For some reasons I need a file pointer (FILE*) that points to nothing. It means I can pass it to fprintf function and fprintf ignore the file pointer.
for example:
Although as NirMH said, you can enclose it in if (nothing != NULL), there is another way. You can open a file in read mode (with "r") and when you send it to fprintf, the write is ignored (Edit: as discussed in the comments, remember to set n=0 if it was negative as returned by fprintf).
I personally suggest the first method though. The only reason I might do the second is if I can't change the function.
If you don't care if your code is system dependent, you can use /dev/null in linux, nul in windows etc,