[Go] Preliminary knowledge-memory allocation of the program

Because of the classic, so forward.

Preliminary knowledge-memory allocation of a program a program that is compiled by C + + has a memory that is divided into the following sections

1. Stack-the compiler automatically allocates releases, stores the function's parameter values, the values of local variables, and so on. It operates in a manner similar to a stack in a data structure.
2. Heap area-typically released by the programmer, if the programmer does not release, the program may end up with the OS back
   Inbox Note that it is not the same as the heap in the data structure, but the distribution is similar to the linked list.
3. Global Zone (static zone)--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 area adjacent. -released by the system after the program is finished.
4. Literal constant area-the constant string is put here. Released by the system after the program is finished.
5. Program code area-the binary code that holds the body of the function.

Ii. Examples of procedures

It was written by a predecessor, very detailed

//main.cpp int a = 0;                           //Global initialization Zone char *p1; //Global uninitialized area 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); //allocated 10 and 20 bytes of the area in the heap area. strcpy (P1,  "123456"); //123456/0 is placed in a constant area, the compiler may optimize it with the "123456" pointed to by P3 as a place. } 

Ii. theoretical knowledge of heaps and stacks 2.1 application method

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 = new CHAR[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 of all should know that the operating system has a record of the free memory address of the list, when the system receives the application of the program, it will traverse the list, look for the first space is larger than the requested space of the heap node, and then delete the node from the list of idle nodes, and the node's space allocated to the program, in addition, The size of this allocation is recorded at the first address in this memory space, so that the DELETE statement in the code can properly free up the 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 retain a piece of 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 the programmer.

2.6 Comparison of access efficiency

  Char   s1[]   =   "aaaaaaaaaaaaaaa";     char   *s2   =   "bbbbbbbbbbbbbbbbb";     

AAAAAAAAAAA is assigned at run time, while 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   =      

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, and the second one reads the pointer value to
In edx, it is obviously slower 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. Classic )

Category: Stacks

[Go] Preliminary knowledge-memory allocation of the program