\input texinfo
@c %**start of header
@setfilename gnunet-c-tutorial.info
@documentencoding UTF-8
@settitle GNUnet C Tutorial
@c %**end of header

@c including 'version.texi' makes makeinfo throw errors.
@include version2.texi

@copying
Copyright @copyright{} 2001-2017 GNUnet e.V.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
copy of the license is included in the section entitled ``GNU Free
Documentation License''.

A copy of the license is also available from the Free Software
Foundation Web site at @url{http://www.gnu.org/licenses/fdl.html}.

Alternately, this document is also available under the General
Public License, version 3 or later, as published by the Free Software
Foundation.  A copy of the license is included in the section entitled
``GNU General Public License''.

A copy of the license is also available from the Free Software
Foundation Web site at @url{http://www.gnu.org/licenses/gpl.html}.
@end copying

@dircategory Tutorial
@direntry
* GNUnet-C-Tutorial: (gnunet-c-tutorial).       C Tutorial for GNunet
@end direntry


@titlepage
@title GNUnet C Tutorial
@subtitle A Tutorial for GNUnet @value{VERSION} (C version)
@author The GNUnet Developers

@page
@vskip 0pt plus 1filll

@insertcopying
@end titlepage

@contents

@c **** TODO
@c 1. Update content?
@c 2. Either reference main documentation or
@c 3. Merge this into main documentation

@node Top
@top Introduction

This tutorials explains how to install GNUnet on a
GNU/Linux system and gives an introduction on how
GNUnet can be used to develop a Peer-to-Peer application.
Detailed installation instructions for
various operating systems and a detailed list of all
dependencies can be found on our website at
@uref{https://gnunet.org/installation} and in our
Reference Documentation (GNUnet Handbook).

Please read this tutorial carefully since every single step is
important and do not hesitate to contact the GNUnet team if you have
any questions or problems! Check here how to contact the GNUnet
team: @uref{https://gnunet.org/contact_information}

@menu

* Vocabulary::                          Vocabulary used throughout this document
* Installing GNUnet::                   Installing GNUnet
* Introduction to GNUnet Architecture:: Introduction to GNUnet Architecture
* First Steps with GNUnet::             First Steps with GNUnet
* Developing Applications::             Developing Applications

@detailmenu
 --- The Detailed Node Listing ---

Vocabulary

Installing GNUnet

* Obtaining a stable version::
* Installing Build Tool Chain and Dependencies::
* Obtaining the latest version from Git::
* Compiling and Installing GNUnet::
* Common Issues - Check your GNUnet installation::

Introduction to GNUnet Architecture

First Steps with GNUnet

* Configure your peer::
* Start a peer::
* Monitor a peer::
* Starting Two Peers by Hand::
* Starting Peers Using the Testbed Service::

Developing Applications

* gnunet-ext::
* Adapting the Template::
* Writing a Client Application::
* Writing a Service::
* Interacting directly with other Peers using the CORE Service::
* Storing peer-specific data using the PEERSTORE service::
* Using the DHT::
* Debugging with gnunet-arm::

@end detailmenu
@end menu

@node Vocabulary
@chapter Vocabulary

@menu
* Words and characters::
* Technical Assumptions::
@end menu

@node Words and characters
@section Words and characters

Throughout this document we use certain words and characters.

@enumerate
@item
@c ``@command{#}'' in example code blocks describes commands you execute as root.
``@command{#}'' in example code blocks describes commands, ie comments.

@example
# Do the foobar thing:
$ make foobar
@end example

@item
Dollarsign ``@command{$}'' in example code blocks describes commands you execute as
unprivileged users.

@example
$ cd foo; ./configure --example-switch
@end example

@item
Backslash ``@command{\}'' describes linebreaks.

@example
./configure --foo --bar --baz \
 --short-loop
@end example

...expands to @code{./configure --foo --bar --baz --short-loop}

@end enumerate

@node Technical Assumptions
@section Technical Assumptions

@c Is it really assuming Bash (ie Bash extensions of POSIX being used)?
The shell on GNU systems is assumed to be Bash.

@node Installing GNUnet
@chapter Installing GNUnet

First of all you have to install a current version of GNUnet.
You can download a tarball of a stable version from GNU FTP mirrors
or obtain the latest development version from our Git repository.

Most of the time you should prefer to download the stable version
since with the latest development version things can be broken,
functionality can be changed or tests can fail. You should only use
the development version if you know that you require a certain
feature or a certain issue has been fixed since the last release.

@menu
* Obtaining a stable version::
* Installing Build Tool Chain and Dependencies::
* Obtaining the latest version from Git::
* Compiling and Installing GNUnet::
* Common Issues - Check your GNUnet installation::
@end menu

@node Obtaining a stable version
@section Obtaining a stable version

Download the tarball from
@indicateurl{https://ftp.gnu.org/gnu/gnunet/gnunet-@value{VERSION}.tar.gz}.

Make sure to download the associated @file{.sig} file and to verify the
authenticity of the tarball against it, like this:

@example
$ wget https://ftp.gnu.org/gnu/gnunet/gnunet-@value{VERSION}.tar.gz.sig
$ gpg --verify-files gnunet-@value{VERSION}.tar.gz.sig
@end example

@noindent
If this command fails because you do not have the required public key,
then you need to run this command to import it:

@example
$ gpg --keyserver keys.gnupg.net --recv-keys 48426C7E
@end example

@noindent
and rerun the @code{gpg --verify-files} command.

Now you can extract the tarball and rename the resulting
directory to @i{gnunet} which we will be using in the
remainder of this document.

@example
$ tar xvzf gnunet-@value{VERSION}.tar.gz
$ mv gnunet-@value{VERSION} gnunet
$ cd gnunet
@end example

@noindent
However, please note that stable versions can be very outdated.
As a developer you are @b{strongly} encouraged to use the version
from @uref{https://gnunet.org/git/, git}.

