distinction between heap and stack

前不久在一个博客看到一篇文章谈到 stack（栈）与heap（堆）的区别问题，stack我是知道的（废话，学过数据结构都懂）。heap想想确实也太不清楚，于是一顿google。发现这条链接的解释还不错http://www.maxi-pedia.com/what+is+heap+and+stack

What is heap and stack?(原文标题)

　　The stack is a place in the computer memory where all the variables that are declared and initialized before runtime are stored. The heap is the section of computer memory where all the variables created or initialized at runtime are stored.

What are the memory segments?

The distinction between stack and heap relates to programming. When you look at your computer memory, it is organized into three segments:

• text (code) segment
• stack segment
• heap segment

　　The text segment (often called code segment) is where the compiled code of the program itself resides. When you open some EXE file in Notepad, you can see that it includes a lot of "Gibberish" language, something that is not readable to human. It is the machine code, the computer representation of the program instructions. This includes all user defined as well as system functions.

Now let's get to some details.

What is stack?

　　The two sections other from the code segment in the memory are used for data. The stack is the section of memory that is allocated for automatic variables within functions.

　　Data is stored in stack using the Last In First Out (LIFO) method. This means that storage in the memory is allocated and deallocated at only one end of the memory called the top of the stack. Stack is a section of memory and its associated registers that is used for temporary storage of information in which the most recently stored item is the first to be retrieved.

What is the heap?

　　On the other hand, heap is an area of memory used for dynamic memory allocation. Blocks of memory are allocated and freed in this case in an arbitrary order. The pattern of allocation and size of blocks is not known until run time. Heap is usually being used by a program for many different purposes.The stack is much faster than the heap but also smaller and more expensive.

Heap and stack from programming perspective

Most object-oriented languages have some defined structure, and some come with so-called main() function. When a program begins running, the system calls the function main() which marks the entry point of the program. For example every C, C++, or C# program must have one function named main(). No other function in the program can be called main(). Before we start explaining, let's take a look at the following example:

int x;                           /* static stack storage */
void main() {   
int y;                           /* dynamic stack storage */   
char str;                      /* dynamic stack storage */   
str = malloc(50);          /* allocates 50 bytes of dynamic heap storage */   
size = calcSize(10);       /* dynamic heap storage */

　　When a program begins executing in the main() function, all variables declared within main() will be stored on the stack.

　　If the main() function calls another function in the program, for example calcSize(), additional storage will be allocated for the variables in calcSize(). This storage will be allocated in the heap memory segment.

　　Notice that the parameters passed by main() to calcSize() are also stored on the stack. If the calcSize() function calls to any additional functions, more space would be allocated at the heap again.

　　When the calcSize() function returns the value, the space for its local variables at heap is then deallocated and heap clears to be available for other functions.

　　The memory allocated in the heap area is used and reused during program execution.

　　It should be noted that memory allocated in heap will contain garbage values left over from previous usage.

　　Memory space for objects is always allocated in heap. Objects are placed on the heap.

　　Built-in datatypes like int, double, float and parameters to methods are allocated on the stack.

　　Even though objects are held on heap, references to them are also variables and they are placed on stack.

　　The stack segment provides more stable storage of data for a program. The memory allocated in the stack remains in existence for the duration of a program. This is good for global and static variables. Therefore, global variables and static variables are allocated on the stack.

Why is stack and heap important?

　　When a program is loaded into memory, it takes some memory management to organize the process. If memory management was not present in your computer memory, programs would clash with each other leaving the computer non-functional.

Heap and stack in Java

　　When you create an object using the new operator, for example myobj = new Object();, it allocates memory for the myobj object on the heap. The stack memory space is used when you declare automatic variables.

　　Note, when you do a string initialization, for example String myString;, it is a reference to an object so it will be created using new and hence it will be placed on the heap.

 总结一下：

预备知识：Memory segment

计算机内存分为三个segments：

（1）text/code segment-存放编译后的代码；

（2）heap segment- 一般由程序员分配释放， 若程序员不释放，程序结束时可能由OS回收 。注意它与数据结构中的堆是两回事，分配方式倒是类似于链表；

（3）stack segment-由编译器自动分配释放 ，存放函数的参数值，局部变量的值等。其操作方式类似于数据结构中的栈。

分配用途：

（1）Stack: allocated for automatic variables within functions

（2）Heap: used for dynamic memory allocation

存取方式比较：

（1）Stack: 后进先出（LIFO）

（2）Heap: 随意分配和释放

效率和空间比较：

The stack is much faster than the heap but also smaller and more expensive

一般对应的分配：

heap:

（1）main()中调用的其他函数，其他函数在调用的其他函数，都是heap

（2）objects也是heap（所以CString也是heap型）

Stack:

（1）内置数据类型（Built-in datatypes）如int，float等都是stack

（2）reference to objects是stack

（3）global variables and static variables are allocated on the stack

 

其他的一些总结：

http://zhidao.baidu.com/question/167720536.html

2.2 申请后系统的响应

栈：只要栈的剩余空间大于所申请空间，系统将为程序提供内存，否则将报异常提示栈溢出。

堆：首先应该知道操作系统有一个记录空闲内存地址的链表，当系统收到程序的申请时，

会遍历该链表，寻找第一个空间大于所申请空间的堆结点，然后将该结点从空闲结点链表中删除，并将该结点的空间分配给程序，另外，对于大多数系统，会在这块内存空间中的首地址处记录本次分配的大小，这样，代码中的delete语句才能正确的释放本内存空间。另外，由于找到的堆结点的大小不一定正好等于申请的大小，系统会自动的将多余的那部分重新放入空闲链表中。

2.3申请大小的限制

栈：在Windows下,栈是向低地址扩展的数据结构，是一块连续的内存的区域。这句话的意思是栈顶的地址和栈的最大容量是系统预先规定好的，在WINDOWS下，栈的大小是2M（也有的说是1M，总之是一个编译时就确定的常数），如果申请的空间超过栈的剩余空间时，将提示overflow。因此，能从栈获得的空间较小。

堆：堆是向高地址扩展的数据结构，是不连续的内存区域。这是由于系统是用链表来存储的空闲内存地址的，自然是不连续的，而链表的遍历方向是由低地址向高地址。堆的大小受限于计算机系统中有效的虚拟内存。由此可见，堆获得的空间比较灵活，也比较大。

2.4申请效率的比较：

栈由系统自动分配，速度较快。但程序员是无法控制的。

堆是由new分配的内存，一般速度比较慢，而且容易产生内存碎片,不过用起来最方便.

另外，在WINDOWS下，最好的方式是用VirtualAlloc分配内存，他不是在堆，也不是在栈是直接在进程的地址空间中保留一快内存，虽然用起来最不方便。但是速度快，也最灵活。

2.5堆和栈中的存储内容

栈： 在函数调用时，第一个进栈的是主函数中后的下一条指令（函数调用语句的下一条可执行语句）的地址，然后是函数的各个参数，在大多数的C编译器中，参数是由右往左入栈的，然后是函数中的局部变量。注意静态变量是不入栈的。

当本次函数调用结束后，局部变量先出栈，然后是参数，最后栈顶指针指向最开始存的地址，也就是主函数中的下一条指令，程序由该点继续运行。

堆：一般是在堆的头部用一个字节存放堆的大小。堆中的具体内容有程序员安排。