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Copyright © 2001-2018 GNUnet e.V.

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   A copy of the license is also available from the Free Software
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INFO-DIR-SECTION Tutorial
START-INFO-DIR-ENTRY
* GNUnet-C-Tutorial: (gnunet-c-tutorial).       C Tutorial for GNunet
END-INFO-DIR-ENTRY


File: gnunet-tutorial.info,  Node: Top,  Next: Installing GNUnet,  Up: (dir)

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 <https://docs.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!  Visit this link in your webbrowser to learn
how to contact the GNUnet team: <https://gnunet.org/en/contact.html>

* Menu:


* Installing GNUnet::                   Installing GNUnet
* Introduction to GNUnet Architecture:: Introduction to GNUnet Architecture
* First Steps with GNUnet::             First Steps with GNUnet
* Developing Applications::             Developing Applications
* GNU Free Documentation License::      The license of this manual

 — The Detailed Node Listing —

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::



File: gnunet-tutorial.info,  Node: Installing GNUnet,  Next: Introduction to GNUnet Architecture,  Prev: Top,  Up: Top

1 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::


File: gnunet-tutorial.info,  Node: Obtaining a stable version,  Next: Installing Build Tool Chain and Dependencies,  Up: Installing GNUnet

1.1 Obtaining a stable version
==============================

Download the tarball from
‘https://ftp.gnu.org/gnu/gnunet/gnunet-0.11.6.tar.gz’.

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

     $ wget https://ftp.gnu.org/gnu/gnunet/gnunet-0.11.6.tar.gz.sig
     $ gpg --verify-files gnunet-0.11.6.tar.gz.sig

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

     $ gpg --keyserver keys.gnupg.net --recv-keys 48426C7E

and rerun the ‘gpg --verify-files’ command.

   Note: The pub key to sign the 0.10.1 release has been revoked.  You
will get an error message stating that there is no known public key or
that it has been revoked.  The next release of GNUnet will have a valid
signature again.  We are sorry for the inconvenience this causes.
Another possible source you could use is our "gnunet" git repository
which, since the change from SVN to git in 2016, has mandatory signed
commits by every developer.

   After verifying the signature you can extract the tarball.  The
resulting directory will be renamed to ‘gnunet’, which we will be using
in the remainder of this document to refer to the root of the source
directory.

     $ tar xvzf gnunet-0.11.6.tar.gz
     $ mv gnunet-0.11.6 gnunet

However, please note that stable versions can be very outdated.  As a
developer you are strongly encouraged to use the version from the git
server (https://git.gnunet.org/).


File: gnunet-tutorial.info,  Node: Installing Build Tool Chain and Dependencies,  Next: Obtaining the latest version from Git,  Prev: Obtaining a stable version,  Up: Installing GNUnet

1.2 Installing Build Tool Chain and Dependencies
================================================

To successfully compile GNUnet, you need the tools to build GNUnet and
the required dependencies.  Please take a look at the GNUnet Reference
Documentation (*note The GNUnet Reference Documentation:
(gnunet)Dependencies.) for a list of required dependencies and (*note
The GNUnet Reference Documentation: (gnunet)Generic installation
instructions.) read its Installation chapter 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 libgnurl (https://gnunet.org/en/gnurl.html).  For the
filesharing service you should install at least one of the datastore
backends (MySQL, SQlite and PostgreSQL are supported).


File: gnunet-tutorial.info,  Node: Obtaining the latest version from Git,  Next: Compiling and Installing GNUnet,  Prev: Installing Build Tool Chain and Dependencies,  Up: Installing GNUnet

1.3 Obtaining the latest version from Git
=========================================

The latest development version can be obtained from our Git repository.
To get the code you need to have ‘Git’ installed.  Usually your
Operating System package manager should provide a suitable distribution
of git (otherwise check out Guix or Nix).  If you are using an Operating
System based on Debian’s apt:

     $ sudo apt-get install git

   This is required for obtaining the repository, which is achieved with
the following command:

     $ git clone https://git.gnunet.org/gnunet

After cloning the repository, you have to execute the ‘bootstrap’ script
in the new directory:

     $ cd gnunet
     $ ./bootstrap

The remainder of this tutorial will assume that you have the Git branch
“master” checked out.


File: gnunet-tutorial.info,  Node: Compiling and Installing GNUnet,  Next: Common Issues - Check your GNUnet installation,  Prev: Obtaining the latest version from Git,  Up: Installing GNUnet

1.4 Compiling and Installing GNUnet
===================================

Note: This section is a duplication of the more in depth *note The
GNUnet Reference Documentation: (gnunet)GNUnet Installation Handbook.

   First, you need to install libgnupgerror 1.27 (or later) and
libgcrypt 1.7.6 (or later):

     $ 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
     $ make
     $ sudo make install
     $ cd ..

     $ 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
     $ make
     $ sudo make install
     $ cd ..

* Menu:

* Installation::


File: gnunet-tutorial.info,  Node: Installation,  Up: Compiling and Installing GNUnet

1.4.1 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 ‘--prefix’
value when calling ‘./configure’.  If you do not specify a prefix,
GNUnet is installed in the directory ‘/usr/local’.  When developing new
applications you may want to enable verbose logging by adding
‘--enable-logging=verbose’:

     $ export PREFIX=$HOME
     $ ./configure --prefix=$PREFIX --enable-logging
     $ make
     $ make install

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

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


File: gnunet-tutorial.info,  Node: Common Issues - Check your GNUnet installation,  Prev: Compiling and Installing GNUnet,  Up: Installing GNUnet

1.5 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.

