Problem with C fwrite

What the fuck? Why, when I try to write a string to a text file using C, does it change the first character to "ÿ"?

e.g.

ÿ10
ÿhis
ÿololol
ÿhis

(all input on different runs of the program).

I'm using "a+" if that makes any difference, though I've tried with "r+" aswell and that does the same thing.

Any help?

>>32
Somehow I've imagined that C development environments for 8-bit microcontrollers and such have 8-bit ints but actually I don't know since I prefer to use toy languages and stay away from the embedded market.