dhoward@ready.eng.ready.com (David Howard) (05/18/91)
Questions on const and volatile: I am using the latest release of the Microtec ANSI C cross compiler, PC hosted, 680x0 target. I am attempting to use const and volatile and getting results that are slightly different from what I expect. If anyone out ther has a comment, please do: const ----- I declare: const unsigned char *device = (unsigned char *)0x600000b; Then I do: *device = 0x01; The compiler complains: 'attempt to modify a const' what I want is for the pointer to be const, but not what it points to! Basically, I want all my 'consts' in ROM so they won't get trashed, but I wish to access variable data thru them. I tried various combinations of parentheses and got nowhere. By the way, the compiler also complains if I do: device = (unsigned char *)0x600000d; but that is what I expect. It doesn't seem right that the declaration should make BOTH the pointer and the pointee const. likewise with volatile, I get a reverse situation. I want to point at a device register that is *really* volatile. In this case I want the pointee to be volatile but not the pointer. If I declare the pointer: volatile unsigned char *device = 0x600000e; the compiler treats BOTH the pointer and pointee as volatile. By that I mean, when I use the pointer, the compiler always reloads the pointer itself, and also always reaccesses the dereferenced location. In this case it works OK, but generates extra code to reload the pointer with 0x600000e every time I use it. It appears that since the specification of const and volatile is implementation dependent, the Microtec guys intepreted it in a strange way. I guess what I really want is the pointer to be const, and the pointee to be volatile. Anyone have a suggestion on how to declare the pointer to get that result? What do other ANSI compilers do?
worley@compass.com (Dale Worley) (05/21/91)
In article <1991May17.213923.17879@ready.eng.ready.com> dhoward@ready.eng.ready.com (David Howard) writes:
I declare:
const unsigned char *device = (unsigned char *)0x600000b;
Then I do:
*device = 0x01;
The compiler complains:
'attempt to modify a const'
what I want is for the pointer to be const, but not what it points to!
Say: "unsigned char * const device = ...;" Obvious, isn't it? :-)
If I declare the pointer:
volatile unsigned char *device = 0x600000e;
the compiler treats BOTH the pointer and pointee as volatile. By that
I mean, when I use the pointer, the compiler always reloads the pointer
itself, and also always reaccesses the dereferenced location. In this
case it works OK, but generates extra code to reload the pointer with
0x600000e every time I use it.
The compiler is conforming, but it is wasting effort. Hope that the
next version of the compiler is more efficient.
Dale Worley Compass, Inc. worley@compass.com
--
Conspiracy is the opiate of the asses.steve@taumet.com (Stephen Clamage) (05/22/91)
dhoward@ready.eng.ready.com (David Howard) writes: >Questions on const and volatile: >I declare: > const unsigned char *device = (unsigned char *)0x600000b; >Then I do: > *device = 0x01; >The compiler complains: > 'attempt to modify a const' The compiler is correct. You say later that you want the pointer to be const, not what it points to. In that case, you need to say this: unsigned char * const device = (unsigned char *)0x600000b; This makes 'device' const, rather than what it points to. You will now be able to write *device = <something> but not device = <some address> The same thing applies to volatile, where T is some type: volatile T *p; /* p not volatile, points to a volatile object */ T * volatile p; /* p is volatile, the object is not */ For a pointer to a memory-mapped input device or clock, you might want something like this: const volatile T * const p = (const volatile T*) 0xc000abcd; This says that p cannot be assigned to (it is const), and that it points to an object which cannot be assigned to, and which might change spontaneously. That is: p = <some address>; /* illegal: p is const */ *p = <some value>; /* illegal: *p is const */ a = *p; b = *p; /* must read p twice, cannot optimize */ -- Steve Clamage, TauMetric Corp, steve@taumet.com