HowToUseLongParameters.md

How to use long parameters

Description: This tutorial will tell you how to use long parameters (which are not supported by the parameter server).

Tutorial Level: EASY

Problem

xmlrpclib.py as used by roslaunch does not support type long (64 bit integers, i.e. in (-2**63,2**63-1) but outside (-2**31,2**31-1)):

<arg name="long_param" value="12345678910111213" />

results in an error.

Background

see

• https://github.com/python/cpython/blob/3.7/Lib/xmlrpc/client.py#L158
• https://docs.python.org/3.6/library/xmlrpc.client.html#module-xmlrpc.client
• http://xmlrpc.com/spec.md (search for "32-bit")

Workaround

We use the following workaround:

• Add the substring L to the long parameter, making it a string (in case it is outside the valid integer range), e.g.

gen.add("long_param", paramtype="int64_t", description="A long parameter", default="12345678910111213L")
# or
gen.add("long_param", paramtype="int64_t", description="A long parameter", default="1L")
# or
gen.add("long_param", paramtype="int64_t", description="A long parameter", default=1)

• when setting it via an launchfile argument:

<arg name="long_param" value="12345678910111213L" />
<!-- or -->
<arg name="long_param" value="1L" />
<!-- or -->
<arg name="long_param" value="1" />

• when setting it via a yaml:

long_param: "12345678910111213L"
# or
long_param: "1L"
# or
long_param: 1

• For long parameters, a string is automatically cast to a 64bit integer when reading from the parameter server
• Any long parameter is stored as string when writing to the parameter server