How do I clear the whole contents of a file in C?

As @stefan said using fopen() with "w" mode will do the job for you. When you open a file with "w" flag it creates an empty file for writing. If a file with the same name already exists its contents are erased and the file is treated as an empty new file.

If the file is already open you can use freopen() function from stdio.h with "w" mode as it will first close the file and then reopen it for writing erasing whatever was in the file previously.