| C와 C++관련 |
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| 제목 | volatile 1998/07/29 (14:30) |
| 이름 | swindler |
| 번호 | 13 |
| 조회 | 704 |
| 본문 |
volatile은 컴파일 최적화 과정에서도 레지스터에 변수가 저장되지 않도록 한다. 만약에 메모리가 아닌 레지스터에 변수가 로드된다면, 멀티 스레딩과 같은 경우에 문제가 생긴다. assembler코드를 뜯어보면 알 수 있지만, 변수가 레지스터에 로드되고 레지스터가 증가된 다음에는 레지스터의 값이 다시 변수에 저장된다. 한 스레드에서 변수명을 레지스터에 로드시킨다면, 다른 스레드가 메모리에서 값을 변경한다 하더라도 원래 스레드에는 아무런 영향을 미치지 못하게 된다. volatile을 선언하면, 변수가 레지스터에 저장되지 않도록 할 수 있다. volatile형은 현재 수행중인 프로그램의 제어를 벗어난 작업에 의해 예기치 못한 수정으로 인한 문제를 해결해 준다. volatile키워드는 수행중인 프로그램 뿐만 아니라 인터럽트에 의해 액세스 될 수 있는 변수의 임시성에 대해 컴파일러에게 알려주므로써 volatile형 변수가 정확한 값을 사용하게 된다. 밑은 MFC Help 에 있는 원문이다. 1. volatile volatile declarator The volatile keyword is a type qualifier used to declare that an object can be modified in the program by something other than statements, such as the operating system, the hardware, or a concurrently executing thread. The following example declares a volatile integer nVint whose value can be modified by external processes: int volatile nVint; Objects declared as volatile are not used in optimizations because their value can change at any time. The system always reads the current value of a volatile object at the point it is requested, even if the previous instruction asked for a value from the same object. Also, the value of the object is written immediately on assignment. One use of the volatile qualifier is to provide access to memory locations used by asynchronous processes such as interrupt handlers. 2.Type of this Pointer The this pointer's type can be modified in the function declaration by the const and volatile keywords. To declare a function as having the attributes of one or more of these keywords, use the cv-mod-list grammar. Syntax cv-mod-list: cv-qualifier cv-mod-listopt cv-qualifier: const volatile Consider this example: class Point { unsigned X() const; }; The preceding code declares a member function, X, in which the this pointer is treated as a const pointer to a const object. Combinations of cv-mod-list options can be used, but they always modify the object pointed to by this, not the this pointer itself. Therefore, the following declaration declares function X; the this pointer is a const pointer to a const object: class Point { unsigned X() __far const; }; The type of this is described by the following syntax, where cv-qualifier-list can be const or volatile, class-type is the name of the class: cv-qualifier-listopt class-type * const this Table 8.2 explains more about how these modifiers work. Table 8.2 Semantics of this Modifiers < Modifier Meaning > <const> Cannot change member data; cannot invoke member functions that are not const. <volatile> Member data is loaded from memory each time it is accessed; disables certain optimizations. It is an error to pass a const object to a member function that is not const. Similarly, it is an error to pass a volatile object to a member function that is not volatile. Member functions declared as const cannot change member data -in such functions, the this pointer is a pointer to a const object. Note Constructors and destructors cannot be declared as const or volatile. They can, however, be invoked on const or volatile objects. |
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