C Case Label

case constant-expression: statement

​

Technical Constraints

• Constant Expression Requirement: The expression following the case keyword must evaluate to an integer type (int, char, enum) at compile time. Variables, floating-point values, string literals, and function calls are strictly prohibited.
• Uniqueness: Every case label within a single switch block must evaluate to a unique integer value. Duplicate values will trigger a compilation error.
• Type Promotion: According to the C standard, integer promotions are first performed on the switch statement’s controlling expression. Subsequently, each case constant expression is converted to the promoted type of the controlling expression before the comparison occurs.
• Scope and Placement: case labels have no lexical scope of their own. They must be enclosed within a switch statement, but C syntax permits them to be nested within other control structures inside that switch (a mechanic famously exploited by Duff’s Device).

​

Execution Mechanics

switch (x) {
    case 1: 
        /* Execution starts here if x == 1 */
        x = 10; 
        /* Falls through to case 2 unless a break is present */
    case 2: 
        x = 20;
        break;
}

​

Lexical Rules Regarding Declarations

switch (x) {
    case 1: {
        /* Universally valid (C89+): Declaration inside a compound statement */
        int y = 5; 
        x = y;
        break;
    }
    case 2: ;      
        /* Valid only in C99+: Empty statement precedes declaration */
        int z = 10;
        x = z;
        break;
}