FizzBuzz without variables(argc is a parameter set by OS) and single printf in 83 bytes
https://github.com/FrozenVoid/C-techniques/blob/master/demos/fizzbuzz/fizzbuzz-oneprintf.c
FizzBuzz as single 64-bit int
https://github.com/FrozenVoid/C-techniques/blob/master/demos/fizzbuzz/fizzbuzz1.c
FizzBuzz as single 64-bit int and without printf
https://github.com/FrozenVoid/C-techniques/blob/master/demos/fizzbuzz/fizzbuzz2.c
FizzBuzz as two ints fizz/buzz
https://github.com/FrozenVoid/C-techniques/blob/master/demos/fizzbuzz/fizzbuzz3.c
FizzBuzz single 64-bit int and without printf in hex
https://github.com/FrozenVoid/C-techniques/blob/master/demos/fizzbuzz/hexbuzz.c
Bonus:FizzBuzz as single string without variables,printf,separate strings,if/ternary/branches(except loop) or modulus/division/multiply use.
https://github.com/FrozenVoid/C-techniques/blob/master/demos/fizzbuzz/stringbuzz.c