Function S_MgBYTEPOS 

pub unsafe fn S_MgBYTEPOS(
    my_perl: *mut PerlInterpreter,
    mg: *mut MAGIC,
    sv: *mut SV,
    s: *const c_char,
    len: STRLEN,
) -> STRLEN

thx shim for crate::S_MgBYTEPOS.