Function vfprintf 

pub unsafe extern "C" fn vfprintf(
    f: *mut FILE,
    fmt: *const c_char,
    ap: VaList<'_>,
) -> c_int

Writes VaList formatted output to f, matching <stdio.h> vfprintf.

Safety

f must point to a valid FILE. fmt must be non-null and point to a valid NUL-terminated C format string whose format specifiers match the arguments in ap.