SEI CERT C Coding Standard

Declarations and Initialization (DCL) - DCL30-C. Declare objects with appropriate storage durations
3 Declarations and Initialization (DCL)
3.1 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.)
3.1.1 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
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;
}
SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems 32
Software Engineering Institute | Carnegie Mellon University
[DISTRIBUTION STATEMENT A] Approved for public release and unlimited distribution.