DCL41-C. Do not declare variables inside a switch statement before the first case label

According to the C Standard, 6.8.5.3, paragraph 4 [ISO/IEC 9899:2024],

A switch statement causes control to jump to, into, or past the statement that is the switch body, depending on the value of a controlling expression, and on the presence of a default label and the values of any case labels on or in the switch body. A case or default label is accessible only within the closest enclosing switch statement.

If a programmer declares variables, initializes them before the first case statement, and then tries to use them inside any of the case statements, those variables will have scope inside the switch block but will not be initialized and will consequently contain indeterminate values.  Reading such values also violates EXP33-C. Do not read uninitialized memory.

Noncompliant Code Example

This noncompliant code example declares variables and contains executable statements before the first case label within the switch statement:

#include <stdio.h>
 
extern void f(int i);
 
void func(int expr) {
  switch (expr) {
    int i = 4;
    f(i);
  case 0:
    i = 17;
    /* Falls through into default code */
  default:
    printf("%d\n", i);
  }
}

Implementation Details

When the preceding example is executed on GCC 4.8.1, the variable i is instantiated with automatic storage duration within the block, but it is not initialized. Consequently, if the controlling expression expr has a nonzero value, the call to printf() will access an indeterminate value of i. Similarly, the call to f() is not executed.

Compliant Solution

In this compliant solution, the statements before the first case label occur before the switch statement:

#include <stdio.h>
 
extern void f(int i);
 
int func(int expr) {
  /*
   * Move the code outside the switch block; now the statements
   * will get executed.
   */
  int i = 4;
  f(i);

  switch (expr) {
    case 0:
      i = 17;
      /* Falls through into default code */
    default:
      printf("%d\n", i);
  }
  return 0;
}

Risk Assessment

Using test conditions or initializing variables before the first case statement in a switch block can result in unexpected behavior and undefined behavior 20.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

Bibliography