Comment by Peter - Reinstate Monica on fopen not seeing my file despite double checking its location and creating a new one just in case right before it

Relative paths are notoriously unreliable when working with build systems. (What is the "current directory"? The location of the exe? The location of the project? The directory from which the IDE was stared?). You can double check by creating the file first in the same program; that one you'll surely be able to open with the same path, relative or not. Then search in the file system where it actually was created, that'll give you a hint. But it's probably better to use an absolute path.