Question

Please help me understand 2 things I found in this C code:

First, there is the whole code:

usbMsgLen_t usbFunctionSetup(uchar data[8])  
{  
usbRequest_t    *rq = (void *)data;  
static uchar    dataBuffer[4];  /* buffer must stay valid when usbFunctionSetup returns */  
if(rq->bRequest == CUSTOM_RQ_ECHO){ /* echo -- used for reliability tests */  
        dataBuffer[0] = rq->wValue.bytes[0];  
        dataBuffer[1] = rq->wValue.bytes[1];  
        dataBuffer[2] = rq->wIndex.bytes[0];  
        dataBuffer[3] = rq->wIndex.bytes[1];  
        usbMsgPtr = dataBuffer;         /* tell the driver which data to return */  
        return 4;  
    }else if(rq->bRequest == CUSTOM_RQ_SET_STATUS){  
        if(rq->wValue.bytes[0] & 1){    /* set LED */  
            LED_PORT_OUTPUT |= _BV(LED_BIT);  
        }else{                          /* clear LED */  
            LED_PORT_OUTPUT &= ~_BV(LED_BIT);  
        }  
    }else if(rq->bRequest == CUSTOM_RQ_GET_STATUS)  {  
        dataBuffer[0] = ((LED_PORT_OUTPUT & _BV(LED_BIT)) != 0);  
        usbMsgPtr = dataBuffer;         /* tell the driver which data to return */  
        return 1;                       /* tell the driver to send 1 byte */  
    }  
    return 0;   /* default for not implemented requests: return no data back to host */  
}  

Now, usbFunctionSetup gets array of 8 unsigned chars. Now there comes the line:

usbRequest_t    *rq = (void *)data;

So, I get the left side of the statement, but what is on the right? I know that (void *) is cast to this type, but why?

And second question is, isnt this code inefficient? Because first function receives 8 bytes of data, and than it creates additional pointer to them. And that additional pointer is created, at least if I am right, just to be able to access individual data by its name defined in usbRequest_t struct. Wouldn’t be simpler and more efficient to just use in code instead of rq->bRequest == something just for example
data[2]==something or if bRequest is bigger than one byte, for example data[1] == low_byte_of_something && data[2]== high_byte_of_something?

Answer 1

The function is getting a ‘raw’ buffer of data, and the line:

usbRequest_t    *rq = (void *)data;

is just creating a pointer to that buffer to access it using the data layout specified by the usbRequest_t struct.

There’s absolutely nothing expensive about this operation – the compiler will likely not even actually create a new pointer variable in an optimized build.

On there other hand, there may well be portability concerns, but that might not be important for your particular application.