@node  Installing Build Tool Chain and Dependencies
@section Installing Build Tool Chain and Dependencies

To successfully compile GNUnet you need the tools to build GNUnet and
the required dependencies. Please have a look at
@uref{https://gnunet.org/dependencies} for a list of required dependencies
and @uref{https://gnunet.org/generic_installation} for specific
instructions for your operating system. Please check the notes at
the end of the configure process about required dependencies.

For GNUnet bootstrapping support and the http(s) plugin you should
install @uref{https://gnunet.org/gnurl, libgnurl}.
For the filesharing service you should install at least one of the
datastore backends. MySQL, SQlite and PostgreSQL are supported.

@node Obtaining the latest version from Git
@section Obtaining the latest version from Git

The latest development version can obtained from our Git repository.
To obtain the code you need Git installed and checkout the repository
using:

@example
$ git clone https://gnunet.org/git/gnunet
@end example

@noindent
After cloning the repository you have to execute the @file{bootstrap}
script in the directory:

@example
$ cd gnunet ; ./bootstrap
@end example

@noindent
The remainder of this tutorial assumes that you have the Git branch
``master'' checked out.

@node Compiling and Installing GNUnet
@section Compiling and Installing GNUnet

First, you need to install at least libgnupgerror 1.27 and
libgcrypt 1.7.6.

@example
$ export GNUPGFTP="https://www.gnupg.org/ftp/gcrypt"
$ wget $GNUPGFTP/libgpg-error/libgpg-error-1.27.tar.bz2
$ tar xf libgpg-error-1.27.tar.bz2
$ cd libgpg-error-1.27
$ ./configure
$ sudo make install
$ cd ..
@end example

@example
$ export GNUPGFTP="https://www.gnupg.org/ftp/gcrypt"
$ wget $GNUPGFTP/libgcrypt/libgcrypt-1.7.6.tar.bz2
$ tar xf libgcrypt-1.7.6.tar.bz2
$ cd libgcrypt-1.7.6
$ ./configure
$ sudo make install
$ cd ..
@end example

@menu
* Installation::
@end menu

@node Installation
@subsection Installation
Assuming all dependencies are installed, the following commands will
compile and install GNUnet in your home directory. You can specify the
directory where GNUnet will be installed by changing the
@code{--prefix} value when calling @command{./configure}.  If
you do not specifiy a prefix, GNUnet is installed in the directory
@file{/usr/local}. When developing new applications you may want
to enable verbose logging by adding @code{--enable-logging=verbose}:

@example
$ ./configure --prefix=$PREFIX --enable-logging
$ make
$ make install
@end example

@noindent
After installing GNUnet you have to add your GNUnet installation
to your path environmental variable. In addition you have to
create the @file{.config} directory in your home directory
(unless it already exists) where GNUnet stores its data and an
empty GNUnet configuration file:

@example
$ export PATH=$PATH:$PREFIX/bin
$ echo export PATH=$PREFIX/bin:\\$PATH >> ~/.bashrc
$ mkdir ~/.config/
$ touch ~/.config/gnunet.conf
@end example

@node Common Issues - Check your GNUnet installation
@section Common Issues - Check your GNUnet installation

You should check your installation to ensure that installing GNUnet
was successful up to this point. You should be able to access GNUnet's
binaries and run GNUnet's self check.

@example
$ which gnunet-arm
@end example

@noindent
should return $PREFIX/bin/gnunet-arm. It should be located in your
GNUnet installation and the output should not be empty.
If you see an output like:

@example
$ which gnunet-arm
@end example

@noindent
check your PATH variable to ensure GNUnet's @file{bin} directory is
included.

GNUnet provides tests for all of its subcomponents. Run

@example
$ make check
@end example

@noindent
to execute tests for all components. @command{make check} traverses all
subdirectories in @file{src}. For every subdirectory you should
get a message like this:

@example
make[2]: Entering directory `/home/$USER/gnunet/contrib'
PASS: test_gnunet_prefix
=============
1 test passed
=============
@end example

@node Introduction to GNUnet Architecture
@chapter Introduction to GNUnet Architecture

GNUnet is organized in layers and services. Each service is composed of a
main service implementation and a client library for other programs to use
the service's functionality, described by an API.
@c This approach is shown in
@c FIXME: enable this once the commented block below works:
@c figure~\ref fig:service.
Some services provide an additional command line tool to enable the user
to interact with the service.

Very often it is other GNUnet services that will use these APIs to build
the higher layers of GNUnet on top of the lower ones. Each layer expands
or extends the functionality of the service below (for instance, to build
a mesh on top of a DHT).
@c FXIME: See comment above.
@c See figure ~\ref fig:interaction for an illustration of this approach.

@c ** @image filename[, width[, height[, alttext[, extension]]]]
@c FIXME: Texlive (?) 20112 makes the assumption that this means
@c 'images/OBJECTNAME.txt' but later versions of it (2017) use this
@c syntax as described below.
@c TODO: Checkout the makedoc script Guile uses.

@image{images/gnunet-tutorial-service,,5in,Service with API and network protocol,.png}

@image{images/gnunet-tutorial-system,,5in,The layered system architecture of GNUnet,.png}

@c \begin{figure}[!h]
@c   \begin{center}
@c %  \begin{subfigure}
@c         \begin{subfigure}[b]{0.3\textwidth}
@c                 \centering
@c                 \includegraphics[width=\textwidth]{figs/Service.pdf}
@c                 \caption{Service with API and network protocol}
@c                 \label{fig:service}
@c         \end{subfigure}
@c         ~~~~~~~~~~
@c         \begin{subfigure}[b]{0.3\textwidth}
@c                 \centering
@c                 \includegraphics[width=\textwidth]{figs/System.pdf}
@c                 \caption{Service interaction}
@c                 \label{fig:interaction}
@c         \end{subfigure}
@c   \end{center}
@c   \caption{GNUnet's layered system architecture}
@c \end{figure}

The main service implementation runs as a standalone process in the
operating system and the client code runs as part of the client program,
so crashes of a client do not affect the service process or other clients.
The service and the clients communicate via a message protocol to be
defined and implemented by the programmer.

