Not a constant initializer element?

Question

I encountered a confusing case when I was doing semantic analysis for my compiler course.

#include <stdio.h>

int a = "abcd"[2];

int main()
{
    char b = "abcd"[2];

    printf("%d\n%c\n", a, b);

    return 0;
}

GCC says "error: initializer element is not constant" for variable "a".

Why?

Answer 1

The C language requires initializers for global variables to be constant expressions. The motivation behind this is for the compiler to be able to compute the expression at compile time and write the computed value into the generated object file.

The C standard provides specific rules for what is a constant expression:

6. An integer constant expression¹¹⁷⁾ shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, _Alignof expressions, and floating constants that are the immediate operands of casts. Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to the sizeof or _Alignof operator .
7. More latitude is permitted for constant expressions in initializers. Such a constant expression shall be, or evaluate to, one of the following:
   • an arithmetic constant expression,
   • a null pointer constant,
   • an address constant, or
   • an address constant for a complete object type plus or minus an integer constant expression.

As you can see non of the cases include an array access expression or a pointer dereference. So "abcd"[2] does not qualify as a constant expression per the standard.

Now the standard also says:

10. An implementation may accept other forms of constant expressions.

So it would not violate the standard to allow "abcd"[1] as a constant expression, but it's also not guaranteed to be allowed.

So it's up to you whether or not to allow it in your compiler. It will be standard compliant either way (though allowing it is more work as you need another case in your isConstantExpression check and you need to actually be able to evaluate the expression at compile time, so I'd go with disallowing it).