     $ which gnunet-arm
     $PREFIX/bin/gnunet-arm

should return $PREFIX/bin/gnunet-arm (where $PREFIX is the location you
have set earlier).  It should be located in your GNUnet installation and
the output should not be empty.

   If you see an output like:

     $ which gnunet-arm

check your PATH variable to ensure GNUnet’s ‘bin’ directory is included.

   GNUnet provides tests for all of its subcomponents.  Assuming you
have successfully built GNUnet, run

     $ cd gnunet
     $ make check

to execute tests for all components.  ‘make check’ traverses all
subdirectories in ‘src’.  For every subdirectory you should get a
message like this:

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


File: gnunet-tutorial.info,  Node: Introduction to GNUnet Architecture,  Next: First Steps with GNUnet,  Prev: Installing GNUnet,  Up: Top

2 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. 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).

   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.


File: gnunet-tutorial.info,  Node: First Steps with GNUnet,  Next: Developing Applications,  Prev: Introduction to GNUnet Architecture,  Up: Top

3 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::


File: gnunet-tutorial.info,  Node: Configure your peer,  Next: Start a peer,  Up: First Steps with GNUnet

3.1 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
‘$PREFIX/share/gnunet/config.d’ directory.  When starting a peer, you
can specify a customized configuration using the the ‘-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:

     $ mkdir ~/gnunet1/
     $ touch peer1.conf

Now add the following lines to ‘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:

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


File: gnunet-tutorial.info,  Node: Start a peer,  Next: Monitor a peer,  Prev: Configure your peer,  Up: First Steps with GNUnet

3.2 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
“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 ‘gnunet-arm’ tool.  GNUnet can
then be started with ‘gnunet-arm -s’ and stopped with ‘gnunet-arm -e’.
An additional service not automatically started can be started using
‘gnunet-arm -i <service name>’ and stopped using ‘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:

     $ gnunet-peerinfo -s

to obtain the public key of your peer.

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

     I am peer `0PA02UVRKQTS2C .. JL5Q78F6H0B1ACPV1CJI59MEQUMQCC5G'.


File: gnunet-tutorial.info,  Node: Monitor a peer,  Next: Starting Two Peers by Hand,  Prev: Start a peer,  Up: First Steps with GNUnet

3.3 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 ‘gnunet-dht-put’ and
‘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:

     # 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

Now open a separate terminal and change again to the ‘gnunet/src/dht’
directory:

     $ 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


File: gnunet-tutorial.info,  Node: Starting Two Peers by Hand,  Next: Starting Peers Using the Testbed Service,  Prev: Monitor a peer,  Up: First Steps with GNUnet

3.4 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 (*note
Starting Peers Using the Testbed Service::).

* Menu:

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


File: gnunet-tutorial.info,  Node: Setup a second peer,  Next: Start the second peer and connect the peers,  Up: Starting Two Peers by Hand

3.4.1 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 ‘~/.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
‘$PREFIX/share/gnunet/config.d’ directory.

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

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

Now you have to edit ‘peer2.conf’ and change:

   • ‘GNUNET\_TEST\_HOME’ under ‘PATHS’
   • Every (uncommented) value for “‘PORT’” (add 10000) in any section
     (the option may be commented out if ‘PORT’ is prefixed by "\#", in
     this case, UNIX domain sockets are used and the PORT option does
     not need to be touched)
   • Every value for “‘UNIXPATH’” in any section (e.g.  by adding a
     "-p2" suffix)

   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 ‘-c peer2.conf’ as an additional command line argument.

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

     $ gnunet-peerinfo -s -c peer2.conf

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).


File: gnunet-tutorial.info,  Node: Start the second peer and connect the peers,  Next: How to connect manually,  Prev: Setup a second peer,  Up: Starting Two Peers by Hand

3.4.2 Start the second peer and connect the peers
-------------------------------------------------

Then, you can start a second peer using:

     $ 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

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

   • UDP neighbour discovery (automatic)
   • Setup a bootstrap server
   • Connect manually

   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
‘peer1.conf’ to enable bootstrapping server:

     [hostlist]
     OPTIONS = -p

Then change ‘peer2.conf’ and replace the “‘SERVERS’” line in the
“‘[hostlist]’” section with “‘http://localhost:8080/’”.  Restart both
peers using:

     # 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

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.


File: gnunet-tutorial.info,  Node: How to connect manually,  Prev: Start the second peer and connect the peers,  Up: Starting Two Peers by Hand

3.4.3 How to connect manually
-----------------------------

If you want to use the ‘peerinfo’ tool to connect your peers, you
should:

   • Set ‘IMMEDIATE_START = NO’ in section ‘hostlist’ (to not connect to
     the global GNUnet)
   • Start both peers running ‘gnunet-arm -c peer1.conf -s’ and
     ‘gnunet-arm -c peer2.conf -s’
   • Get ‘HELLO’ message of the first peer running ‘gnunet-peerinfo -c
     peer1.conf -g’
   • Give the output to the second peer by running ‘gnunet-peerinfo -c
     peer2.conf -p '<output>'’

   Check that they are connected using ‘gnunet-core -c peer1.conf’,
which should give you the other peer’s peer identity:

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


File: gnunet-tutorial.info,  Node: Starting Peers Using the Testbed Service,  Prev: Starting Two Peers by Hand,  Up: First Steps with GNUnet

3.5 Starting Peers Using the Testbed Service
============================================

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
‘include/gnunet\_testbed\_service.h’.  The API provides many routines
for managing a group of peers.  It also provides a helper function
‘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
‘SERVICEHOME’, ‘PORT’, ‘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
‘src/testbed/testbed.conf’.

