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Why do i get error "error: expression must have a constant value"

PB
Hi !

I have the following code, which I am using in an Embedded systems,
c-compiler.. However I see the same problem with GCC too..

I need the last 10 bits of an address pointer, which is completly know
at link time, but will be symbols at the compile time. I used the
following code for this initalization..
uint32 data_ptr_mask = ((uint32) data) & 0x3ff;

for illustration purpose, I have also included
uint32 data_ptr_off = ((uint32 ) data) +0x3ff;

When I compile this code (see the example routine below) I get the
following error with gcc
"error: expression must have a constant value"

Can somebody tell me why the bit operators, "*" and "/" don't work
while "+" and "-" do.

TIA

-P.B. Srinivas
/****** Start of CODE ***/
#include <stdio.h>

typedef int int32;
typedef unsigned uint32;

/* Global variable declarations **/
int32 data[64];

/** this works **/
uint32 data_ptr = (uint32 ) data;

/** this also works **/
uint32 data_ptr_off = ((uint32 ) data) +0x3ff;

/** this doesn't work , only +/- operators **/
/** seem to work, why is this ? ****/
uint32 data_ptr_mask = ((const uint32) data) & 0x3ff;

int main()
{
printf("Pointer Addition %x %x\n",data_ptr, data_ptr_off);
printf("Pointer Masking %x %x\n",data_ptr, data_ptr_mask);

}
/*********** End of code *************/

Mar 15 '06
10 21181
On 14 Mar 2006 19:13:45 -0800, "PB" <pb******@gmail .com> wrote:
Hi !

I have the following code, which I am using in an Embedded systems,
c-compiler.. However I see the same problem with GCC too..

I need the last 10 bits of an address pointer, which is completly know
at link time, but will be symbols at the compile time. I used the
Right. In general this is and only can be determined at link time.
following code for this initalization..
uint32 data_ptr_mask = ((uint32) data) & 0x3ff;

for illustration purpose, I have also included
uint32 data_ptr_off = ((uint32 ) data) +0x3ff;

When I compile this code (see the example routine below) I get the
following error with gcc
"error: expression must have a constant value"

Can somebody tell me why the bit operators, "*" and "/" don't work
while "+" and "-" do.

Because 6.6p7 allows "an address constant for an object type plus or
minus an integer constant" <OT> in turn because linkers traditionally
support that but not portably other operations like your bitwise. </>
(Actually what you wrote isn't strictly an address constant but rather
the conversion of an address constant; however, on most systems,
including the ones gcc supports, this is effectively an identity.)

<OT> Depending on your linker, you may be able to cause (all or
specific) data to aligned at a 10-bit boundary, or even placed at a
fixed location; in either case the offset is known in advance and you
can compile it as a constant. </> Otherwise you'll have to do this
computation at run time, probably early in your initialization code.

- David.Thompson1 at worldnet.att.ne t
Mar 27 '06 #11

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