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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 removed from a program.

Non-Compliant Code Example 1

This example, inspired by Fortify demonstrates how dead code can be introduced into a program. 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 = malloc(10);
        if (s == NULL) {
           /* Handle Error */
        }
        /* Process s */
        return 0;
    }
    /* ... */
    if (s) {
        /* This code is never reached */
    }
    return 0;
}

Compliant Solution 1

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

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

Non-Compliant Code Example 2

In this example, the strlen() function is used to limit the number of times the function string_loop() will iterate. However, the programmer mistakenly subtracts 1 from the result of strlen(). As a result, the last character before the terminating null character will never be processed.

int string_loop(char *str) {
    size_t i;
    for (i=0; i < strlen(str)-1; i++) {
        /* Process str */
    }
    return 0;
}

Compliant Solution 2

In this case, removing the dead code requires us to adjust the terminating condition of the loop so that every element in str is processed. This can be done by not subtracting 1 from the result of strlen().

int string_loop(char *str) {
    size_t i;
    for (i=0; i < strlen(str); i++) {
        /* Process str */
    }
    return 0;
}

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 in to a program and the effort required to remove it can be complex. Given this, resolving dead code can be an in-depth process requiring significant changes.

References

\[[Fortify 06|AA. C References#Fortify 06]\] Code Quality, "Dead Code"