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Code that has no effect or is never executed (that is, dead or unreachable code) is typically the result of a coding error and can cause unexpected behavior. Such code is usually optimized out of a program during compilation. However, to improve readability and ensure that logic errors are resolved, it should be identified, understood, and eliminated.

Statements or expressions that have no effect should be identified and removed from code. Most modern compilers, in many cases, can warn about code that has no effect or is never executed. (See MSC00-C. Compile cleanly at high warning levels.) 

Noncompliant Code Example

This noncompliant code example demonstrates how dead code can be introduced into a program [Fortify 2006]. The second conditional statement, if (s), will never evaluate true because it requires that s not be assigned NULL, and the only path where s can be assigned a non-null value ends with a return statement.

int func(int condition) {
    char *s = NULL;
    if (condition) {
        s = (char *)malloc(10);
        if (s == NULL) {
           /* Handle Error */
        }
        /* Process s */
        return 0;
    }
    /* ... */
    if (s) {
        /* This code is unreachable */
    }
    return 0;
}

Compliant Solution

Remediation of dead code requires the programmer to determine why the code is never executed and then to resolve the situation appropriately. To correct the preceding noncompliant code, the return is removed from the body of the first conditional statement.

int func(int condition) {
    char *s = NULL;
    if (condition) {
        s = (char *)malloc(10);
        if (s == NULL) {
           /* Handle error */
        }
        /* Process s */
    }
    /* ... */
    if (s) {
        /* This code is now reachable */
    }
    return 0;
}

Noncompliant Code Example

In this example, the strlen() function is used to limit the number of times the function s_loop() will iterate. The conditional statement inside the loop evaluates to true when the current character in the string is the null terminator. However, because strlen() returns the number of characters that precede the null terminator, the conditional statement never evaluates true.

int s_loop(char *s) {
    size_t i;
    size_t len = strlen(s);
    for (i=0; i < len; i++) {
        /* ... */
	  if (s[i] == '\0') {
	    /* This code is never reached */
      }
    }
    return 0;
}

Compliant Solution

Removing the dead code depends on the intent of the programmer. Assuming the intent is to flag and process the last character before the null terminator, the conditional is adjusted to correctly determine if the i refers to the index of the last character before the null terminator.

int s_loop(char *s) {
    size_t i;
    size_t len = strlen(s);
    for (i=0; i < len; i++) {
        /* ... */
	  if (s[i+1] == '\0') {
	    /* This code is now reached */
      }
    }
    return 0;
}

Noncompliant Code Example (Assignment)

In this noncompliant code example, the comparison of a to b has no effect:

int a;
int b;
/* ... */
a == b;

This code is likely a case of the programmer mistakenly using the equals operator == instead of the assignment operator =.

Compliant Solution (Assignment)

The assignment of b to a is now properly performed:

int a;
int b;
/* ... */
a = b;

Noncompliant Code Example (Dereference)

In this example, a pointer increment and then a dereference occur, but the dereference has no effect:

int *p;
/* ... */
*p++;

Compliant Solution (Dereference)

Correcting this example depends on the intent of the programmer. For example, if dereferencing p was a mistake, then p should not be dereferenced.

int *p;
/* ... */
++p;

If the intent was to increment the value referred to by p, then parentheses can be used to ensure p is dereferenced and then incremented. (See EXP00-C. Use parentheses for precedence of operation.)

int *p;
/* ... */
(*p)++;


Another possibility is that p is being used to reference a memory-mapped device. In this case, the variable p should be declared as volatile.

volatile int *p;
/* ... */
(void) *(p++);

Noncompliant Code Example (if/else if)

A chain of if/else if statements is evaluated from top to bottom. At most, only one branch of the chain will be executed: the first one with a condition that evaluates to true. Consequently, duplicating a condition in a sequence of if/else if statements automatically leads to dead code.

if (param == 1)
   openWindow();
 else if (param == 2)
   closeWindow();
 else if (param == 1) /* Duplicated condition */
   moveWindowToTheBackground();

Compliant Solution (if/else if)

In this compliant solution, the third conditional expression has been corrected.

if (param == 1)
   openWindow();
 else if (param == 2)
   closeWindow();
 else if (param == 3)
   moveWindowToTheBackground();

Noncompliant Code Example (logical operators)

Using the same subexpression on either side of a logical operator is almost always a mistake.  In this noncompliant code example, the rightmost subexpression of the controlling expression of each if statement has no effect.  

if (a == b && a == b) { // if the first one is true, the second one is too
  do_x();
}
if (a == c || a == c ) { // if the first one is true, the second one is too
  do_w();
}

Compliant Solution (logical operators)

In this compliant solution, the rightmost subexpression of the controlling expression of each if statement has been removed.

if (a == b) { 
  do_x();
}
if (a == c) { 
  do_w();
}

Noncompliant Code Example (Unconditional Jump)

Unconditional jump statements typically has no effect.  