@node First Steps with GNUnet
@chapter First Steps with GNUnet

@menu
* Configure your peer::
* Start a peer::
* Monitor a peer::
* Starting Two Peers by Hand::
* Starting Peers Using the Testbed Service::
@end menu

@node Configure your peer
@section Configure your peer

First of all we need to configure your peer. Each peer is started with
a configuration containing settings for GNUnet itself and its services.
This configuration is based on the default configuration shipped with
GNUnet and can be modified. The default configuration is located in the
@file{$PREFIX/share/gnunet/config.d} directory. When starting a peer, you
can specify a customized configuration using the the @command{-c} command
line switch when starting the ARM service and all other services. When
using a modified configuration the default values are loaded and only
values specified in the configuration file will replace the default
values.

Since we want to start additional peers later, we need some modifications
from the default configuration. We need to create a separate service
home and a file containing our modifications for this peer:

@example
$ mkdir ~/gnunet1/
$ touch peer1.conf
@end example

@noindent
Now add the following lines to @file{peer1.conf} to use this directory.
For simplified usage we want to prevent the peer to connect to the GNUnet
network since this could lead to confusing output. This modifications
will replace the default settings:

@example
[PATHS]
# Use this directory to store GNUnet data
GNUNET_HOME = ~/gnunet1/
[hostlist]
# prevent bootstrapping
SERVERS =
@end example

@node Start a peer
@section Start a peer
Each GNUnet instance (called peer) has an identity (peer ID) based on a
cryptographic public private key pair. The peer ID is the printable hash
of the public key.

GNUnet services are controlled by a master service, the so called
@dfn{Automatic Restart Manager} (ARM). ARM starts, stops and even
restarts services automatically or on demand when a client connects.
You interact with the ARM service using the @command{gnunet-arm} tool.
GNUnet can then be started with @command{gnunet-arm -s} and stopped with
@command{gnunet-arm -e}.  An additional service not automatically started
can be started using @command{gnunet-arm -i <service name>} and stopped
using @command{gnunet-arm -k <servicename>}.

Once you have started your peer, you can use many other GNUnet commands
to interact with it.  For example, you can run:

@example
$ gnunet-peerinfo -s
@end example

@noindent
to obtain the public key of your peer.

You should see an output containing the peer ID similar to:

@example
I am peer `0PA02UVRKQTS2C .. JL5Q78F6H0B1ACPV1CJI59MEQUMQCC5G'.
@end example

@node Monitor a peer
@section Monitor a peer

In this section, we will monitor the behaviour of our peer's DHT
service with respect to a specific key. First we will start
GNUnet and then start the DHT service and use the DHT monitor tool
to monitor the PUT and GET commands we issue ussing the
@command{gnunet-dht-put} and @command{gnunet-dht-get} commands.
Using the ``monitor'' line given below, you can observe the behavior
of your own peer's DHT with respect to the specified KEY:

@example
# start gnunet with all default services:
$ gnunet-arm -c ~/peer1.conf -s
# start DHT service:
$ gnunet-arm -c ~/peer1.conf -i dht
$ cd ~/gnunet/src/dht;
$ ./gnunet-dht-monitor -c ~/peer1.conf -k KEY
@end example

@noindent
Now open a separate terminal and change again to
the @file{gnunet/src/dht} directory:

@example
$ cd ~/gnunet/src/dht
# put VALUE under KEY in the DHT:
$ ./gnunet-dht-put -c ~/peer1.conf -k KEY -d VALUE
# get key KEY from the DHT:
$ ./gnunet/src/dht/gnunet-dht-get -c ~/peer1.conf -k KEY
# print statistics about current GNUnet state:
$ gnunet-statistics -c ~/peer1.conf
# print statistics about DHT service:
$ gnunet-statistics -c ~/peer1.conf -s dht
@end example

@node Starting Two Peers by Hand
@section Starting Two Peers by Hand

This section describes how to start two peers on the same machine by hand.
The process is rather painful, but the description is somewhat
instructive. In practice, you might prefer the automated method
(@pxref{Starting Peers Using the Testbed Service}).

@menu
* Setup a second peer::
* Start the second peer and connect the peers::
* How to connect manually::
@end menu

@node Setup a second peer
@subsection Setup a second peer
We will now start a second peer on your machine.
For the second peer, you will need to manually create a modified
configuration file to avoid conflicts with ports and directories.
A peers configuration file is by default located
in @file{~/.gnunet/gnunet.conf}. This file is typically very short
or even empty as only the differences to the defaults need to be
specified.  The defaults are located in many files in the
@file{$PREFIX/share/gnunet/config.d} directory.

To configure the second peer, use the files
@file{$PREFIX/share/gnunet/config.d} as a template for your main
configuration file:

@example
$ cat $PREFIX/share/gnunet/config.d/*.conf > peer2.conf
@end example

@noindent
Now you have to edit @file{peer2.conf} and change:

@itemize
@item @code{GNUNET\_TEST\_HOME} under @code{PATHS}
@item Every (uncommented) value for ``@code{PORT}'' (add 10000) in any
section (the option may be commented out if @code{PORT} is
prefixed by "\#", in this case, UNIX domain sockets are used
and the PORT option does not need to be touched)
@item Every value for ``@code{UNIXPATH}'' in any section
(e.g. by adding a "-p2" suffix)
@end itemize

to a fresh, unique value.  Make sure that the PORT numbers stay
below 65536. From now on, whenever you interact with the second peer,
you need to specify @command{-c peer2.conf} as an additional
command line argument.

Now, generate the 2nd peer's private key:

@example
$ gnunet-peerinfo -s -c peer2.conf
@end example

@noindent
This may take a while, generate entropy using your keyboard or mouse
as needed.  Also, make sure the output is different from the
gnunet-peerinfo output for the first peer (otherwise you made an
error in the configuration).

@node Start the second peer and connect the peers
@subsection Start the second peer and connect the peers

Then, you can start a second peer using:

@example
$ gnunet-arm -c peer2.conf -s
$ gnunet-arm -c peer2.conf -i dht
$ ~/gnunet/src/dht/gnunet-dht-put -c peer2.conf -k KEY -d VALUE
$ ~/gnunet/src/dht/gnunet-dht-get -c peer2.conf -k KEY
@end example

If you want the two peers to connect, you have multiple options:

@itemize
@item UDP neighbour discovery (automatic)
@item Setup a bootstrap server
@item Connect manually
@end itemize

To setup peer 1 as bootstrapping server change the configuration of
the first one to be a hostlist server by adding the following lines to
@file{peer1.conf} to enable bootstrapping server:

@example
[hostlist]
OPTIONS = -p
@end example

@noindent
Then change @file{peer2.conf} and replace the ``@code{SERVERS}''
line in the ``@code{[hostlist]}'' section with
``@code{http://localhost:8080/}''.  Restart both peers using:

@example
# stop first peer
$ gnunet-arm -c peer1.conf -e
# start first peer
$ gnunet-arm -c peer1.conf -s
# start second peer
$ gnunet-arm -c peer2.conf -s
@end example

@noindent
Note that if you start your peers without changing these settings, they
will use the ``global'' hostlist servers of the GNUnet P2P network and
likely connect to those peers.  At that point, debugging might become
tricky as you're going to be connected to many more peers and would
likely observe traffic and behaviors that are not explicitly controlled
by you.

@node How to connect manually
@subsection How to connect manually

If you want to use the @code{peerinfo} tool to connect your
peers, you should:

@itemize
@item Set @code{FORCESTART = NO} in section @code{hostlist}
(to not connect to the global GNUnet)
@item Start both peers running @command{gnunet-arm -c peer1.conf -s}
and @command{gnunet-arm -c peer2.conf -s}
@item Get @code{HELLO} message of the first peer running
@command{gnunet-peerinfo -c peer1.conf -g}
@item Give the output to the second peer by running
@command{gnunet-peerinfo -c peer2.conf -p '<output>'}
@end itemize

Check that they are connected using @command{gnunet-core -c peer1.conf},
which should give you the other peer's peer identity:

@example
$ gnunet-core -c peer1.conf
Peer `9TVUCS8P5A7ILLBGO6 [...shortened...] 1KNBJ4NGCHP3JPVULDG'
@end example

@node Starting Peers Using the Testbed Service
@section Starting Peers Using the Testbed Service
@c \label{sec:testbed}

GNUnet's testbed service is used for testing scenarios where
a number of peers are to be started.  The testbed can manage peers
on a single host or on multiple hosts in a distributed fashion.
On a single affordable computer, it should be possible to run
around tens of peers without drastically increasing the load on the
system.

The testbed service can be access through its API
@file{include/gnunet\_testbed\_service.h}.  The API provides many
routines for managing a group of peers.  It also provides a helper
function @code{GNUNET\_TESTBED\_test\_run()} to quickly setup a
minimalistic testing environment on a single host.

This function takes a configuration file which will be used as a
template configuration for the peers.  The testbed takes care of
modifying relevant options in the peers' configuration such as
@code{SERVICEHOME}, @code{PORT}, @code{UNIXPATH} to unique values
so that peers run without running into conflicts.  It also checks
and assigns the ports in configurations only if they are free.

Additionally, the testbed service also reads its options from the
same configuration file.  Various available options and details
about them can be found in the testbed default configuration file
@file{src/testbed/testbed.conf}.

With the testbed API, a sample test case can be structured as follows:

@example
@verbatiminclude testbed_test.c
@end example

@noindent
The source code for the above listing can be found at
@uref{https://gnunet.org/git/gnunet.git/tree/doc/testbed_test.c}
or in the @file{doc/} folder of your repository check-out.
After installing GNUnet, the above source code can be compiled as:

@example
$ export CPPFLAGS="-I/path/to/gnunet/headers"
$ export LDFLAGS="-L/path/to/gnunet/libraries"
$ gcc $CPPFLAGS $LDFLAGS -o testbed-test testbed_test.c \
 -lgnunettestbed -lgnunetdht -lgnunetutil
# Generate (empty) configuration
$ touch template.conf
# run it (press CTRL-C to stop)
$ ./testbed-test
@end example

@noindent
The @code{CPPFLAGS} and @code{LDFLAGS} are necessary if GNUnet
is installed into a different directory other than @file{/usr/local}.

All of testbed API's peer management functions treat management
actions as operations and return operation handles.  It is expected
that the operations begin immediately, but they may get delayed (to
balance out load on the system). The program using the API then has
to take care of marking the operation as ``done'' so that its
associated resources can be freed immediately and other waiting
operations can be executed.  Operations will be canceled if they are
marked as ``done'' before their completion.

An operation is treated as completed when it succeeds or fails.
Completion of an operation is either conveyed as events through
@i{controller event callback} or through respective operation
completion callbacks.  In functions which support completion
notification through both controller event callback and operation
completion callback, first the controller event callback will be
called.  If the operation is not marked as done in that callback
or if the callback is given as NULL when creating the operation,
the operation completion callback will be called.  The API
documentation shows which event are to be expected in the
controller event notifications.  It also documents any exceptional
behaviour.

Once the peers are started, test cases often need to connect
some of the peers' services.  Normally, opening a connect to
a peer's service requires the peer's configuration.  While using
testbed, the testbed automatically generates per-peer configuration.
Accessing those configurations directly through file system is
discouraged as their locations are dynamically created and will be
different among various runs of testbed.  To make access to these
configurations easy, testbed API provides the function
@code{GNUNET\_TESTBED\_service\_connect()}.  This function fetches
the configuration of a given peer and calls the @i{Connect Adapter}.
In the example code, it is the @code{dht\_ca}.  A connect adapter is
expected to open the connection to the needed service by using the
provided configuration and return the created service connection handle.
Successful connection to the needed service is signaled through
@code{service\_connect\_comp\_cb}.

A dual to connect adapter is the @i{Disconnect Adapter}.  This callback
is called after the connect adapter has been called when the operation
from @code{GNUNET\_TESTBED\_service\_connect()} is marked as ``done''.
It has to disconnect from the service with the provided service
handle (@code{op\_result}).

