int
my_xs_func(p_struct, OUTLIST buffer, len)
        void *p_struct;
        SV *buffer;
        int len;
    PREINIT:
        char *tmp_buffer;
    CODE:
        tmp_buffer = (char *)malloc(len);
        RETVAL = my_func(p_struct, tmp_buffer, len);
        buffer = sv_2mortal(newSVpv(tmp_buffer, 0));
        free(tmp_buffer);
    OUTPUT:
        RETVAL