It is guaranteed per C99. It's a compound initializer. Also if you set a struct with {1} the first field is set to 1 and the rest to 0. AFAIR it was always lik

It is guaranteed per C99. It's a compound initializer. Also if you set a struct with {1} the first field is set to 1 and the rest to 0. AFAIR it was always like that (at least since standardized C, but probably earlier).