through each line in the main function of the client, which puts the number into the utcb, sends it to the correct server, receives the answer and prints it.

Afterwards we look at the dispatch method, which prints the number and answers with the double of the number.

Finally we see the main function of the server, which prepares the server to be ready for receiving numbers.


First we look at the configuration file which sets up both tasks.

local ld = L4.default_loader
local channel = ld:new_channel()
            caps = {
               my_server_side = channel:svr()
         }, "rom/simple-server")
            caps = {
               my_client_side = channel
         }, "rom/simple-client")

For this example we start two tasks, the simple-server and simple-client task. They are connected via channel, an IPC Gate, with each other. my_server_side is the name of the capability selector in the simple-server task, which gets full access to the IPC Gate. The full access was granted cause of the svr method. The capability selector in the other task is named my_client_side and gets the least priviliges. These capability selectors are named to retrieve them later from the caps table.


We now want to look at the straight forward client.

Cap<void> server = Env::env()->get_cap<void>("my_client_side");

We get the capability selector, which we declared for the simple-client in its caps table in the ned.lua file. Therefor we use the Env::env() object, the Initial Environment of this task given by Ned. With this object, we can access the caps table with get_cap. The returned Cap will be used to refer to the server, the receiver of this message.

Ipc::Iostream ios(l4_utcb());

The Ipc::Iostream ios handles the values in the UTCB and calls the kernel for sending these values and receiving other. It is basically a wrapper around the kernel calls and around the placement of values in the UTCB. In the constructor of ios we provide the UTCB in which the Iostream should write and read the values. With l4_utcb() we get the current UTCB of the running thread.

ios << n;

This writes the number into the UTCB at the current location as managed by ios. The current location here is the first and it is moved after writing the number.


call will send and than receive data. First it sends the data stored in the UTCB to the thread were the server capability comes from. Second it blocks the current thread and makes a closed wait for the thread behind the server capability. cap() just returns the capability selector from the Cap. The thread will be unblocked if an IPC arrives, in this case the answer of the simple-server containing a number.

ios >> n;

After the answer from server has arrived, we read some number out of it.

cout << n << "\n";

And finally we print this number. Done.


The more complex server contains a very important pattern. It defines a Server_object which does the actual work of answering requests and a Registry_server which dispatches IPCs to different Server_objects.

struct SimpleServer : Server_object

We define our SimpleServer as a Server_object and need to implement its very simple interface.

int dispatch(l4_umword_t, Ipc::Iostream &ios)

dispatch is the only method needed to implement the interface of Server_object. We discard the first argument, cause we don't need it. The second argument is more interesting. It is an already set up Ipc::Iostream, which points to the UTCB containing the earliear send number.

ios >> n;

We can read this number. Particularly sizeof(n) bytes are read from the UTCB to n from the current position and the current position is forward by this number of bytes. No we can print the number and send the answer.

ios << n * 2;
return L4_EOK;

The answer is written to the UTCB and with L4_EOK we signal to the one who called dispatch that everything is alright.

In our main function we set up our server and the Server_object we implemented and let the server loop forever for accepting incoming IPCs and answering them.

Util::Registry_server<> server;

We instantiate a Registry_server which will be the heart of our server task. It maintains a registry, where objects of type Server_object are registered and has the loop method, which accepts an incoming IPC containing the capability selector used to identify the registered Server_object. Now the Server_object can be found in the registry and the incoming IPC can be forwarded as an Iostream to the dispatch method of the Server_object, the method above in this file.

Cap<void> cap = server.registry()->register_obj(&simple_server, "my_server_side");

This is the call which actually registers the our instance of SimpleServer in the registry() of the Registry_server. As we already saw in the ned.lua file, there is the name "my_server_side" and we use it here to say that whenever an IPC is made to the IPC-Gate named "my_server_side" the IPC is forwarded to the dispatch method of simple_server.

Now we have pretty much set up everything we need to let interaction of a client and a server happen.


We can now loop forever and deal with IPCs by forwarding them to our own Server_object and send our answer back.


We have seen a simple setup for a client and a server task in L4Re. With Ipc::Iostream you can read and write to an UTCB. Methods like call will initiate an IPC. You automatically get an Ipc::Iostream when you implement Server_object. Server_object itself is used handle IPCs and actually do something with requests and construct answers. Registry_server bundles the loop which receives IPCs and dispatches them to different Server_objects. Adding Server_objects to the server is as simple as registering them at the Registry_server.