libplayerc example

An example. More...

An example.

libplayerc is based on a device proxy model, in which the client maintains a local proxy for each of the devices on the remote server. Thus, for example, one can create local proxies for the position2d and laser devices. There is also a special client proxy, used to control the Player server itself.

Programs using libplayerc will generally have the following structure:

#include <stdio.h>
#include <libplayerc/playerc.h>

int
main(int argc, const char **argv)
{
  int i;
  playerc_client_t *client;
  playerc_position2d_t *position2d;

  // Create a client and connect it to the server.
  client = playerc_client_create(NULL, "localhost", 6665);
  if (0 != playerc_client_connect(client))
    return -1;

  // Create and subscribe to a position2d device.
  position2d = playerc_position2d_create(client, 0);
  if (playerc_position2d_subscribe(position2d, PLAYER_OPEN_MODE))
    return -1;

  // Make the robot spin
  playerc_position2d_enable(position2d, 1);
  playerc_position2d_set_speed(position2d, 0, 0, 0.1);

  for (i = 0; i < 200; i++)
  {
    // Wait for new data from server
    playerc_client_read(client);

    // Print current robot pose
    printf("position2d : %f %f %f\n",
           position2d->px, position2d->py, position2d->pa);
  }

  // Shutdown
  playerc_position2d_unsubscribe(position2d);
  playerc_position2d_destroy(position2d);
  playerc_client_disconnect(client);
  playerc_client_destroy(client);

  return 0;
}

This example can be built using the command:

$ gcc -o simpleclient `pkg-config --cflags playerc` simpleclient.c `pkg-config --libs playerc`

Make sure that libplayerc is installed somewhere that pkg-config can find it.

The above program can be broken into six steps, as follows.

• Create and connect a client proxy.

  client = playerc_client_create(NULL, "localhost", 6665);
  playerc_client_connect(client);
  

  The create function creates a new client proxy and returns a pointer to be used in future function calls (localhost should be replaced with the network host name of the robot). The connect function notifies the Player server that a new client wishes to recieve data.

• Create and subscribe a device proxy.

  position2d = playerc_position2d_create(client, 0);
  playerc_position2d_subscribe(position2d, PLAYERC_OPEN_MODE);
  

  The create function creates a new position2d device proxy and returns a pointer to be used in future function calls. The subscribe function notifies the Player server that the client is using the position2d device, and that the client expects to both send commands and recieve data (PLAYERC_OPEN_MODE).

• Configure the device, send commands.

  playerc_position2d_enable(position2d, 1);
  playerc_position2d_set_speed(position2d, 0, 0, 0.1);
  

  The enable function sends a configuration request to the server, changing the robot's motor state from off to on, thereby allowing the robot to move. The setspeed function sends a new motor speed, in this case commanding the robot to turn on the spot.

  Note that most Player devices will accept both asynchronous command and synchronous configuration requests. Sending commands is analogous using the standard Unix write device interface, while sending configuration requests is analogous to using the ioctl interface. For the most part, libplayerc hides the distinction between these two interfaces. Users should be aware, however, that while commands are always handled promptly by the server, configuration requests may take significant time to complete. If possible, configuration requests should therefore be restricted to the initialization phase of the program.

• Read data from the device.

  playerc_client_read(client);
  printf("position : %f %f %f\n", position2d->px, ... );
  

  The read function blocks until new data arrives from the Player server. This data may be from one of the subscribed devices, or it may be from the server itself (which sends regular synchronization messages to all of its clients). The read function inspects the incoming data and automatically updates the elements in the appropriate device proxy. This function also returns a pointer to the proxy that was updated, so that user programs may, if desired, trigger appropriate events on the arrival of different kinds of data.

  In pull mode read will process a full set of messages from the server, in push mode read will process a single message.

• Unsubscribe and destroy the device proxy.

  playerc_position2d_unsubscribe(position2d);
  playerc_position2d_destroy(position2d);
  

  The unsubscribe function tells the Player server that the client is no longer using this device. The destroy function then frees the memory associated with the device proxy; the device pointer is now invalid and should be not be re-used.

• Disconnect and destroy the client proxy.

  playerc_client_disconnect(client);
  playerc_client_destroy(client);
  

  The disconnect function tells the server that the client is shutting down. The destroy function then frees the memory associated with the client proxy; the client pointer is now invalid and should be not be re-used.