Code:

static int dataSent; // for USB_DATA_LONGOUT

USB_PUBLIC usbMsgLen_t usbFunctionSetup(uchar data[8]) {
    usbRequest_t *rq = (void *)data; // cast data to correct type
   
    if(rq->bRequest == USB_DATA_LONGOUT) {
        dataSent = 0;
        return USB_NO_MSG;
    }
}

USB_PUBLIC uchar usbFunctionRead(uchar *data, uchar len) {
    uchar i;

    for(i = 0; dataSent < 128 && i < len; i++, dataSent++)
        data[i] = '0'+i;
    
    if(i && dataSent == 128) 
        data[i-1] = '\0';
      
    return i; // equals the amount of bytes written
}

Code:

char buffer[256] = "";

nBytes = usb_control_msg(handle, USB_TYPE_VENDOR | USB_RECIP_DEVICE | USB_ENDPOINT_IN, 
    USB_DATA_LONGOUT, 0, 0, (char *)buffer, sizeof(buffer), 5000);

printf("Received %d bytes: %s\n", nBytes, buffer);
   
if(nBytes < 0)
    fprintf(stderr, "USB error: %s\n", usb_strerror());
else
    printf("Received %d bytes: %s\n", nBytes, buffer);