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It's suggested that an external program link against the Gazebo libraries, specifically the transport library, and send messages using the interface provided by Gazebo Nodes.

You can send messages to Gazebo using your own transport mechanism. Construct your message with:

1. An 8byte header that is the size of the message
2. The message data (which must be a serialized protobuf message that Gazebo understands).

The following is a simple example in Python. Here we are assuming the python script is waiting for incoming requests from Gazebo, and then responds with a new message.

#!/usr/bin/env python

import socket
import struct
from sys import *
from xml.dom import minidom

import request_pb2

TCP_IP = '127.0.0.1'
TCP_PORT = 50199
BUFFER_SIZE = 1024

models = {}

#################################################
# Read the main manifest file
def read_manifest():
  xmldoc = minidom.parse('manifest.xml')
  model_list = xmldoc.getElementsByTagName("model")

  for model in model_list:
    model_name = model.getElementsByTagName("name")[0].childNodes[0].data
    model_path = model.getElementsByTagName("path")[0].childNodes[0].data
    models[model_name] = model_path

read_manifest()

print 'here'
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((TCP_IP, TCP_PORT))
s.listen(1)
print 'here2'

conn, addr = s.accept()
print 'Connection address:', addr
while 1:
  request = request_pb2.Request()
  data = conn.recv(BUFFER_SIZE)
  if not data:
    break

  request.ParseFromString(data)

  print "Request: ", request.request

  response = request_pb2.Response()
  response.id = request.id
  response.request = request.request

  if request.request == "models":
    response.response = "success"

    model_list_msg = request_pb2.ModelList()
    response.type = model_list_msg.DESCRIPTOR.full_name
    for key in models:
      model_list_msg.model.append(key)
    response.data = model_list_msg.SerializeToString()

  print "Response Size:", len(response.SerializeToString())
  hex_size = hex(len(response.SerializeToString()))
  conn.send(hex_size.rjust(8))
  conn.send(response.SerializeToString())  # echo
conn.close()