DCL30-C. Declare objects with appropriate storage durations

Every object has a storage duration that determines its lifetime: static, thread, automatic, or allocated.

According to the C Standard, 6.2.4, paragraph 2 [ISO/IEC 9899:2011],

The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime.

Do not attempt to access an object outside of its lifetime. Attempting to do so is undefined behavior and can lead to an exploitable vulnerability (see also undefined behavior 9 in the C Standard, Annex J).

Noncompliant Code Example (Differing Storage Durations)

In this noncompliant code example, the address of the variable c_str with automatic storage duration is assigned to the variable p, which has static storage duration. The assignment itself is valid, but it is invalid for c_str to go out of scope while p holds its address, as happens at the end of dont_do_this().

#include <stdio.h>
 
const char *p;
void dont_do_this(void) {
  const char c_str[] = "This will change";
  p = c_str; /* Dangerous */
}

void innocuous(void) {
  printf("%s\n", p);
}

int main(void) {
  dont_do_this();
  innocuous();
  return 0;
}

Compliant Solution (Same Storage Durations)

In this compliant solution, p is declared with the same storage duration as c_str, preventing p from taking on an indeterminate value outside of this_is_OK():

void this_is_OK(void) {
  const char c_str[] = "Everything OK";
  const char *p = c_str;
  /* ... */
}
/* p is inaccessible outside the scope of string c_str */

Alternatively, both p and c_str could be declared with static storage duration.

Compliant Solution (Differing Storage Durations)

If it is necessary for p to be defined with static storage duration but c_str with a more limited duration, then p can be set to NULL before c_str is destroyed. This practice prevents p from taking on an indeterminate value, although any references to p must check for NULL.

const char *p;
void is_this_OK(void) {
  const char c_str[] = "Everything OK?";
  p = c_str;
  /* ... */
  p = NULL;
}

Noncompliant Code Example (Return Values)

In this noncompliant code sample, the function init_array() returns a pointer to a character array with automatic storage duration, which is accessible to the caller:

char *init_array(void) {
  char array[10];
  /* Initialize array */
  return array;
}

Some compilers generate a diagnostic message when a pointer to an object with automatic storage duration is returned from a function, as in this example. Programmers should compile code at high warning levels and resolve any diagnostic messages (see MSC00-C. Compile cleanly at high warning levels).

Compliant Solution (Return Values)

The solution, in this case, depends on the intent of the programmer. If the intent is to modify the value of array and have that modification persist outside the scope of init_array(), the desired behavior can be achieved by declaring array elsewhere and passing it as an argument to init_array():

#include <stddef.h>
void init_array(char *array, size_t len) {
  /* Initialize array */
  return;
}

int main(void) {
  char array[10];
  init_array(array, sizeof(array) / sizeof(array[0]));
  /* ... */
  return 0;
}

Noncompliant Code Example (Output Parameter)

In this noncompliant code example, the function squirrel_away() stores a pointer to local variable local into a location pointed to by function parameter ptr_param. Upon the return of squirrel_away(), the pointer ptr_param points to a variable that has an expired lifetime.

void squirrel_away(char **ptr_param) {
  char local[10];
  /* Initialize array */
  *ptr_param = local;
}

void rodent(void) {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is live but invalid here */
}

Compliant Solution (Output Parameter)

In this compliant solution, the variable local has static storage duration; consequently, ptr can be used to reference the local array within the rodent() function:

char local[10];
 
void squirrel_away(char **ptr_param) {
  /* Initialize array */
  *ptr_param = local;
}

void rodent(void) {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is valid in this scope */
}

Risk Assessment

Referencing an object outside of its lifetime can result in an attacker being able to execute arbitrary code.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography