Can I force a word read-modify-write instread of a byte write?

[Phil Endecott]

Ian Lance Taylor wrote:
> "Phil Endecott" <spam_from_gcc_help@chezphil.org> writes:
>
>> I know that in some circumstances gcc will generate read-modify-write
>> sequences to access memory locations, for example if I have a bitfield
>> struct.  Is there a way in which I can tell it to only do (32-bit)
>> word writes to a region of memory, and to do read-modify-write
>> sequences instead of byte writes, even though the processor does have
>> byte write instructions?
>
> Sure, this is what the volatile qualifier is for.  If you always
> access the memory through volatile qualified pointers, the compiler
> will always access the memory precisely as you write in the program.

Hi Ian, thanks for replying.

Could you help me out with an example?  Say I have a uint8_t* that I 
want to write to:

uint8_t* ptr = ......;
*ptr = 123;

How do I change that so that it does a read/modify/write?  Are you 
saying that I can do something like

volatile uint32_t* ptr = .....;
uint8_t* ptr2 = ptr;
*ptr2 = 123;

with appropriate extra casts and 'volatile's, and it will do what I want?

Cheers,

Phil.