void spiDataRW(int channel, SV* byte_ref, int len){

     /*
      * Custom wrapper for wiringPiSPIDataRW() as we
      * need to translate an aref into an unsigned char *,
      * and then send back an array containing the bytes
      * read from the device
      */ 

    if (channel != 0 && channel != 1){
        croak("channel param must be 0 or 1\n");
    }

    if (! SvROK(byte_ref) || SvTYPE(SvRV(byte_ref)) != SVt_PVAV){
        croak("data param must be an array reference\n");
    }

    AV* bytes = (AV*)SvRV(byte_ref);

    int num_bytes = av_len(bytes) + 1;

    if (len != num_bytes){
        croak("len param doesn't match element count in data\n");
    }

    unsigned char buf[num_bytes];

    int i;

    for (i=0; i<len; i++){
        SV** elem = av_fetch(bytes, i, 0);

        int elem_int = (int)SvNV(*elem);
        
        if (elem_int < 0 || elem_int > 255){
            printf("byte %d in data param out of range: (%d)\n", i, elem_int);
            exit(1);
        }

        buf[i] = (unsigned char)SvNV(*elem);
    }
    
    if (wiringPiSPIDataRW(channel, buf, len) < 0){
        croak("failed to write to the SPI bus\n");
    }

    inline_stack_vars;
    inline_stack_reset;

    int x;
    for (x=0; x<len; x++){
        inline_stack_push(sv_2mortal(newSViv(buf[x])));
    } 

    inline_stack_done;
}