[sdiy] Pointers in C

[MTG]

+1  for unions. On an 8-bitter it will be the smallest and fastest.

GB

----- Original Message ----- 
From: "Jay Vaughan" <jayv at synth.net>
To: "Magnus Danielson" <magnus at rubidium.dyndns.org>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, December 31, 2011 8:36 AM
Subject: Re: [sdiy] Pointers in C


> Best thing is to use a union though - this will let you set up some 
> aliases to individual parts of the word without arithmetic and pointer 
> mess.  It is definitely good to understand the increment/decrement rules 
> that the compiler follows for type sizes, but beyond that: use a union. 
> It's clean and relatively painless.
>
> ;
> --
> seclorum
>
> On Dec 31, 2011, at 17:23, Magnus Danielson <magnus at rubidium.dyndns.org> 
> wrote:
>
>> On 12/31/2011 12:42 PM, Paul Maddox wrote:
>>> All,
>>>
>>>   Pointers have always confused me, though I'm getting better slowly.
>>>
>>>   But I have a question, if I have a pointer to an unsigned long int 
>>> (32bits) and I want to access it as four unsigned chars, do I just read 
>>> a char and increase the pointer? for example;
>>>
>>> int *my_ptr = my_unsigned_long_int;
>>> unsigned char FirstByte = *my_ptr;
>>> my_ptr++;
>>> unsigned char SecondByte = *my_ptr;
>>> my_ptr++;
>>> unsigned char ThirdByte = *my_ptr;
>>> my_ptr++;
>>> unsigned char FourthByte = *my_ptr;
>>>
>>>   or am I nuts?
>>
>> Hi nuts,
>>
>> since you declared a int pointer the pointer arithmetic is in the size of 
>> that. In this case you use int and assuming that the int has size of 16 
>> bits, then you will increment in 2 byte steps with each ++.
>>
>> The code you wrote is equalent to
>>
>> char * my_ptr;
>> my_ptr = (char *)my_unsigned_long_int_ptr;
>> unsigned char FirstByte = *my_ptr;
>> my_ptr = my_ptr + 2;
>> unsigned char SecondByte = *my_ptr
>> my_ptr = my_ptr + 2;
>>
>> What you want is to convert your pointer to a pointer of the size you 
>> want, char, and then you can access it sequentially as above. Thus:
>>
>> char *my_ptr = (char *)my_unsigned_long_int_ptr;
>>
>> Further, if you indeed have a long int variable which you want to pick 
>> the bytes from, then you would do this:
>>
>> char *my_ptr = (char *)&my_unsigned_long_int;
>>
>> Where the & operator gives you the address of the variable after it of 
>> the type of that variable, in this case you get a long int * from this, 
>> which is then type-converted in the (char *) expression.
>>
>> Pointer arithmetic in C is a bit confusing at first, but it is kind of 
>> neat. Another way to write the same thing would be
>>
>> unsigned char FirstByte = my_ptr[0];
>> unsigned char SecondByte = my_ptr[1];
>> ...
>>
>> which is the same as
>>
>> unsigned char FirstByte = *(my_ptr+0);
>> unsigned char SecondByte = *(my_ptr+1);
>> ...
>>
>> Cheers,
>> Magnus
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