switch case: error: case label does not reduce to an integer constant

Let me chip in with an example. The following was tested on gcc version 4.6.3 with the flags -std=c99 -pedantic set:

#define SOME_HARDCODED_CONSTANT 0 //good
int foo(int i, int b){
    const int c=0; //bad
    int a=0; //bad

    switch(i){
        case c:     //compile error
        case a:     //compile error.
        case (b+a): //compile error
        case SOME_HARDCODED_CONSTANT: //all good
        case 5: //all good
    }
}

As others have noted, case arguments cannot be evaluated at runtime. Use an if-else block to do that.

switch labels must be constant expressions, they have to be evaluated at compile time. If you want to branch on run-time values, you must use an if.

A const-qualified variable is not a constant expression, it is merely a value you are not allowed to modify.

The form of integer constant expressions is detailed in 6.6 (6) [C99 and the n1570 draft of the C2011 standard]:

6 An integer constant expression 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.

The restriction that only sizeof expressions whose result is an integer constant are allowed rules out sizeof expressions whose operand is a variable length array.