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

     #include <unistd.h>
     #include <gnunet/platform.h>
     #include <gnunet/gnunet_util_lib.h>
     #include <gnunet/gnunet_testbed_service.h>
     #include <gnunet/gnunet_dht_service.h>
     
     #define NUM_PEERS 20
     
     static struct GNUNET_TESTBED_Operation *dht_op;
     
     static struct GNUNET_DHT_Handle *dht_handle;
     
     
     struct MyContext
     {
       int ht_len;
     } ctxt;
     
     
     static int result;
     
     
     static void
     shutdown_task (void *cls)
     {
       if (NULL != dht_op)
       {
         GNUNET_TESTBED_operation_done (dht_op);
         dht_op = NULL;
         dht_handle = NULL;
       }
       result = GNUNET_OK;
     }
     
     
     static void
     service_connect_comp (void *cls,
                           struct GNUNET_TESTBED_Operation *op,
                           void *ca_result,
                           const char *emsg)
     {
       GNUNET_assert (op == dht_op);
       dht_handle = ca_result;
       // Do work here...
       GNUNET_SCHEDULER_shutdown ();
     }
     
     
     static void *
     dht_ca (void *cls, const struct GNUNET_CONFIGURATION_Handle *cfg)
     {
       struct MyContext *ctxt = cls;
     
       dht_handle = GNUNET_DHT_connect (cfg, ctxt->ht_len);
       return dht_handle;
     }
     
     
     static void
     dht_da (void *cls, void *op_result)
     {
       struct MyContext *ctxt = cls;
     
       GNUNET_DHT_disconnect ((struct GNUNET_DHT_Handle *) op_result);
       dht_handle = NULL;
     }
     
     
     static void
     test_master (void *cls,
                  struct GNUNET_TESTBED_RunHandle *h,
                  unsigned int num_peers,
                  struct GNUNET_TESTBED_Peer **peers,
                  unsigned int links_succeeded,
                  unsigned int links_failed)
     {
       ctxt.ht_len = 10;
       dht_op = GNUNET_TESTBED_service_connect
           (NULL, peers[0], "dht",
            &service_connect_comp, NULL,
            &dht_ca, &dht_da, &ctxt);
       GNUNET_SCHEDULER_add_shutdown (&shutdown_task, NULL);
     }
     
     
     int
     main (int argc, char **argv)
     {
       int ret;
     
       result = GNUNET_SYSERR;
       ret = GNUNET_TESTBED_test_run
           ("awesome-test", "template.conf",
            NUM_PEERS, 0LL,
            NULL, NULL, &test_master, NULL);
       if ( (GNUNET_OK != ret) || (GNUNET_OK != result) )
         return 1;
       return 0;
     }

The source code for the above listing can be found at
<https://git.gnunet.org/gnunet.git/tree/doc/documentation/testbed_test.c>
or in the ‘doc/’ folder of your repository check-out.  After installing
GNUnet, the above source code can be compiled as:

     $ 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

The ‘CPPFLAGS’ and ‘LDFLAGS’ are necessary if GNUnet is installed into a
different directory other than ‘/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
“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 ‘GNUNET\_TESTBED\_service\_connect()’.  This
function fetches the configuration of a given peer and calls the
“Connect Adapter”.  In the example code, it is the ‘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 ‘service\_connect\_comp\_cb’.

   A dual to connect adapter is the “Disconnect Adapter”.  This callback
is called after the connect adapter has been called when the operation
from ‘GNUNET\_TESTBED\_service\_connect()’ is marked as “done”.  It has
to disconnect from the service with the provided service handle
(‘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.  *Note The GNUnet Reference
Documentation: (gnunet)Supported Topologies, then use the DHT API to
store and retrieve values in the network.


File: gnunet-tutorial.info,  Node: Developing Applications,  Next: GNU Free Documentation License,  Prev: First Steps with GNUnet,  Up: Top

4 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::


File: gnunet-tutorial.info,  Node: gnunet-ext,  Next: Adapting the Template,  Up: Developing Applications

4.1 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:

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

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:

   • the GNUnet service (gnunet-ext/src/ext/gnunet-service-ext.c)
   • the client API (gnunet-ext/src/ext/ext_api.c)
   • the client application using the service API
     (gnunet-ext/src/ext/gnunet-ext.c)

   The interfaces for these entities are defined in:

   • client API interface (gnunet-ext/src/ext/ext.h)
   • the service interface
     (gnunet-ext/src/include/gnunet_service_SERVICE.h)
   • the P2P protocol (gnunet-ext/src/include/gnunet_protocols_ext.h)

   In addition the ext systems provides:

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


File: gnunet-tutorial.info,  Node: Adapting the Template,  Next: Writing a Client Application,  Prev: gnunet-ext,  Up: Developing Applications

4.2 Adapting the Template
=========================

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

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


File: gnunet-tutorial.info,  Node: Writing a Client Application,  Next: Writing a Service,  Prev: Adapting the Template,  Up: Developing Applications

4.3 Writing a Client Application
================================

When writing any client application (for example, a command-line tool),
the basic structure is to start with the ‘GNUNET\_PROGRAM\_run’
function.  This function will parse command-line options, setup the
scheduler and then invoke the ‘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):

