Thread: Constant value not known

Hi,

When compiling any of these codes, I get the message: "constant value not known" which seems weird to me... at least for the second one I don't understand why.

Thanks for any help.

F.

Code:

////////////////////////////////////////////////////////////// V.1

const int size_input_bits = 12382;
const int size_input_ints = ceil((float)size_input_bits/32);
int h_input [size_input_ints];

////////////////////////////////////////////////////////////// V.2

#define size_input_bits 12382;
...........
const int size_input_ints = ceil((float)size_input_bits/32);
int h_input [size_input_ints];

Find my comments Inline.

Code:

////////////////////////////////////////////////////////////// V.1

const int size_input_bits = 12382;
const int size_input_ints = ceil((float)size_input_bits/32);
int h_input [size_input_ints];

////////////////////////////////////////////////////////////// V.2

#define size_input_bits 12382;
U dont need to put ' ; ' when defining a variable.
...........
const int size_input_ints = ceil((float)size_input_bits/32);
--I think initializers of global variables cannot contain function
calls in C. You can switch to C++, that introduces the ability to do dynamic initialization of global variables.

int h_input [size_input_ints];

> #define size_input_bits 12382;
#defines don't normally have trailing ;

> int h_input [size_input_ints];
Standard (C89) C doesn't have variable length arrays of any persuasion, even if you declare the size as an int constant. As far as C is concerned, it is a dynamic run-time expression (which always yields the same value).

The C and C++ ideas about const are very different.

Code:

#define size_input_bits 12382
...........
int h_input [ size_input_bits / 32 ];

This should work in C.

http://david.tribble.com/text/cdiffs...-const-linkage
In C++, the compiler is free to substitute the known value at compile time, thus achieving what #define does in C.

In C, const just means "warn me if I try to modify this variable". For example, this is valid C.
const volatile int clock;
A read-only variable which is constantly changing.