Exercise: Find out how many peers you can run on your system.

Exercise: Find out how to create a 2D torus topology by changing the
options in the configuration file.
See @uref{https://gnunet.org/supported-topologies}, then use the
DHT API to store and retrieve values in the network.

@node Developing Applications
@chapter Developing Applications

@menu
* gnunet-ext::
* Adapting the Template::
* Writing a Client Application::
* Writing a Service::
* Interacting directly with other Peers using the CORE Service::
* Storing peer-specific data using the PEERSTORE service::
* Using the DHT::
* Debugging with gnunet-arm::
@end menu

@node gnunet-ext
@section gnunet-ext
To develop a new peer-to-peer application or to extend GNUnet we provide
a template build system for writing GNUnet extensions in C. It can be
obtained as follows:

@example
$ git clone https://gnunet.org/git/gnunet-ext
$ cd gnunet-ext/
$ ./bootstrap
$ ./configure --prefix=$PREFIX --with-gnunet=$PREFIX
$ make
$ make install
$ make check
@end example

@noindent
The GNUnet ext template includes examples and a working buildsystem
for a new GNUnet service. A common GNUnet service consists of the
following parts which will be discussed in detail in the remainder
of this document. The functionality of a GNUnet service is implemented in:

@itemize
@item the GNUnet service (gnunet-ext/src/ext/gnunet-service-ext.c)
@item the client API (gnunet-ext/src/ext/ext_api.c)
@item the client application using the service API
(gnunet-ext/src/ext/gnunet-ext.c)
@end itemize

The interfaces for these entities are defined in:

@itemize
@item client API interface (gnunet-ext/src/ext/ext.h)
@item the service interface (gnunet-ext/src/include/gnunet_service_SERVICE.h)
@item the P2P protocol (gnunet-ext/src/include/gnunet_protocols_ext.h)
@end itemize


In addition the ext systems provides:

@itemize
@item a test testing the API (gnunet-ext/src/ext/test_ext_api.c)
@item a configuration template for the service
(gnunet-ext/src/ext/ext.conf.in)
@end itemize

@node Adapting the Template
@section Adapting the Template

The first step for writing any extension with a new service is to
ensure that the @file{ext.conf.in} file contains entries for the
@code{UNIXPATH}, @code{PORT} and @code{BINARY} for the service in a
section named after the service.

If you want to adapt the template rename the @file{ext.conf.in} to
match your services name, you have to modify the @code{AC\_OUTPUT}
section in @file{configure.ac} in the @file{gnunet-ext} root.

@node Writing a Client Application
@section Writing a Client Application

When writing any client application (for example, a command-line
tool), the basic structure is to start with the
@code{GNUNET\_PROGRAM\_run} function.  This function will parse
command-line options, setup the scheduler and then invoke the
@code{run} function (with the remaining non-option arguments)
and a handle to the parsed configuration (and the configuration
file name that was used, which is typically not needed):

@example
@verbatiminclude tutorial-examples/001.c
@end example

@menu
* Handling command-line options::
* Writing a Client Library::
* Writing a user interface::
@end menu

@node Handling command-line options
@subsection Handling command-line options

Options can then be added easily by adding global variables and
expanding the @code{options} array.  For example, the following would
add a string-option and a binary flag (defaulting to @code{NULL} and
@code{GNUNET\_NO} respectively):

@example
@verbatiminclude tutorial-examples/002.c
@end example

Issues such as displaying some helpful text describing options using
the @code{--help} argument and error handling are taken care of when
using this approach.  Other @code{GNUNET\_GETOPT\_}-functions can be used
to obtain integer value options, increment counters, etc.  You can
even write custom option parsers for special circumstances not covered
by the available handlers. To check if an argument was specified by the
user you initialize the variable with a specific value (e.g. NULL for
a string and GNUNET\_SYSERR for a integer) and check after parsing
happened if the values were modified.

Inside the @code{run} method, the program would perform the
application-specific logic, which typically involves initializing and
using some client library to interact with the service.  The client
library is supposed to implement the IPC whereas the service provides
more persistent P2P functions.

Exercise: Add a few command-line options and print them inside
of @code{run}.  What happens if the user gives invalid arguments?

@node Writing a Client Library
@subsection Writing a Client Library

The first and most important step in writing a client library is to
decide on an API for the library.  Typical API calls include
connecting to the service, performing application-specific requests
and cleaning up.  Many examples for such service APIs can be found
in the @file{gnunet/src/include/gnunet\_*\_service.h} files.

Then, a client-service protocol needs to be designed.  This typically
involves defining various message formats in a header that will be
included by both the service and the client library (but is otherwise
not shared and hence located within the service's directory and not
installed by @command{make install}).  Each message must start with a
@code{struct GNUNET\_MessageHeader} and must be shorter than 64k.  By
convention, all fields in IPC (and P2P) messages must be in big-endian
format (and thus should be read using @code{ntohl} and similar
functions and written using @code{htonl} and similar functions).
Unique message types must be defined for each message struct in the
@file{gnunet\_protocols.h} header (or an extension-specific include
file).

@menu
* Connecting to the Service::
* Sending messages::
* Receiving Replies from the Service::
@end menu

@node Connecting to the Service
@subsubsection Connecting to the Service

Before a client library can implement the application-specific protocol
with the service, a connection must be created:

@example
@verbatiminclude tutorial-examples/003.c
@end example

@noindent
As a result a @code{GNUNET\_MQ\_Handle} is returned
which can to used henceforth to transmit messages to the service.
The complete MQ API can be found in @file{gnunet\_mq\_lib.h}.
The @code{hanlders} array in the example above is incomplete.
Here is where you will define which messages you expect to
receive from the service, and which functions handle them.
The @code{error\_cb} is a function that is to be called whenever
there are errors communicating with the service.

@node Sending messages
@subsubsection Sending messages

In GNUnet, messages are always sent beginning with a
@code{struct GNUNET\_MessageHeader} in big endian format.
This header defines the size and the type of the
message, the payload follows after this header.

