Function Perl_my_vsnprintf 

pub unsafe fn Perl_my_vsnprintf(
    my_perl: *mut PerlInterpreter,
    buffer: *mut c_char,
    len: usize,
    format: *const c_char,
    ap: *mut __va_list_tag,
) -> c_int

thx shim for crate::Perl_my_vsnprintf.