What are the conventions for indicating programming variables within text?
It is common to typeset code with a monospaced font (a font whose characters have all the same width), like get_config_value. 

If you're using LaTeX to write your thesis, you can use the command \texttt for inline code. For clarity and flexibility, it's probably better to define a new command, e.g. \newcommand{\code}[1]{\texttt{#1}}. You can use the listings package for longer pieces of code.

For other typesetting systems or word processors, check their manual.