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This noncompliant code example demonstrates how dead code can be introduced into a program \[[Fortify 2006|AA. Bibliography#Fortify 06]\]. 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. |
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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 the situation appropriately. To correct the preceding noncompliant code, the return is removed from the body of the first conditional statement.
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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 string_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.
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int string_loop(char *str) {
size_t i;
size_t len = strlen(str);
for (i=0; i < len; i++) {
/* ... */
if (str[i] == '\0')
/* This code is never reached */
}
return 0;
}
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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.
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int string_loop(char *str) {
size_t i;
size_t len = strlen(str);
for (i=0; i < len; i++) {
/* ... */
if (str[i+1] == '\0')
/* This code is now reached */
}
return 0;
}
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MSC07-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 recommendation
MSC01-C. Strive for logical completeness.) Since valid values of an enumerated type include all those of its underlying integer type, unless enumeration constants have been provided for all those values the
default label is appropriate and necessary.
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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));
}
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