#include <stdio.h>
 
for (int i = 0; i < 10; ++i) {
  printf("i is %d", i);
  continue;  // this is meaningless; the loop would continue anyway
}

Compliant Solution (Unconditional Jump)

The continue statement has been removed from this compliant solution.

#include <stdio.h>
 
for (int i = 0; i < 10; ++i) {
   printf("i is %d", i); 
}

Exceptions

MSC12-EX1: In some situations, seemingly dead code may make software resilient. An example is the default label in a switch statement whose controlling expression has an enumerated type and that specifies labels for all enumerations of the type. (See MSC01-C. Strive for logical completeness.) Because valid values of an enumerated type include all those of its underlying integer type, unless enumeration constants are provided for all those values, the default label is appropriate and necessary.

typedef enum { Red, Green, Blue } Color;
const char* f(Color c) {
  switch (c) {
    case Red: return "Red";
    case Green: return "Green";
    case Blue: return "Blue";
    default: return "Unknown color";   /* Not dead code */
  }
}

void g() {
  Color unknown = (Color)123;
  puts(f(unknown));
}

MSC12-EX2: It is permissible to temporarily remove code that may be needed later. (See MSC04-C. Use comments consistently and in a readable fashion for an illustration.)

MSC12-EX3: Unused functions and variables that are part of an exported library do not violate this guideline.  Likewise, code that is never executed because it is #ifdefed out does not violate this guideline, on the grounds that it could be subsequently used in another application, or built on a different platform.

Risk Assessment

The presence of code that has no effect or is never executed can indicate logic errors that may result in unexpected behavior and vulnerabilities. Such code can be introduced into programs in a variety of ways and eliminating it can require significant analysis.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

MSC12-C

Low

Unlikely

Medium

P2

L3

Automated Detection

Tool

Version

Checker

Description

Astrée
17.04i

unreachable-code

statement-sideeffect

Partially checked
CodeSonar
5.0p0

DIAG.UNEX.*
LANG.STRUCT.EBS
LANG.STRUCT.RC
MISC.NOEFFECT
LANG.STRUCT.UC
LANG.STRUCT.UA
LANG.STRUCT.UULABEL

LANG.STRUCT.UUMACRO
LANG.STRUCT.UUPARAM
LANG.STRUCT.UUTAG
LANG.STRUCT.UUTYPE
LANG.STRUCT.UUVAR

Code not exercised by analysis
Empty branch statement checks
Redundant condition
Function call has no effect
Unreachable code checks
Useless assignment
Unused Label
Unused Macro
Unused Parameter
Unused Tag
Unused Type
Unused Variable

Coverity

2017.07

NO_EFFECT


DEADCODE

 

UNREACHABLE

Finds statements or expressions that do not accomplish anything or statements that perform an unintended action.

Can detect the specific instance where code can never be reached because of a logical contradiction or a dead "default" in switch statement

Can detect the instances where code block is unreachable because of the syntactic structure of the code

ECLAIR

1.2

CC2.MSC12

Partially implemented

GCC

3.0

-Wunused-value
-Wunused-parameter

Options detect unused local variables, nonconstant static variables and unused function parameters, or unreachable code respectively.

Klocwork
2018

CWARN.NOEFFECT.SELF_ASSIGN
CWARN.NOEFFECT.UCMP.GE
CWARN.NOEFFECT.UCMP.GE.MACRO
CWARN.NOEFFECT.UCMP.LT
CWARN.NOEFFECT.UCMP.LT.MACRO
CWARN.NULLCHECK.FUNCNAME
EFFECT
INVARIANT_CONDITION.UNREACH
LA_UNUSED
MISRA.STMT.NO_EFFECT
UNREACH.GEN
UNREACH.RETURN
UNREACH.SIZEOF
VA_UNUSED.GEN
VA_UNUSED.INIT


LDRA tool suite
9.7.1


8 D, 65 D, 105 D, I J, 139 S, 140 S, 57 S

Partially implemented

Parasoft C/C++test
10.4

CERT_C-MSC12-a
CERT_C-MSC12-b
CERT_C-MSC12-c
CERT_C-MSC12-d
CERT_C-MSC12-e
CERT_C-MSC12-f
CERT_C-MSC12-g
CERT_C-MSC12-h

There shall be no unreachable code in "else" block
There shall be no unreachable code after 'return', 'break', 'continue', and 'goto' statements
There shall be no unreachable code in "if/else/while/for" block
There shall be no unreachable code in switch statement
There shall be no unreachable code in 'for' loop
There shall be no unreachable code after 'if' or 'switch' statement
There shall be no unreachable code after "if" or "switch" statement inside while/for/do...while loop
Avoid switch with unreachable branches

