variably modified 'stack' at file scope [duplicate]

Question

int const a=9;
int stack[a];
int main()
{
    return 0;
}

The above code gives an error:variably modified 'stack' at file scope

But when I change the code to :

#define b 3
int stack[b];
int main()
{
    return 0;
}

It compiles without error.While both #define and const variable are used for defining the constant identifier then why there is error when I use the const var instead of #define.I searched the similar question but all they gave solution about the error but no reason to it.

Searched about the const and #define and found that sometimes gcc compiler recognizes const as readonly but it is too confusing.

Answer 1

In C language static storage variables can have size defined by the constant expression. Using the variable (even the constant one) as the size is not such a expression.

The error is 100 percent correct.

The second case: proprocessor replaces textually the b with 3 which is the constant expression

Constant expression is something which is evaluated during the compilation. Variable value can be only evaluated runtime.