@example
@verbatiminclude tutorial-examples/004.c
@end example

@noindent
Existing message types are defined in @file{gnunet\_protocols.h}.
A common way to create a message is with an envelope:

@example
@verbatiminclude tutorial-examples/005.c
@end example

@noindent
Exercise: Define a message struct that includes a 32-bit
unsigned integer in addition to the standard GNUnet MessageHeader.
Add a C struct and define a fresh protocol number for your message.
Protocol numbers in gnunet-ext are defined
in @file{gnunet-ext/src/include/gnunet_protocols_ext.h}

Exercise: Find out how you can determine the number of messages
in a message queue.

Exercise: Find out how you can determine when a message you
have queued was actually transmitted.

Exercise: Define a helper function to transmit a 32-bit
unsigned integer (as payload) to a service using some given client
handle.

@node Receiving Replies from the Service
@subsubsection Receiving Replies from the Service

Clients can receive messages from the service using the handlers
specified in the @code{handlers} array we specified when connecting
to the service.  Entries in the the array are usually created using
one of two macros, depending on whether the message is fixed size
or variable size.  Variable size messages are managed using two
callbacks, one to check that the message is well-formed, the other
to actually process the message.  Fixed size messages are fully
checked by the MQ-logic, and thus only need to provide the handler
to process the message.  Note that the prefixes @code{check\_}
and @code{handle\_} are mandatory.

@example
@verbatiminclude tutorial-examples/006.c
@end example

@noindent
Exercise: Expand your helper function to receive a response message
(for example, containing just the @code{struct GNUnet MessageHeader}
without any payload).  Upon receiving the service's response, you
should call a callback provided to your helper function's API.

Exercise: Figure out where you can pass values to the
closures (@code{cls}).

@node Writing a user interface
@subsection Writing a user interface

Given a client library, all it takes to access a service now is to
combine calls to the client library with parsing command-line
options.

Exercise: Call your client API from your @code{run()} method in your
client application to send a request to the service.  For example,
send a 32-bit integer value based on a number given at the
command-line to the service.

@node Writing a Service
@section Writing a Service

Before you can test the client you've written so far, you'll
need to also implement the corresponding service.

@menu
* Code Placement::
* Starting a Service::
@end menu

@node Code Placement
@subsection Code Placement

New services are placed in their own subdirectory under
@file{gnunet/src}. This subdirectory should contain the API
implementation file @file{SERVICE\_api.c}, the description of
the client-service protocol @file{SERVICE.h} and P2P protocol
@file{SERVICE\_protocol.h}, the implementation of the service itself
@file{gnunet-service-SERVICE.h} and several files for tests,
including test code and configuration files.

@node Starting a Service
@subsection Starting a Service

The key API definition for creating a service is the
@code{GNUNET\_SERVICE\_MAIN} macro:

@example
@verbatiminclude tutorial-examples/007.c
@end example

@noindent
In addition to the service name and flags, the macro takes three
functions, typically called @code{run}, @code{client\_connect\_cb} and
@code{client\_disconnect\_cb} as well as an array of message handlers
that will be called for incoming messages from clients.

A minimal version of the three central service funtions would look
like this:

@example
@verbatiminclude tutorial-examples/008.c
@end example

@noindent
Exercise: Write a stub service that processes no messages at all
in your code.  Create a default configuration for it, integrate it
with the build system and start the service from
@command{gnunet-service-arm} using @command{gnunet-arm -i NAME}.

Exercise: Figure out how to set the closure (@code{cls}) for handlers
of a service.

Exercise: Figure out how to send messages from the service back to the
client.

Each handler function in the service @b{must} eventually (possibly in some
asynchronous continuation) call
@code{GNUNET\_SERVICE\_client\_continue()}. Only after this call
additional messages from the same client may
be processed. This way, the service can throttle processing messages
from the same client.

Exercise: Change the service to ``handle'' the message from your
client (for now, by printing a message).  What happens if you
forget to call @code{GNUNET\_SERVICE\_client\_continue()}?

@node Interacting directly with other Peers using the CORE Service
@section Interacting directly with other Peers using the CORE Service

FIXME: This section still needs to be updated to the lastest API!

One of the most important services in GNUnet is the @code{CORE} service
managing connections between peers and handling encryption between peers.

One of the first things any service that extends the P2P protocol
typically does is connect to the @code{CORE} service using:

@example
@verbatiminclude tutorial-examples/009.c
@end example

@menu
* New P2P connections::
* Receiving P2P Messages::
* Sending P2P Messages::
* End of P2P connections::
@end menu

@node New P2P connections
@subsection New P2P connections

Before any traffic with a different peer can be exchanged, the peer must
be known to the service. This is notified by the @code{CORE}
@code{connects} callback, which communicates the identity of the new
peer to the service:

@example
@verbatiminclude tutorial-examples/010.c
@end example

@noindent
Note that whatever you return from @code{connects} is given as the
@i{cls} argument to the message handlers for messages from
the respective peer.

Exercise: Create a service that connects to the @code{CORE}.  Then
start (and connect) two peers and print a message once your connect
callback is invoked.

@node Receiving P2P Messages
@subsection Receiving P2P Messages

To receive messages from @code{CORE}, you pass the desired
@i{handlers} to the @code{GNUNET\_CORE\_connect()} function,
just as we showed for services.

It is your responsibility to process messages fast enough or
to implement flow control. If an application does not process
CORE messages fast enough, CORE will randomly drop messages
to not keep a very long queue in memory.

Exercise: Start one peer with a new service that has a message
handler and start a second peer that only has your ``old'' service
without message handlers.  Which ``connect'' handlers are invoked when
the two peers are connected?  Why?

@node Sending P2P Messages
@subsection Sending P2P Messages

You can transmit messages to other peers using the @i{mq} you were
given during the @code{connect} callback.  Note that the @i{mq}
automatically is released upon @code{disconnect} and that you must
not use it afterwards.

It is your responsibility to not over-fill the message queue, GNUnet
will send the messages roughly in the order given as soon as possible.

