I want to generate empty implementations of procedures defined in a header file. Ideally they should return NULL for pointers, 0 for integers, etc, and, in an ideal world, also
UML modeling tools are capable of generating default implementation in the language of choice. Generally there is also a support for importing source code (including C headers). You can try to import your headers and generate source code from them. I personally have experience with Enterprise Architect and it supports both of these operations.