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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

Automated Detection

Tool

Version

Checker

Description

Astrée20.10

switch-skipped-code

Fully checked
Axivion Bauhaus Suite6.9.0CertC-DCL41Fully implemented
Clang3.9-Wsometimes-uninitialized

Coverity2017.07

MISRA C 2004 Rule 15.0

MISRA C 2012 Rule 16.1

Implemented
LDRA tool suite 9.7.1385 SFully implemented
Parasoft C/C++test
10.4.2
CERT_C-DCL41-a

A switch statement shall only contain switch labels and switch clauses, and no other code

Polyspace Bug Finder

R2020a

CERT C: Rule DCL41-C

Checks for ill-formed switch statements (rule partially covered)

PRQA QA-C
9.7

3234
2008
2882

Partially implemented

PVS-Studio

7.07

V622
RuleChecker

20.10

switch-skipped-code

Fully checked
TrustInSoft Analyzer

1.38

initialisationExhaustively detects undefined behavior (see the compliant and the non-compliant example).

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)

Taxonomy

Taxonomy item

Relationship

MISRA C:2012Rule 16.1 (required)Prior to 2018-01-12: CERT: Unspecified Relationship

Bibliography

[ISO/IEC 9899:2011]6.8.4.2, "The switch Statement"



10 Comments

  1. Should this be cross referenced with avoid dead code?

    1. Offhand, I think this should be incorporated into that rule. MSC07-C. Detect and remove dead code. (I'd still consider this a complete rule for the purposes of this assignment.)

      Also the 'Implmenetation Details' section needs to specify which platform produced the results shown here.

  2. Under Implementation Details, should this sentence

    Similarly, the call to the function will never be executed either.

    say

    Similarly, the call to function f will never be executed either.

    instead?

    1. He is definitely referring to the function f(). I'm not sure about the sentence. Maybe something like "Similarly, the call to f() is not executed.".

  3. This rule seems to be written more broadly than the actual problem. Consider this silly example:

    switch(x) {
    int y;
    case 1:
    y = 10;
    f(&y);
    /* FALLTHROUGH */
    case 2:
    y = 20;
    g(&y);
    }

    That's perfectly fine, though a bit strange-looking. Shouldn't our rule just be to not initialize variables before the first case statement, rather than to not declare them there?

    1. Well, we have a rule EXP33-C. Do not read uninitialized memory.  I've cited EXP33-C in the introduction.

      I'd say this is still a rule because no one has seriously tried to argue that declaring variables before the first case label is useful. Your code example is safe, but also silly, and it is easy to fix. Do you have a non-silly instance of code that would justify demoting this to a recommendation?


      1. Is it easy to fix, though? The "obvious" fix of moving y out of the switch statement would violate DCL19-C. Minimize the scope of variables and functions. (I'll also try to come up with a less silly example, but I don't have one offhand.)

        1. The rules are more important than the recommendations. Violating a recommendation is permitted, especially if the alternative is to violate a rule.

      2. Also, to clarify, my first choice with this rule wouldn't be to demote it to recommendation. I'd rather keep it a rule, but change it to only ban initialization there, and to allow declaration without initialization.

        1. Personally, upon seeing your comment, my impulse was to fold this rule into EXP33-C, because if your code is not safe, then it violates that rule. This rule suggests that safe code, such as your silly example, is still insecure, because it could easy lead to reading uninitialized memory. Again, I would want to see a serious argument (aka non-silly code example) that would justify changing or demoting this rule.