     #include <gnunet/platform.h>
     #include <gnunet/gnunet_util_lib.h>
     
     static int ret;
     
     static void
     run (void *cls,
          char *const *args,
          const char *cfgfile,
          const struct GNUNET_CONFIGURATION_Handle *cfg)
     {
       // main code here
       ret = 0;
     }
     
     int
     main (int argc, char *const *argv)
     {
       struct GNUNET_GETOPT_CommandLineOption options[] = {
         GNUNET_GETOPT_OPTION_END
       };
       return (GNUNET_OK ==
               GNUNET_PROGRAM_run (argc,
                                   argv,
                                   "binary-name",
                                   gettext_noop ("binary description text"),
                                   options, &run, NULL)) ? ret : 1;
     }
     

* Menu:

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


File: gnunet-tutorial.info,  Node: Handling command-line options,  Next: Writing a Client Library,  Up: Writing a Client Application

4.3.1 Handling command-line options
-----------------------------------

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

     static char *string_option;
     static int a_flag;
     
     // ...
       struct GNUNET_GETOPT_CommandLineOption options[] = {
         GNUNET_GETOPT_option_string ('s', "name", "SOMESTRING",
          gettext_noop ("text describing the string_option NAME"),
          &string_option},
         GNUNET_GETOPT_option_flag ('f', "flag",
          gettext_noop ("text describing the flag option"), 
          &a_flag),
         GNUNET_GETOPT_OPTION_END
       };
       string_option = NULL;
       a_flag = GNUNET_SYSERR;
     // ...
     

   Issues such as displaying some helpful text describing options using
the ‘--help’ argument and error handling are taken care of when using
this approach.  Other ‘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 ‘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
‘run’.  What happens if the user gives invalid arguments?


File: gnunet-tutorial.info,  Node: Writing a Client Library,  Next: Writing a user interface,  Prev: Handling command-line options,  Up: Writing a Client Application

4.3.2 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
‘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 ‘make install’).  Each message must start with a ‘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 ‘ntohl’ and similar functions and written using
‘htonl’ and similar functions).  Unique message types must be defined
for each message struct in the ‘gnunet\_protocols.h’ header (or an
extension-specific include file).

* Menu:

* Connecting to the Service::
* Sending messages::
* Receiving Replies from the Service::


File: gnunet-tutorial.info,  Node: Connecting to the Service,  Next: Sending messages,  Up: Writing a Client Library

4.3.2.1 Connecting to the Service
.................................

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

     struct GNUNET_MQ_MessageHandlers handlers[] = {
         // ...
       GNUNET_MQ_handler_end ()
     };
     struct GNUNET_MQ_Handle *mq;
     
     mq = GNUNET_CLIENT_connect (cfg,
                                 "service-name",
                                 handlers,
                                 &error_cb,
                                 NULL);

As a result a ‘GNUNET\_MQ\_Handle’ is returned which can to used
henceforth to transmit messages to the service.  The complete MQ API can
be found in ‘gnunet\_mq\_lib.h’.  The ‘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 ‘error\_cb’ is a function that is to be called whenever there are
errors communicating with the service.


File: gnunet-tutorial.info,  Node: Sending messages,  Next: Receiving Replies from the Service,  Prev: Connecting to the Service,  Up: Writing a Client Library

4.3.2.2 Sending messages
........................

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

     struct GNUNET_MessageHeader
     {
       uint16_t size GNUNET_PACKED;
       uint16_t type GNUNET_PACKED;
     };

Existing message types are defined in ‘gnunet\_protocols.h’.  A common
way to create a message is with an envelope:

     struct GNUNET_MQ_Envelope *env;
     struct GNUNET_MessageHeader *msg;
     
     env = GNUNET_MQ_msg_extra (msg, payload_size, GNUNET_MY_MESSAGE_TYPE);
     GNUNET_memcpy (&msg[1],
                    &payload,
                    payload_size);
     // Send message via message queue 'mq'
     GNUNET_mq_send (mq, env);

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
‘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.


File: gnunet-tutorial.info,  Node: Receiving Replies from the Service,  Prev: Sending messages,  Up: Writing a Client Library

4.3.2.3 Receiving Replies from the Service
..........................................

Clients can receive messages from the service using the handlers
specified in the ‘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 ‘check\_’ and ‘handle\_’ are mandatory.

     static void
     handle_fix (void *cls, const struct MyMessage *msg)
     {
       // process 'msg'
     }
     
     static int
     check_var (void *cls, const struct MyVarMessage *msg)
     {
       // check 'msg' is well-formed
       return GNUNET_OK;
     }
     
     static void
     handle_var (void *cls, const struct MyVarMessage *msg)
     {
       // process 'msg'
     }
     
     struct GNUNET_MQ_MessageHandler handlers[] = {
       GNUNET_MQ_hd_fixed_size (fix,
                               GNUNET_MESSAGE_TYPE_MY_FIX,
                               struct MyMessage,
                               NULL),
       GNUNET_MQ_hd_fixed_size (var,
                               GNUNET_MESSAGE_TYPE_MY_VAR,
                               struct MyVarMessage,
                               NULL),
     
       GNUNET_MQ_handler_end ()
     };

Exercise: Expand your helper function to receive a response message (for
example, containing just the ‘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
(‘cls’).


File: gnunet-tutorial.info,  Node: Writing a user interface,  Prev: Writing a Client Library,  Up: Writing a Client Application

4.3.3 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 ‘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.


