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Howto:Parsing binary data received via UDP in C

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Note  The code shown here is currently platform specific (Windows) and still needs to be ported to use the SimGear wrappers to be truly cross-platform (i.e. compile on Linux/Unix and Mac OS). Equally, it would be a good idea to add self-contained snippets of code that actually compile (including the corresponding CMakeLists.txt). In the long term, it would be great to commit this as a demo to the contrib directory in $FG_SRC (see the talk page for pointers). Any help with this would be appreciated!

Contributed by: OsoMoore (12/2014)

I am parsing binary data sent by FlightGear via UDP. I am receiving the data with a C++ application written in Visual Studio 2013.

Here is my protocol XML (to be put into $FG_ROOT/Protocols), which determines the layout of each packet (see $FG_ROOT/Docs/README.IO for details):

<?xml version="1.0"?>
<PropertyList>

 <generic>

  <output>
   <binary_mode>true</binary_mode>

  <!-- note that chunk ordering/typing is critical here and needs to match the C struct below -->
   <chunk>
    <name>latitude</name>
    <type>float</type>
    <node>/position/latitude-deg</node>
   </chunk>

   <chunk>
    <name>longitude</name>
    <type>float</type>
    <node>/position/longitude-deg</node>
   </chunk>

   <chunk>
    <name>altitude</name>
    <type>int</type>
    <node>/position/altitude-ft</node>
   </chunk>

   <chunk>
    <name>airspeed</name>
    <type>int</type>
    <node>/velocities/airspeed-kt</node>
   </chunk>

   <chunk>
    <name>roll</name>
    <type>float</type>
    <node>/orientation/roll-deg</node>
   </chunk>

   <chunk>
    <name>pitch</name>
    <type>float</type>
    <node>/orientation/pitch-deg</node>
   </chunk>

   <chunk>
    <name>heading</name>
    <type>float</type>
    <node>/orientation/heading-deg</node>
   </chunk>

  <!-- any chunks added here, also need to be added to the C struct below -->

  </output>

 </generic>

</PropertyList>


Here is the struct into which I receive the data. The #pragma should result in the structure being packed tightly in memory.

#if defined(_WIN32) || defined(WIN32) 
#pragma pack(push, 1)
  typedef struct {
#else
  typedef struct __attribute__((__packed__))  {
#endif     
      // NOTE: ordering/typing needs to match the protocol spec above
      float latitude;
      float longitude;
      int altitude;
      int airspeed;
      float roll;
      float pitch;
      float heading;
   } flightgear_packet;
#if defined(_WIN32) || defined(WIN32) 
#pragma pack(pop)
#endif


Added this endian swapper function:

float float_swap(float value){
   int temp = htonl(*(unsigned int*)&value);
   return *(float*)&temp;
};

Here is the code which receives the packet and converts it into host byte ordering:

      flightgear_packet buff, buff2;

      //  initialize buffer (fill with 0)
      memset(&buff, 0, sizeof(buff));

         bytes_read = recvfrom(SOCKET(sock), (char*)(&buff), sizeof(buff), 0, (struct sockaddr *) &si_other, &slen);
         
         if (bytes_read > 0)
         {

            // populate structture
            buff2.airspeed = ntohl(buff.airspeed);
            buff2.altitude = ntohl(buff.altitude);
            buff2.heading = float_swap(buff.heading);
            buff2.latitude = float_swap(buff.latitude);
            buff2.longitude = float_swap(buff.longitude);
            buff2.pitch = float_swap(buff.pitch);
            buff2.roll = float_swap(buff.roll);

            // print out data
            printf("Airspeed %i, Altitude %i\n", buff2.airspeed, buff2.altitude);
            printf("Position %3.2f, %3.2f\n", buff2.latitude, buff2.longitude);
            printf("Pitch/Roll/Heading %3.2f, %3.2f, %3.2f\n", buff2.pitch, buff2.roll, buff2.heading);

         }

Hopefully this will be useful to someone else down the line.