Using libgazebo

External programs can use libgazebo to interact with the Gazebo simulator. libgazebo is a simple C library that allows other programs to peek and poke into a running simulation; through this library, programs may read sensor data from and send commands to simulated devices. The Player device server, for example, uses libgazebo in this way.

Normal users will interact with Gazebo via player and need not be aware of this library. This chapter is included primarily to aid those who either wish to add a new type of interface to the simulator, or who wish to write their own custom control programs (by-passing player entirely).

Python bindings for libgazebo are also available.

Architecture

libgazebo uses interprocess communication (IPC) to allow a program to exchange data with a running simulator. Hereafter, we shall refer to the simulator as the server and the program as the client. Thus, for example, when using player to interact with Gazebo, Gazebo is the server and player is the client.

The details of the underlying IPC are entirely hidden by libgazebo, which should be treated as a black box for passing data back and forth to the server. Currently, the only limitation users need be aware of is that both server and client must reside on the same machine; they cannot be run in separate locations across a network.

Devices and Interfaces

libgazebo makes the familiar distinction between devices and interfaces. In Gazebo, a device is a fully parameterized model, representing a particular real-world object such as a Pioneer2AT robot or a SICK LMS200 laser. An interface, on the other hand, is a general specification for an entire class of devices, such as position or laser. Thus, a Pioneer2AT device will present a position interface, while the SICK LMS200 will present a laser interface.

The complete set of interfaces supported by libgazebo is described here. Note that these interfaces should not be confused with the interfaces defined by Player. Currently, these two sets of interfaces are fairly similar, but there is no guarantee that they will remain so in future.

Using libgazebo

Client programs must connect both to a specific server and a specific set of devices within that server. The following code snippet illustrates the general behavior:

// simple.c - works with simple.world
// compile with:
//    gcc -Wall -g3 `pkg-config --cflags gazebo` -c -o simple.o simple.c
//    gcc simple.o -o simple `pkg-config --libs gazebo` 
// Thanks to Kevin Knoedler for various fixes.

#include <stdio.h>
#include <gazebo.h>

int main (int argc, char *argv[])
{
  int i;
  gz_client_t *client;
  gz_position_t *position;
  int server_id;
  int client_id;

  // Assume one server and one client
  server_id = 0;
  client_id = 0;

  // Print all errors
  gz_error_init(1, 9);

  // Create a client object
  client = gz_client_alloc();  

  // Connect to the server
  if (gz_client_connect_wait(client, server_id, client_id) != 0)
    return -1;

  // Create a position interface
  position = gz_position_alloc();

  // Connect to device on server
  if (gz_position_open(position, client, "robot1") != 0)
    return -1;

  // Read some data
  for (i = 0; i < 10; i++)
  {
    // Wait for new data
    gz_client_wait(client);
    
    // Lock it
    gz_position_lock(position, 1);

    // Print the current odometric position
    printf("%0.3f %0.3f\n", 
           position->data->pos[0], position->data->pos[1]); 

    // Unlock it
    gz_position_unlock(position);
  }

  // Close the interface and free object
  gz_position_close(position);
  gz_position_free(position);

  // Close the connection and free the client object
  gz_client_disconnect(client);
  gz_client_free(client);

  return 0;
}

There are several points to note:

The value of server_id variable must correspond to the server ID specified on the Gazebo command line. By default, the ID is 0, but other values may be specified if the user is running multiple instances of Gazebo simultaneously.

The value of client_id variable must be unique for each client, and bounded in the range GZ_CLIENT_ID_FIRST to GZ_CLIENT_ID_LAST.

The device_id variable specifies a unique device ID; the value of this variable must correspond to the device ID specified in the Gazebo world file.

Any attempt to connect to a device using an unsupported interface will result in the open function returning an error. For example, if the device specified by device_id is actually a laser, attempting to open it using gz_position_open() will result in an error.

Do not use the create and destroy functions associated with each interface; these are reserved for use by Gazebo only. Use open and close instead.

This example can be demonstrated using the follow world file:

<?xml version="1.0"?>

<gz:world 
  xmlns:gz='http://playerstage.sourceforge.net/gazebo/xmlschema/#gz'
  xmlns:model='http://playerstage.sourceforge.net/gazebo/xmlschema/#model'
  xmlns:sensor='http://playerstage.sourceforge.net/gazebo/xmlschema/#sensor'
  xmlns:window='http://playerstage.sourceforge.net/gazebo/xmlschema/#window'
  xmlns:param='http://playerstage.sourceforge.net/gazebo/xmlschema/#params'
  xmlns:ui='http://playerstage.sourceforge.net/gazebo/xmlschema/#params'>

  <model:ObserverCam>
    <id>userCam0</id>
    <xyz>-0.506 0.894 0.865</xyz>
    <rpy>0 26 -23</rpy>
    <imageSize>640 480</imageSize>
  </model:ObserverCam>

  <model:LightSource>
    <id>light1</id>
    <xyz>0.0 10.0 100.0</xyz>
  </model:LightSource>

  <model:GroundPlane>
    <id>ground1</id>
    <color>0 0.5 0</color>
  </model:GroundPlane>

  <model:Pioneer2AT>
    <id>robot1</id>
    <xyz>0.0 0.0 0</xyz>
  </model:Pioneer2AT>

</gz:world>

Compiling and Linking Programs With libgazebo

All public functions are declared in gazebo.h and defined in libgazebo.a. The default install scripts put these in sensible places, so your compiler should pick them up automatically. Nevertheless, the recommended way to build applications is using the pkg-config mechanism; the compile and link lines should look something like this:

gcc -Wall -g3 `pkg-config --cflags gazebo` -c -o simple.o simple.c
gcc simple.o -o simple `pkg-config --libs gazebo`

Last updated 12 September 2005 21:38:45