File: gnunet-tutorial.info,  Node: Writing a Service,  Next: Interacting directly with other Peers using the CORE Service,  Prev: Writing a Client Application,  Up: Developing Applications

4.4 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::


File: gnunet-tutorial.info,  Node: Code Placement,  Next: Starting a Service,  Up: Writing a Service

4.4.1 Code Placement
--------------------

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


File: gnunet-tutorial.info,  Node: Starting a Service,  Prev: Code Placement,  Up: Writing a Service

4.4.2 Starting a Service
------------------------

The key API definition for creating a service is the
‘GNUNET\_SERVICE\_MAIN’ macro:

     GNUNET_SERVICE_MAIN
     ("service-name",
      GNUNET_SERVICE_OPTION_NONE,
      &run,
      &client_connect_cb,
      &client_disconnect_cb,
      NULL,
      GNUNET_MQ_hd_fixed_size (...),
      GNUNET_MQ_hd_var_size (...),
      GNUNET_MQ_handler_end ());

In addition to the service name and flags, the macro takes three
functions, typically called ‘run’, ‘client\_connect\_cb’ and
‘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 functions would look
like this:

     static void
     run (void *cls,
          const struct GNUNET_CONFIGURATION_Handle *c,
          struct GNUNET_SERVICE_Handle *service)
     {
     }
     
     static void *
     client_connect_cb (void *cls,
                        struct GNUNET_SERVICE_Client *c,
                        struct GNUNET_MQ_Handle *mq)
     {
       return c;
     }
     
     static void
     client_disconnect_cb (void *cls,
                           struct GNUNET_SERVICE_Client *c,
                           void *internal_cls)
     {
       GNUNET_assert (c == internal_cls);
     }

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 ‘gnunet-service-arm’ using
‘gnunet-arm -i NAME’.

   Exercise: Figure out how to set the closure (‘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 must eventually (possibly in
some asynchronous continuation) call
‘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
‘GNUNET\_SERVICE\_client\_continue()’?


File: gnunet-tutorial.info,  Node: Interacting directly with other Peers using the CORE Service,  Next: Storing peer-specific data using the PEERSTORE service,  Prev: Writing a Service,  Up: Developing Applications

4.5 Interacting directly with other Peers using the CORE Service
================================================================

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

   One of the most important services in GNUnet is the ‘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 ‘CORE’ service using:

     #include <gnunet/gnunet_core_service.h>
     
     struct GNUNET_CORE_Handle *
     GNUNET_CORE_connect (const struct GNUNET_CONFIGURATION_Handle *cfg,
                          void *cls,
                          GNUNET_CORE_StartupCallback init,
                          GNUNET_CORE_ConnectEventHandler connects,
                          GNUNET_CORE_DisconnectEventHandler disconnects,
                          const struct GNUNET_MQ_MessageHandler *handlers);

* Menu:

* New P2P connections::
* Receiving P2P Messages::
* Sending P2P Messages::
* End of P2P connections::


File: gnunet-tutorial.info,  Node: New P2P connections,  Next: Receiving P2P Messages,  Up: Interacting directly with other Peers using the CORE Service

4.5.1 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 ‘CORE’ ‘connects’
callback, which communicates the identity of the new peer to the
service:

     void *
     connects (void *cls,
               const struct GNUNET_PeerIdentity *peer,
               struct GNUNET_MQ_Handle *mq)
     {
       return mq;
     }
     

Note that whatever you return from ‘connects’ is given as the ‘cls’
argument to the message handlers for messages from the respective peer.

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


File: gnunet-tutorial.info,  Node: Receiving P2P Messages,  Next: Sending P2P Messages,  Prev: New P2P connections,  Up: Interacting directly with other Peers using the CORE Service

4.5.2 Receiving P2P Messages
----------------------------

To receive messages from ‘CORE’, you pass the desired ‘handlers’ to the
‘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?


File: gnunet-tutorial.info,  Node: Sending P2P Messages,  Next: End of P2P connections,  Prev: Receiving P2P Messages,  Up: Interacting directly with other Peers using the CORE Service

4.5.3 Sending P2P Messages
--------------------------

You can transmit messages to other peers using the ‘mq’ you were given
during the ‘connect’ callback.  Note that the ‘mq’ automatically is
released upon ‘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?


File: gnunet-tutorial.info,  Node: End of P2P connections,  Prev: Sending P2P Messages,  Up: Interacting directly with other Peers using the CORE Service

4.5.4 End of P2P connections
----------------------------

If a message handler returns ‘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:

     void
     disconnects (void *cls,
                  const struct GNUNET_PeerIdentity * peer)
     {
         /* Remove peer's identity from known peers */
         /* Make sure no messages are sent to peer from now on */
     }
     

Exercise: Fix your service to handle peer disconnects.


File: gnunet-tutorial.info,  Node: Storing peer-specific data using the PEERSTORE service,  Next: Using the DHT,  Prev: Interacting directly with other Peers using the CORE Service,  Up: Developing Applications

4.6 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:

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

   The first step is to start a connection to the PEERSTORE service:
     #include "gnunet_peerstore_service.h"
     
     peerstore_handle = GNUNET_PEERSTORE_connect (cfg);
     

   The service handle ‘peerstore_handle’ will be needed for all
subsequent PEERSTORE operations.

