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Code that has no effect or is executed but does not perform any action, or that has an unintended effect, most likely results from a coding 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.)This recommendation is related to MSC07-C. Detect and remove dead code. 

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:

...

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.

...

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

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));
}

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.

Automated Detection

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

Bibliography

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