Function xmlGenericErrorDefaultFunc 

pub unsafe extern "C" fn xmlGenericErrorDefaultFunc() -> c_int

xmlGenericErrorDefaultFunc(void *ctx, const char *msg, ...) — the upstream default generic error handler. Not exported dynamically (matches upstream, where the symbol is internal).

2 fixed args (ctx=rdi, msg=rsi) → gp_offset 16; the va_list pointer is passed as the 3rd arg (rdx) of the receiver.