Look in the draft n1256 (C99 with Technical corrigenda TC1, TC2, and TC3 included) for 6.5.2.2 Function calls
:
For functions without prototype, or parameters corresponding to the ellipsis ...
, the default argument promotions are performed.
Those are: Default integer promotions and promotion of float
to double
.
Default integer promotions: Every integer type of rank less than int
is promoted to int
or unsigned int
.