Note: latest code on my laptop. Refactoring and improving for code release.
Really simple concept: scan an input c file, find the functions that need prototype declarations, and then output the prototype declarations for you to copy and paste into a header or at the top of your file.
I use this simple tool when quickly prototyping microcontroller c code. Just write your code without worrying about declaring function prototypes, then copy and paste the code into this tool and it will create the declarations for all functions that have not been declared yet. No rocket science going on, just saves you some time.
You can also use this tool to merge additions to your header file if you have a number of functions that you forgot to declare. Just paste the header and source files together in here and let the script spit out the missing declarations to add to the header.
Keep in mind that this tool is intended for quick prototyping. If you are making something complicated, best plan out your headers. Don't blame me for helping you be lazy :)