Polyspace Bug Finder

R2018a

Dead code

Unreachable code

Use of memset with size argument zero

MISRA C:2012 Rule 2.1

MISRA C:2012 Rule 2.2

Code does not execute

Default case is missing and may be reached

Size argument of function in memset family is zero

A project shall not contain unreachable code

There shall be no dead code

PRQA QA-C
9.3

3426, 3427, 3307, 3110, 3112, 3404, 1501, 1503, 2008, 2880, 2881, 2882, 2883, 2877, 3196, 3202, 3203, 3205, 3206, 3207, 3210, 3219, 3229, 3404, 3422, 3423, 3425, 3470, 2980, 2981, 2982, 2983, 2984, 2985, 2986

Partially implemented
RuleChecker
17.04i

statement-sideeffect

Partially checked
SonarQube C/C++ Plugin
 3.11
S1764, S2589, S2583, S1116, S1172, S1763, S1862, S1065, S1656, S2754, S1751
Splint
3.1.1

 -standard

The default mode checks for unreachable code.

PVS-Studio

6.23

V551, V606, V649, V779

Related Vulnerabilities

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

CVE-2014-1266 results from a violation of this rule. There is a spurious goto fail statement on line 631 of sslKeyExchange.c. This goto statement gets executed unconditionally, even though it is indented as if it were part of the preceding if statement. As a result, the call to sslRawVerify() (which would perform the actual signature verification) becomes dead code [ImperialViolet 2014].

Related Guidelines

SEI CERT C++ Coding StandardVOID MSC12-CPP. Detect and remove code that has no effect
ISO/IEC TR 24772Unspecified Functionality [BVQ]
Likely Incorrect Expressions [KOA]
Dead and Deactivated Code [XYQ]
MISRA C:2012Rule 2.2 (required)

Bibliography


[Fortify 2006]Code Quality, "Dead Code"
[Coverity 2007]





8 Comments

  1. > *p++;

    This is not an example of dead code, or code with no effect, but *wrong code* - there is no chance that the developer has done this on purpose (or s/he would have done)

    (void)*p++; // trust me, there's a good reason for this
    

    (and I am hard-pressed to think of an actual good reason). 

    So suggesting that this avoids a troublesome technique - which is by nature intentional - misses the fact that it's just wrong in every way.

    One can't code secure if one doesn't actually know how C works. 

    1. If there is an actual reason, it would involve volatile qualification; I think I've seen some device driver code that did this.  (Reading the data reset the interrupt, and the following code wanted to use the next sequential register address.)  Certainly it is too "tricky" and should be rewritten for clarity, or else fully explained in a comment.

  2. Is the statement "Most modern compilers can warn about code that has no effect" too wide? How many would detect the following?

    int has_dead_code ( int para )
    {
       int local = 99;
    
       para  = para + local;
    
       local = para;         // Has no effect on program output
    
       if ( 0 == local )     // Has no effect on program output
       {
          ++local;           // Has no effect on program output
       }
    
       return para;
    }
    




    1. I watered down the statement. It is usually not possible to be too wishy washy (in most cases). 8^)

  3. In "Noncompliant Code Example (Dereference)", the description and the code do not match.  The way the code is written "*p++;", p is dereferenced and then incremented.  In order for p to be incremented and then dereferenced, as the description says, the code would need to say "*++p;".

    1. Sigh...if you know what the code does, the description does match it. But if you don't, it's easy to get the wrong impression from the description. This is a case where the code can not be described by simple English. The C binding rules parse p+ as (p+), which means the increment occurs before the dereference as stated. There is a trade-off between being accurate and being short...hopefully my rewrite succeeds on both counts.

      1. Ok.  I see what you mean.  ANSI X3.159-1989 says that the postincrement operator is applied first to *p++, but the p++ sub-expression's value is the original value of p, so after p is incremented the previous value of p is what is dereferenced. In most cases *p++ looks as if p is dereferenced first, then incremented.

        (When I use the Rich Text tab, some of the backslashes before star and plus are deleted, even after I explicitly added them in Wiki Markup, so some of the pluses are interpreted as text formatting.)

  4. In MISRA C:2012 parlance this appears to be "dead code".