your code does not handle these edge cases safely: empty string ("") and string consisting entirely of exclamation marks ("!!!"). these inputs make your code go out-of-bounds of str_in, towards the left (negative indices). hence the crash.

it is also incorrect because you do not nul-terminate at the right index:

str_out[lenStrIn] = '\0'; 

this cannot be right, since str_out will be shorter than str_in in general

https://docs.codewars.com/training/troubleshooting#print-input

you can use printf to see which values you get and which you return

You did not null-terminate the masked at all.