Can a const variable be used to declare the size of an array in C?

Question

Why does the following code throw an error?

const int a = 5;
int b[a]={1,2,3,4,5};

And also when I tried to compile the above code without "const" keyword, I got the same error:

int a = 5; 
int b[a]={1,2,3,4,5};

why is it so? What is the mistake that I am doing here?

And also another question: When are constants replaced with their actual values in a code, i.e if I declare a variable say: const int x= 5; I know that no memory is allocated in RAM for the variable x, but constant variable area in ROM holds the value 5 and that x is simply replaced by the value 5 everywhere x appears in the code. But when does this happen? Compilation time? Boot up time? Preprocessing time?

PS: I am talking about Embedded C (running on a Microcontroller etc), not C running on my desktop. So the embedded system is bound to have a ROM (Flash, EEPROM...). What would happen then?

Answer 1

It's simply a limitation of the language. The sizes of statically-bounded arrays need to be constant expressions, and unfortunately in C that's only something like a literal constant or a sizeof expression or such like, but not a const-typed variable.

(As Simon pointed out, since C99 there are also runtime-bounded arrays, or "variable-length arrays", whose size can be given by the value of any variable. But that's a different animal.)

You may be interested to hear that the rules are different in C++, where a static const int is indeed a constant expression, and C++11 even adds a new keyword, constexpr, to allow even more general use of constant expressions which encompass more things whose value "could reasonably be determined at compile time".