Exercise: Write a service that upon connect sends messages as
fast as possible to the other peer (the other peer should run a
service that ``processes'' those messages).  How fast is the
transmission?  Count using the STATISTICS service on both ends.  Are
messages lost? How can you transmit messages faster?  What happens if
you stop the peer that is receiving your messages?

@node End of P2P connections
@subsection End of P2P connections

If a message handler returns @code{GNUNET\_SYSERR}, the remote
peer shuts down or there is an unrecoverable network
disconnection, CORE notifies the service that the peer disconnected.
After this notification no more messages will be received from the
peer and the service is no longer allowed to send messages to the peer.
The disconnect callback looks like the following:

@example
@verbatiminclude tutorial-examples/011.c
@end example

@noindent
Exercise: Fix your service to handle peer disconnects.

@node Storing peer-specific data using the PEERSTORE service
@section Storing peer-specific data using the PEERSTORE service

GNUnet's PEERSTORE service offers a persistorage for arbitrary
peer-specific data. Other GNUnet services can use the PEERSTORE
to store, retrieve and monitor data records. Each data record
stored with PEERSTORE contains the following fields:

@itemize
@item subsystem: Name of the subsystem responsible for the record.
@item peerid: Identity of the peer this record is related to.
@item key: a key string identifying the record.
@item value: binary record value.
@item expiry: record expiry date.
@end itemize

The first step is to start a connection to the PEERSTORE service:
@example
@verbatiminclude tutorial-examples/012.c
@end example

The service handle @code{peerstore_handle} will be needed for
all subsequent PEERSTORE operations.

@menu
* Storing records::
* Retrieving records::
* Monitoring records::
* Disconnecting from PEERSTORE::
@end menu

@node Storing records
@subsection Storing records

To store a new record, use the following function:

@example
@verbatiminclude tutorial-examples/013.c
@end example

@noindent
The @code{options} parameter can either be
@code{GNUNET_PEERSTORE_STOREOPTION_MULTIPLE} which means that multiple
values can be stored under the same key combination
(subsystem, peerid, key), or @code{GNUNET_PEERSTORE_STOREOPTION_REPLACE}
which means that PEERSTORE will replace any existing values under the
given key combination (subsystem, peerid, key) with the new given value.

The continuation function @code{cont} will be called after the store
request is successfully sent to the PEERSTORE service. This does not
guarantee that the record is successfully stored, only that it was
received by the service.

The @code{GNUNET_PEERSTORE_store} function returns a handle to the store
operation. This handle can be used to cancel the store operation only
before the continuation function is called:

@example
@verbatiminclude tutorial-examples/013.1.c
@end example

@node Retrieving records
@subsection Retrieving records

To retrieve stored records, use the following function:

@example
@verbatiminclude tutorial-examples/014.c
@end example

@noindent
The values of @code{peer} and @code{key} can be @code{NULL}. This
allows the iteration over values stored under any of the following
key combinations:

@itemize
@item (subsystem)
@item (subsystem, peerid)
@item (subsystem, key)
@item (subsystem, peerid, key)
@end itemize

The @code{callback} function will be called once with each retrieved
record and once more with a @code{NULL} record to signal the end of
results.

The @code{GNUNET_PEERSTORE_iterate} function returns a handle to the
iterate operation. This handle can be used to cancel the iterate
operation only before the callback function is called with a
@code{NULL} record.

@node Monitoring records
@subsection Monitoring records

PEERSTORE offers the functionality of monitoring for new records
stored under a specific key combination (subsystem, peerid, key).
To start the monitoring, use the following function:

@example
@verbatiminclude tutorial-examples/015.c
@end example

@noindent
Whenever a new record is stored under the given key combination,
the @code{callback} function will be called with this new
record. This will continue until the connection to the PEERSTORE
service is broken or the watch operation is canceled:

@example
@verbatiminclude tutorial-examples/016.c
@end example

@node Disconnecting from PEERSTORE
@subsection Disconnecting from PEERSTORE

When the connection to the PEERSTORE service is no longer needed,
disconnect using the following function:

@example
@verbatiminclude tutorial-examples/017.c
@end example

@noindent
If the @code{sync_first} flag is set to @code{GNUNET_YES},
the API will delay the disconnection until all store requests
are received by the PEERSTORE service. Otherwise, it will
disconnect immediately.

@node Using the DHT
@section Using the DHT

The DHT allows to store data so other peers in the P2P network can
access it and retrieve data stored by any peers in the network.
This section will explain how to use the DHT. Of course, the first
thing to do is to connect to the DHT service:

@example
@verbatiminclude tutorial-examples/018.c
@end example

@noindent
The second parameter indicates how many requests in parallel to expect.
It is not a hard limit, but a good approximation will make the DHT more
efficient.

@menu
* Storing data in the DHT::
* Obtaining data from the DHT::
* Implementing a block plugin::
* Monitoring the DHT::
@end menu

@node Storing data in the DHT
@subsection Storing data in the DHT
Since the DHT is a dynamic environment (peers join and leave frequently)
the data that we put in the DHT does not stay there indefinitely. It is
important to ``refresh'' the data periodically by simply storing it
again, in order to make sure other peers can access it.

The put API call offers a callback to signal that the PUT request has been
sent. This does not guarantee that the data is accessible to others peers,
or even that is has been stored, only that the service has requested to
a neighboring peer the retransmission of the PUT request towards its final
destination. Currently there is no feedback about whether or not the data
has been sucessfully stored or where it has been stored. In order to
improve the availablilty of the data and to compensate for possible
errors, peers leaving and other unfavorable events, just make several
PUT requests!

@example
@verbatiminclude tutorial-examples/019.c
@end example

@noindent
Exercise: Store a value in the DHT periodically to make sure it
is available over time. You might consider using the function
@code{GNUNET\_SCHEDULER\_add\_delayed} and call
@code{GNUNET\_DHT\_put} from inside a helper function.

