Go The difference between C + + heap and stack

Tag: OS works with AR Data code new C + + as

First, preparatory knowledge-memory allocation of the program

The memory used by a program compiled by C + + is divided into the following sections

1, stack (stack)-Automatically allocated by the compiler release, storing the function's parameter value, local variable value, etc. It operates in a manner similar to a stack in a data structure.

2, heap area (heap)-generally by the programmer to allocate the release, if the programmer does not release, the end of the program may be recycled by the OS. Note that it is not the same as the heap in the data structure, the distribution is similar to the list, hehe.

3, Global (static)-the storage of global variables and static variables is placed in a block, initialized global variables and static variables in an area, uninitialized global variables and uninitialized static variables in another adjacent area. -System release after the program is finished

4, the literal constant area-the constant string is put here. Released by the system after the program is finished

5. Program code area-binary code that holds the function body.

Ii. Examples of procedures

It was written by a predecessor, very detailed

Main.cpp

int a = 0; Global initialization Zone

Char *p1; Global uninitialized Zone

Main ()

{

int b; Stack

Char s[] = "ABC"; Stack

Char *p2; Stack

Char *p3 = "123456"; 123456\0 in the constant area, p3 on the stack.

static int c = 0; global (static) initialization zone

P1 = (char *) malloc (10);

P2 = (char *) malloc (20);

Areas that are allocated 10 and 20 bytes are in the heap area.

strcpy (P1, "123456"); 123456\0 is placed in a constant area, the compiler may optimize it to a place with the "123456" that P3 points to.

}

Ii. theoretical knowledge of heaps and stacks

2.1 How to apply

Stack

Automatically assigned by the system. For example, declare a local variable int b in the function; The system automatically opens up space for B in the stack

Heap

Requires the programmer to apply himself and indicate the size of the malloc function in C

such as P1 = (char *) malloc (10);

Using the new operator in C + +

such as P2 = (char *) malloc (10);

But note that P1, p2 itself is in the stack.

2.2

Response of the system after application

Stack: As long as the remaining space of the stack is larger than the requested space, the system will provide memory for the program, otherwise it will report the exception prompt stack overflow.

Heap: First you should know that the operating system has a list of idle memory addresses, when the system receives the application of the program,

The list is traversed to find the first heap node that is larger than the requested space, and then the node is removed from the list of idle nodes, and the space of that node is allocated to the program, and for most systems the size of this allocation is recorded at the first address in the memory space, so that The DELETE statement in the code can properly free up this memory space. Also, because the size of the found heap node does not necessarily equal the size of the request, the system automatically re-places the extra portion into the idle list.

2.3 Application Size Limits

Stack: Under Windows, the stack is the data structure to the low address extension, which is a contiguous area of memory. This sentence means that the top of the stack of the address and the maximum capacity of the stack is the system pre-defined, in Windows, the size of the stack is 2M (also said 1M, in short, is a compile-time determination of the constant), if the request for more space than the stack's remaining space, will prompt overflow. Therefore, the space available from the stack is small.

Heap: A heap is a data structure that extends to a high address, and is a discontinuous area of memory. This is because the system is stored with a linked list of free memory address, is naturally discontinuous, and the chain of the list of traversal direction is from the low address to high address. The size of the heap is limited by the valid virtual memory in the computer system. Thus, the space of the heap is more flexible and relatively large.

2.4 Comparison of application efficiency:

The stack is automatically assigned by the system and is faster. But programmers can't control it.

Heap is the memory allocated by new, the general speed is relatively slow, and prone to memory fragmentation, but the most convenient to use.

In addition, under Windows, the best way is to use VirtualAlloc to allocate memory, he is not in the heap, nor in the stack is directly in the process's address space to keep a fast memory, although the most inconvenient to use. But the speed is fast, also the most flexible.

2.5 Storage contents in stacks and stacks

Stack: In a function call, the first stack is the address of the next instruction in the main function (the next executable statement of the function call statement), and then the parameters of the function, in most C compilers, the arguments are left-to-right and then the local variables in the function. Note that static variables are not in the stack.

When the function call is finished, the local variable is first out of the stack, then the parameter, and the last stack pointer points to the first saved address, which is the next instruction in the main function, and the program continues to run from that point.

Heap: The size of a heap is typically stored in a heap at the head of a pile. The concrete contents of the heap are arranged by programmers.

2.6 Comparison of access efficiency

Char s1[] = "AAAAAAAAAAAAAAA";

Char *s2 = "BBBBBBBBBBBBBBBBB";

AAAAAAAAAAA is assigned at run time;

And BBBBBBBBBBB is determined at compile time;

However, in subsequent accesses, the array on the stack is faster than the string that the pointer points to (for example, a heap).

Like what:

#include

void Main ()

{

char a = 1;

Char c[] = "1234567890";

Char *p = "1234567890";

A = c[1];

A = p[1];

Return

}

The corresponding assembly code

10:a = c[1];

00401067 8A 4D F1 mov cl,byte ptr [ebp-0fh]

0040106A 4D FC mov byte ptr [ebp-4],cl

11:a = p[1];

0040106D 8B-EC mov edx,dword ptr [ebp-14h]

00401070 8A mov al,byte ptr [edx+1]

00401073 FC mov byte ptr [ebp-4],al

The first reads the elements in the string directly into the register CL, while the second one reads the pointer values into EDX, which is obviously slow to read the characters according to EdX.

2.7 Summary:

The difference between heap and stack can be seen in the following analogy:

Use the stack like we go to a restaurant to eat, just order (send application), pay, and eat (use), eat enough to go, do not bother to cut vegetables, wash vegetables and other preparation work and washing dishes, brush pots and other finishing work, his advantage is fast, but the freedom is small.

The use of the heap is like a DIY dish that you like to eat, more trouble, but more in line with their own tastes, and great freedom.

Go The difference between C + + heap and stack