CertC-DCL41

Do not declare variables inside a switch statement before the first case label

Required inputs: IR

According to the C Standard, 6.8.4.2, paragraph 4 [ ISO/IEC 9899:2011],

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.

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.

Value of expr
 Output
0 17
Nonzero Indeterminate
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.

Rule Severity Likelihood Remediation Cost Priority Level
DCL41-C Medium Unlikely Medium P4 L3
Related Guidelines
Taxonomy Taxonomy item Relationship
MISRA C:2012 Rule 16.1 (required) Prior to 2018-01-12: CERT: Unspecified Relationship
Bibliography
[ ISO/IEC 9899:2011] 6.8.4.2, "The switch Statement"
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/declarations-and-initialization-dcl/dcl41-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

switch_not_well_formed

Switch has a statement before the first case label.

None

False

Options

acceptable_start_items

acceptable_start_items : set[bauhaus.ir.PIR_Class_Name] = set()

Allowed items before the first case label of a switch.