* Menu:

* Storing records::
* Retrieving records::
* Monitoring records::
* Disconnecting from PEERSTORE::


File: gnunet-tutorial.info,  Node: Storing records,  Next: Retrieving records,  Up: Storing peer-specific data using the PEERSTORE service

4.6.1 Storing records
---------------------

To store a new record, use the following function:

     struct GNUNET_PEERSTORE_StoreContext *
     GNUNET_PEERSTORE_store (struct GNUNET_PEERSTORE_Handle *h,
                             const char *sub_system,
                             const struct GNUNET_PeerIdentity *peer,
                             const char *key,
                             const void *value,
                             size_t size,
                             struct GNUNET_TIME_Absolute expiry,
                             enum GNUNET_PEERSTORE_StoreOption options,
                             GNUNET_PEERSTORE_Continuation cont,
                             void *cont_cls);
     

The ‘options’ parameter can either be
‘GNUNET_PEERSTORE_STOREOPTION_MULTIPLE’ which means that multiple values
can be stored under the same key combination (subsystem, peerid, key),
or ‘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 ‘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 ‘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:

     void
     GNUNET_PEERSTORE_store_cancel (struct GNUNET_PEERSTORE_StoreContext
                                    *sc);


File: gnunet-tutorial.info,  Node: Retrieving records,  Next: Monitoring records,  Prev: Storing records,  Up: Storing peer-specific data using the PEERSTORE service

4.6.2 Retrieving records
------------------------

To retrieve stored records, use the following function:

     struct GNUNET_PEERSTORE_IterateContext *
     GNUNET_PEERSTORE_iterate (struct GNUNET_PEERSTORE_Handle *h,
                               const char *sub_system,
                               const struct GNUNET_PeerIdentity *peer,
                               const char *key,
                               GNUNET_PEERSTORE_Processor callback,
                               void *callback_cls);
     

The values of ‘peer’ and ‘key’ can be ‘NULL’.  This allows the iteration
over values stored under any of the following key combinations:

   • (subsystem)
   • (subsystem, peerid)
   • (subsystem, key)
   • (subsystem, peerid, key)

   The ‘callback’ function will be called once with each retrieved
record and once more with a ‘NULL’ record to signal the end of results.

   The ‘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 ‘NULL’
record.


File: gnunet-tutorial.info,  Node: Monitoring records,  Next: Disconnecting from PEERSTORE,  Prev: Retrieving records,  Up: Storing peer-specific data using the PEERSTORE service

4.6.3 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:

     struct GNUNET_PEERSTORE_WatchContext *
     GNUNET_PEERSTORE_watch (struct GNUNET_PEERSTORE_Handle *h,
                             const char *sub_system,
                             const struct GNUNET_PeerIdentity *peer,
                             const char *key,
                             GNUNET_PEERSTORE_Processor callback,
                             void *callback_cls);
     

Whenever a new record is stored under the given key combination, the
‘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:

     void
     GNUNET_PEERSTORE_watch_cancel (struct GNUNET_PEERSTORE_WatchContext
                                    *wc);
     


File: gnunet-tutorial.info,  Node: Disconnecting from PEERSTORE,  Prev: Monitoring records,  Up: Storing peer-specific data using the PEERSTORE service

4.6.4 Disconnecting from PEERSTORE
----------------------------------

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

     void
     GNUNET_PEERSTORE_disconnect (struct GNUNET_PEERSTORE_Handle *h,
                                  int sync_first);
     

If the ‘sync_first’ flag is set to ‘GNUNET_YES’, the API will delay the
disconnection until all store requests are received by the PEERSTORE
service.  Otherwise, it will disconnect immediately.


File: gnunet-tutorial.info,  Node: Using the DHT,  Next: Debugging with gnunet-arm,  Prev: Storing peer-specific data using the PEERSTORE service,  Up: Developing Applications

4.7 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:

     dht_handle = GNUNET_DHT_connect (cfg, parallel_requests);
     

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::


File: gnunet-tutorial.info,  Node: Storing data in the DHT,  Next: Obtaining data from the DHT,  Up: Using the DHT

4.7.1 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 successfully 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!

     message_sent_cont (void *cls,
                        const struct GNUNET_SCHEDULER_TaskContext *tc)
     {
       // Request has left local node
     }
     
     struct GNUNET_DHT_PutHandle *
     GNUNET_DHT_put (struct GNUNET_DHT_Handle *handle,
                     const struct GNUNET_HashCode *key,
                     uint32_t desired_replication_level,
                     enum GNUNET_DHT_RouteOption options,
                     enum GNUNET_BLOCK_Type type,
                     size_t size,
                     const void *data,
                     struct GNUNET_TIME_Absolute exp,
                     struct GNUNET_TIME_Relative timeout,
                     GNUNET_DHT_PutContinuation cont, void *cont_cls)
     

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


File: gnunet-tutorial.info,  Node: Obtaining data from the DHT,  Next: Implementing a block plugin,  Prev: Storing data in the DHT,  Up: Using the DHT

4.7.2 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
‘GNUNET\_TIME\_UNIT\_FOREVER\_REL’.

   If we give a route option ‘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.

     static void
     get_result_iterator (void *cls, struct GNUNET_TIME_Absolute expiration,
                          const struct GNUNET_HashCode *key,
                          const struct GNUNET_PeerIdentity *get_path,
                          unsigned int get_path_length,
                          const struct GNUNET_PeerIdentity *put_path,
                          unsigned int put_path_length,
                          enum GNUNET_BLOCK_Type type, size_t size,
                          const void *data)
     {
       // Optionally:
       GNUNET_DHT_get_stop (get_handle);
     }
     
     get_handle =
           GNUNET_DHT_get_start (dht_handle,
                                 block_type,
                                 &key,
                                 replication,
                                 GNUNET_DHT_RO_NONE,
                                 NULL,
                                 0,
                                 &get_result_iterator,
                                 cls)
     

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 ‘GNUNET\_i2s’.  Pay
attention to the route option parameters in both calls!