@node Obtaining data from the DHT
@subsection Obtaining data from the DHT

As we saw in the previous example, the DHT works in an asynchronous mode.
Each request to the DHT is executed ``in the background'' and the API
calls return immediately. In order to receive results from the DHT, the
API provides a callback. Once started, the request runs in the service,
the service will try to get as many results as possible (filtering out
duplicates) until the timeout expires or we explicitly stop the request.
It is possible to give a ``forever'' timeout with
@code{GNUNET\_TIME\_UNIT\_FOREVER\_REL}.

If we give a route option @code{GNUNET\_DHT\_RO\_RECORD\_ROUTE}
the callback will get a list of all the peers the data has travelled,
both on the PUT path and on the GET path.

@example
@verbatiminclude tutorial-examples/020.c
@end example

@noindent
Exercise: Store a value in the DHT and after a while retrieve it.
Show the IDs of all the peers the requests have gone through.
In order to convert a peer ID to a string, use the function
@code{GNUNET\_i2s}. Pay attention to the route option parameters
in both calls!

@node Implementing a block plugin
@subsection Implementing a block plugin

In order to store data in the DHT, it is necessary to provide a block
plugin.  The DHT uses the block plugin to ensure that only well-formed
requests and replies are transmitted over the network.

The block plugin should be put in a file @file{plugin\_block\_SERVICE.c}
in the service's respective directory. The
mandatory functions that need to be implemented for a block plugin are
described in the following sections.

@menu
* Validating requests and replies::
* Deriving a key from a reply::
* Initialization of the plugin::
* Shutdown of the plugin::
* Integration of the plugin with the build system::
@end menu

@node Validating requests and replies
@subsubsection Validating requests and replies

The evaluate function should validate a reply or a request. It returns
a @code{GNUNET\_BLOCK\_EvaluationResult}, which is an enumeration. All
possible answers are in @file{gnunet\_block\_lib.h}.  The function will
be called with a @code{reply\_block} argument of @code{NULL} for
requests.  Note that depending on how @code{evaluate} is called, only
some of the possible return values are valid.  The specific meaning of
the @code{xquery} argument is application-specific.  Applications that
do not use an extended query should check that the @code{xquery\_size}
is zero.  The block group is typically used to filter duplicate
replies.

@example
@verbatiminclude tutorial-examples/021.c
@end example

@noindent
Note that it is mandatory to detect duplicate replies in this function
and return the respective status code.  Duplicate detection is
typically done using the Bloom filter block group provided by
@file{libgnunetblockgroup.so}.  Failure to do so may cause replies to
circle in the network.

@node Deriving a key from a reply
@subsubsection Deriving a key from a reply

The DHT can operate more efficiently if it is possible to derive a key
from the value of the corresponding block.  The @code{get\_key}
function is used to obtain the key of a block --- for example, by
means of hashing.  If deriving the key is not possible, the function
should simply return @code{GNUNET\_SYSERR} (the DHT will still work
just fine with such blocks).

@example
@verbatiminclude tutorial-examples/022.c
@end example

@node Initialization of the plugin
@subsubsection Initialization of the plugin

The plugin is realized as a shared C library.  The library must export
an initialization function which should initialize the plugin.  The
initialization function specifies what block types the plugin cares
about and returns a struct with the functions that are to be used for
validation and obtaining keys (the ones just defined above).

@example
@verbatiminclude tutorial-examples/023.c
@end example

@node Shutdown of the plugin
@subsubsection Shutdown of the plugin

Following GNUnet's general plugin API concept, the plugin must
export a second function for cleaning up.  It usually does very
little.

@example
@verbatiminclude tutorial-examples/024.c
@end example

@node Integration of the plugin with the build system
@subsubsection Integration of the plugin with the build system

In order to compile the plugin, the @file{Makefile.am} file for the
service SERVICE should contain a rule similar to this:
@c Actually this is a Makefile not C. But the whole structure of examples
@c must be improved.

@example
@verbatiminclude tutorial-examples/025.c
@end example

@noindent
Exercise: Write a block plugin that accepts all queries
and all replies but prints information about queries and replies
when the respective validation hooks are called.

@node Monitoring the DHT
@subsection Monitoring the DHT

It is possible to monitor the functioning of the local
DHT service. When monitoring the DHT, the service will
alert the monitoring program of any events, both started
locally or received for routing from another peer.
The are three different types of events possible: a
GET request, a PUT request or a response (a reply to a GET).

Since the different events have different associated data,
the API gets 3 different callbacks (one for each message type)
and optional type and key parameters, to allow for filtering of
messages. When an event happens, the appropiate callback is
called with all the information about the event.

@example
@verbatiminclude tutorial-examples/026.c
@end example

@node Debugging with gnunet-arm
@section Debugging with gnunet-arm

Even if services are managed by @command{gnunet-arm}, you can
start them with @command{gdb} or @command{valgrind}.  For
example, you could add the following lines to your
configuration file to start the DHT service in a @command{gdb}
session in a fresh @command{xterm}:

@example
[dht]
PREFIX=xterm -e gdb --args
@end example

@noindent
Alternatively, you can stop a service that was started via
ARM and run it manually:

@example
$ gnunet-arm -k dht
$ gdb --args gnunet-service-dht -L DEBUG
$ valgrind gnunet-service-dht -L DEBUG
@end example

@noindent
Assuming other services are well-written, they will automatically
re-integrate the restarted service with the peer.

GNUnet provides a powerful logging mechanism providing log
levels @code{ERROR}, @code{WARNING}, @code{INFO} and @code{DEBUG}.
The current log level is configured using the @code{$GNUNET_FORCE_LOG}
environmental variable. The @code{DEBUG} level is only available if
@command{--enable-logging=verbose} was used when running
@command{configure}. More details about logging can be found under
@uref{https://gnunet.org/logging}.

You should also probably enable the creation of core files, by setting
@code{ulimit}, and echo'ing @code{1} into
@file{/proc/sys/kernel/core\_uses\_pid}. Then you can investigate the
core dumps with @command{gdb}, which is often the fastest method to
find simple errors.

Exercise: Add a memory leak to your service and obtain a trace
pointing to the leak using @command{valgrind} while running the service
from @command{gnunet-service-arm}.

@bye