In C programming, an object's storage duration determines its lifetime. There are four types of storage durations: static, thread, automatic, and allocated.
Lifetime of an Object
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 (or just past) reaches the end of its lifetime.
Static Storage Duration
An object whose identifier is declared without the storage-class specifier `_Thread_local`, and either with external or internal linkage or with the storage-class specifier `static`, has static storage duration. Its lifetime is the entire execution of the program and its stored value is initialized only once, prior to program startup.
Thread Storage Duration
An object whose identifier is declared with the storage-class specifier `_Thread_local` has thread storage duration. Its lifetime is the entire execution of the thread for which it is created, and its stored value is initialized when the thread is started. There is a distinct object per thread, and use of the declared name in an expression refers to the object associated with the thread evaluating the expression. The result of attempting to indirectly access an object with thread storage duration from a thread other than the one with which the object is associated is implementation-defined.
Automatic Storage Duration
An object whose identifier is declared with no linkage and without the storage-class specifier `static` has automatic storage duration, as do some compound literals. The result of attempting to indirectly access an object with automatic storage duration from a thread other than the one with which the object is associated is implementation-defined.
For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way. If the block is entered recursively, a new instance of the object is created each time. The initial value of the object is indeterminate. If an initialization is specified for the object, it is performed each time the declaration or compound literal is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached.
For such an object that does have a variable length array type, its lifetime extends from the declaration of the object until execution of the program leaves the scope of the declaration. If the scope is entered recursively, a new instance of the object is created each time. The initial value of the object is indeterminate.
Temporary Lifetime
A non-lvalue expression with structure or union type, where the structure or union contains a member with array type (including, recursively, members of all contained structures and unions) refers to an object with automatic storage duration and temporary lifetime. Its lifetime begins when the expression is evaluated and its initial value is the value of the expression. Its lifetime ends when the evaluation of the containing full expression ends. Any attempt to modify an object with temporary lifetime results in undefined behavior. An object with temporary lifetime behaves as if it were declared with the type of its value for the purposes of effective type. Such an object need not have a unique address.