File: gnunet-tutorial.info,  Node: Implementing a block plugin,  Next: Monitoring the DHT,  Prev: Obtaining data from the DHT,  Up: Using the DHT

4.7.3 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 ‘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::


File: gnunet-tutorial.info,  Node: Validating requests and replies,  Next: Deriving a key from a reply,  Up: Implementing a block plugin

4.7.3.1 Validating requests and replies
.......................................

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

     static enum GNUNET_BLOCK_EvaluationResult
     block_plugin_SERVICE_evaluate (void *cls,
                                   enum GNUNET_BLOCK_Type type,
                                   struct GNUNET_BlockGroup *bg,
                                   const GNUNET_HashCode *query,
                                   const void *xquery,
                                   size_t xquery_size,
                                   const void *reply_block,
                                   size_t reply_block_size)
     {
       // Verify type, block and bg
     }
     

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
‘libgnunetblockgroup.so’.  Failure to do so may cause replies to circle
in the network.


File: gnunet-tutorial.info,  Node: Deriving a key from a reply,  Next: Initialization of the plugin,  Prev: Validating requests and replies,  Up: Implementing a block plugin

4.7.3.2 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 ‘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
‘GNUNET\_SYSERR’ (the DHT will still work just fine with such blocks).

     static int
     block_plugin_SERVICE_get_key (void *cls, enum GNUNET_BLOCK_Type type,
                                  const void *block, size_t block_size,
                                  struct GNUNET_HashCode *key)
     {
       // Store the key in the key argument, return GNUNET_OK on success.
     }
     


File: gnunet-tutorial.info,  Node: Initialization of the plugin,  Next: Shutdown of the plugin,  Prev: Deriving a key from a reply,  Up: Implementing a block plugin

4.7.3.3 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).

     void *
     libgnunet_plugin_block_SERVICE_init (void *cls)
     {
       static enum GNUNET_BLOCK_Type types[] =
       {
         GNUNET_BLOCK_TYPE_SERVICE_BLOCKYPE,
         GNUNET_BLOCK_TYPE_ANY
       };
       struct GNUNET_BLOCK_PluginFunctions *api;
     
       api = GNUNET_new (struct GNUNET_BLOCK_PluginFunctions);
       api->evaluate = &block_plugin_SERICE_evaluate;
       api->get_key = &block_plugin_SERVICE_get_key;
       api->types = types;
       return api;
     }
     


File: gnunet-tutorial.info,  Node: Shutdown of the plugin,  Next: Integration of the plugin with the build system,  Prev: Initialization of the plugin,  Up: Implementing a block plugin

4.7.3.4 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.

     void *
     libgnunet_plugin_block_SERVICE_done (void *cls)
     {
       struct GNUNET_TRANSPORT_PluginFunctions *api = cls;
     
       GNUNET_free (api);
       return NULL;
     }
     


File: gnunet-tutorial.info,  Node: Integration of the plugin with the build system,  Prev: Shutdown of the plugin,  Up: Implementing a block plugin

4.7.3.5 Integration of the plugin with the build system
.......................................................

In order to compile the plugin, the ‘Makefile.am’ file for the service
SERVICE should contain a rule similar to this:

       plugindir = $(libdir)/gnunet
     
       plugin_LTLIBRARIES = \
               libgnunet_plugin_block_ext.la
       libgnunet_plugin_block_ext_la_SOURCES = \
               plugin_block_ext.c
       libgnunet_plugin_block_ext_la_LIBADD = \
               $(prefix)/lib/libgnunethello.la \
               $(prefix)/lib/libgnunetblock.la \
               $(prefix)/lib/libgnunetutil.la
       libgnunet_plugin_block_ext_la_LDFLAGS = \
               $(GN_PLUGIN_LDFLAGS)
       libgnunet_plugin_block_ext_la_DEPENDENCIES = \
               $(prefix)/lib/libgnunetblock.la
     

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.


File: gnunet-tutorial.info,  Node: Monitoring the DHT,  Prev: Implementing a block plugin,  Up: Using the DHT

4.7.4 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 appropriate callback is called with all the information
about the event.

     static void
     get_callback (void *cls,
                   enum GNUNET_DHT_RouteOption options,
                   enum GNUNET_BLOCK_Type type,
                   uint32_t hop_count,
                   uint32_t desired_replication_level,
                   unsigned int path_length,
                   const struct GNUNET_PeerIdentity *path,
                   const struct GNUNET_HashCode * key)
     {
     }
     
     
     static void
     get_resp_callback (void *cls,
                        enum GNUNET_BLOCK_Type type,
                        const struct GNUNET_PeerIdentity *get_path,
                        unsigned int get_path_length,
                        const struct GNUNET_PeerIdentity *put_path,
                        unsigned int put_path_length,
                        struct GNUNET_TIME_Absolute exp,
                        const struct GNUNET_HashCode * key,
                        const void *data,
                        size_t size)
     {
     }
     
     
     static void
     put_callback (void *cls,
                   enum GNUNET_DHT_RouteOption options,
                   enum GNUNET_BLOCK_Type type,
                   uint32_t hop_count,
                   uint32_t desired_replication_level,
                   unsigned int path_length,
                   const struct GNUNET_PeerIdentity *path,
                   struct GNUNET_TIME_Absolute exp,
                   const struct GNUNET_HashCode * key,
                   const void *data,
                   size_t size)
     {
     }
     
     
     monitor_handle = GNUNET_DHT_monitor_start (dht_handle,
                                               block_type,
                                               key,
                                               &get_callback,
                                               &get_resp_callback,
                                               &put_callback,
                                               cls);
     


File: gnunet-tutorial.info,  Node: Debugging with gnunet-arm,  Prev: Using the DHT,  Up: Developing Applications

4.8 Debugging with gnunet-arm
=============================

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

     [dht]
     PREFIX=xterm -e gdb --args

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

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

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
‘ERROR’, ‘WARNING’, ‘INFO’ and ‘DEBUG’.  The current log level is
configured using the ‘$GNUNET_FORCE_LOG’ environmental variable.  The
‘DEBUG’ level is only available if ‘--enable-logging=verbose’ was used
when running ‘configure’.  More details about logging can be found under
<https://docs.gnunet.org/#Logging>.

   You should also probably enable the creation of core files, by
setting ‘ulimit’, and echo’ing ‘1’ into
‘/proc/sys/kernel/core\_uses\_pid’.  Then you can investigate the core
dumps with ‘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 ‘valgrind’ while running the service from
‘gnunet-service-arm’.


File: gnunet-tutorial.info,  Node: GNU Free Documentation License,  Prev: Developing Applications,  Up: Top

Appendix A GNU Free Documentation License
*****************************************

                     Version 1.3, 3 November 2008

     Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
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     being considered responsible for modifications made by others.

     This License is a kind of “copyleft”, which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
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     a copy of this License into the extracted document, and follow this
     License in all other respects regarding verbatim copying of that
     document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of a
     storage or distribution medium, is called an “aggregate” if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation’s users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document’s Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled “Acknowledgements”,
     “Dedications”, or “History”, the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided under this License.  Any attempt
     otherwise to copy, modify, sublicense, or distribute it is void,
     and will automatically terminate your rights under this License.

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly and
     finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from you
     under this License.  If your rights have been terminated and not
     permanently reinstated, receipt of a copy of some or all of the
     same material does not give you any rights to use it.

  10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     <http://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License “or any later version” applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If the
     Document does not specify a version number of this License, you may
     choose any version ever published (not as a draft) by the Free
     Software Foundation.  If the Document specifies that a proxy can
     decide which future versions of this License can be used, that
     proxy’s public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

  11. RELICENSING

     “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
     World Wide Web server that publishes copyrightable works and also
     provides prominent facilities for anybody to edit those works.  A
     public wiki that anybody can edit is an example of such a server.
     A “Massive Multiauthor Collaboration” (or “MMC”) contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
     license published by Creative Commons Corporation, a not-for-profit
     corporation with a principal place of business in San Francisco,
     California, as well as future copyleft versions of that license
     published by that same organization.

     “Incorporate” means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is “eligible for relicensing” if it is licensed under this
     License, and if all works that were first published under this
     License somewhere other than this MMC, and subsequently
     incorporated in whole or in part into the MMC, (1) had no cover
     texts or invariant sections, and (2) were thus incorporated prior
     to November 1, 2008.

     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       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''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the “with...Texts.” line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.



Tag Table:
Node: Top1142
Node: Installing GNUnet3017
Node: Obtaining a stable version3929
Node: Installing Build Tool Chain and Dependencies5620
Node: Obtaining the latest version from Git6686
Node: Compiling and Installing GNUnet7696
Node: Installation8727
Node: Common Issues - Check your GNUnet installation9860
Node: Introduction to GNUnet Architecture11113
Node: First Steps with GNUnet12221
Node: Configure your peer12568
Node: Start a peer14037
Node: Monitor a peer15216
Node: Starting Two Peers by Hand16553
Node: Setup a second peer17150
Node: Start the second peer and connect the peers18997
Node: How to connect manually20636
Node: Starting Peers Using the Testbed Service21581
Node: Developing Applications29160
Node: gnunet-ext29634
Node: Adapting the Template31093
Node: Writing a Client Application31717
Node: Handling command-line options33263
Node: Writing a Client Library35288
Node: Connecting to the Service36701
Node: Sending messages37849
Node: Receiving Replies from the Service39460
Node: Writing a user interface41514
Node: Writing a Service42074
Node: Code Placement42479
Node: Starting a Service43037
Node: Interacting directly with other Peers using the CORE Service45380
Node: New P2P connections46660
Node: Receiving P2P Messages47577
Node: Sending P2P Messages48447
Node: End of P2P connections49442
Node: Storing peer-specific data using the PEERSTORE service50330
Node: Storing records51553
Node: Retrieving records53344
Node: Monitoring records54653
Node: Disconnecting from PEERSTORE55846
Node: Using the DHT56520
Node: Storing data in the DHT57333
Node: Obtaining data from the DHT59350
Node: Implementing a block plugin61577
Node: Validating requests and replies62412
Node: Deriving a key from a reply64128
Node: Initialization of the plugin65064
Node: Shutdown of the plugin66145
Node: Integration of the plugin with the build system66731
Node: Monitoring the DHT67874
Node: Debugging with gnunet-arm70645
Node: GNU Free Documentation License72290

End Tag Table


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