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Code that is never executed is known as dead code. Typically, the presence of dead code indicates that a logic error has occurred as a result of changes to a program or the program's environment. Dead code is usually optimized out of a program during compilation. However, to improve readability and ensure that logic errors are resolved, dead code should be identified, understood, and eliminated.

This recommendation is related to MSC12-C. Detect and remove code that has no effect or is never executed.

Noncompliant Code Example

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This noncompliant code example demonstrates how dead code can be introduced into a program \[ [Fortify 06|AA. C References#Fortify 06]\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 -null value ends with a return statement.

Code Block
bgColor #FFCCCC lang c
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 never reached */ } return 0; }

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

Code Block
bgColor #ccccff lang c
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; }

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In this example, the strlen() function is used to limit the number of times the function strings_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.

Code Block
bgColor #FFCCCC lang c
int strings_loop(char *strs) { size_t i; size_t len = strlen(strs); for (i=0; i < len; i++) { /* ... */ if (strs[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 strings_loop(char *strs) {
    size_t i;
    size_t len = strlen(strs);
    for (i=0; i < len; i++) {
        /* ... */
	  if (strs[i+1] == '\0') {
	    /* This code is now reached */
      }
    }
    return 0;
}

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 dead code may indicate logic errors that can lead to unintended program behavior. The ways in which dead code can be introduced into a program and the effort required to remove it can be complex. As a result, resolving dead code can be an in-depth process requiring significant analysis.

Automated Detection

The LDRA tool suite Version 7.6.0 can detect violations of this recommendation.

Splint Version 3.1.1 can detect violations of this recommendation.

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Klocwork Version 8.0.4.16 can detect violations of this rule with the LV_UNUSED.DEAD, PA_UNUSED.DEAD, UNREACH.BREAK, UNREACH.GEN, UNREACH.RETURN, and UNREACH.RETURN0 checkers.

Related Vulnerabilities

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

Other Languages

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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

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References

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Bibliography

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Image Added Image Added Image Added PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "BRS Leveraging human experience," "BVQ Unspecified Functionality," and "XYQ Dead and Deactivated Code" \[[MISRA 04|AA. C References#MISRA 04]\] Rule 2.4 \[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 561|http://cwe.mitre.org/data/definitions/561.html], "Dead Code"MSC06-C. Be aware of compiler optimization when dealing with sensitive data      49. Miscellaneous (MSC)       MSC08-C. Library functions should validate their parameters