The 'String change case user defined function in C' is a programming tool that allows users to convert the case of a given string using a custom-defined function in the C programming language.

This function provides flexibility for changing the case of characters in a string based on the user's requirements. With this tool, programmers can easily manipulate the cases of strings in their C programs, enhancing the overall functionality and usability of their applications.

The program is explained using animation to understand use of pointers and change the case in original string. 

You can modify the function to make the function more robust.