Merging changes to volatile bitfield structs

[Phil Endecott]

John Love-Jensen wrote:
> Hi Phil,
>
>> Can anyone share their preferred approach to this sort of problem?
>
> Reg workingReg = reg;
> workingReg.a = 0;
> workingReg.b = 0;
> workingReg.c = 0;
> reg = workingReg;

I'm having trouble working out the necessary casting:

struct S {
   unsigned a : 1;
   unsigned b : 31;
};

volatile S s;

void f() {
   S x;
   x.a = 0;
   x.b = 0;
   s = x;
}

This fails to compile with g++:

$ g++ -W -Wall -c /tmp/voltest.cc
/tmp/voltest.cc: In function ‘void f()’:
/tmp/voltest.cc:12: error: passing ‘volatile S’ as ‘this’ argument of 
‘S& S::operator=(const S&)’ discards qualifiers

It does compile as C, if I replace 'S' with 'struct S'.  I have tried 
various casts but haven't yet found the right one.  Any ideas anyone?


Thanks,

Phil.