Go C + + heap, stack, free storage, global/static storage, and constant storage

One the memory used by a program compiled by C + + is divided into the following sections:
1. Stack (stack)-Automatically allocated by the compiler to release, store the function parameter value, local variable value and so on. It operates in a manner similar to a stack in a data structure.
2, heap area (heap)-Generally by the programmer assigned to release (Malloc/free, New/delete), if the programmer does not release, after the end of the program may be recycled by the operating system. Note that it is different from the heap stones in the data structure, and the distribution is similar to the linked list. Where there is malloc/free distribution can also be called free storage area.
3. Global zone (Static storage)-Global variables and static variables are stored in a block, initialized global variables and static variables in an area, uninitialized global variables and uninitialized static variables in another area adjacent to. -The system is released after the program finishes (in C + + they initialize and uninitialized a chunk of memory).
4, 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 in the constant area, p3 on the stack.      static int c =0;     Global (static) initialization zone      p1 = (char *) malloc (n);      P2 = (char *) malloc (20);//the area allocated with 10 and 20 bytes is in the heap area.      strcpy (P1, "123456");   123456 in the constant area, the compiler might optimize it to a place with the "123456" that P3 points to.   }  

　　

Third, heap and stack of theoretical knowledge

The difference between heap and stack is mainly a few points:

2.1 Different management methods
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 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 Fragmentation Issues
For the heap, frequent new/delete is bound to cause the memory space discontinuity, thus produces the massive fragment, causes the program efficiency to reduce.
For the stack, there is no problem, because the stack is a last-in, first-out queue, they are so one by one correspondence, so that there is never a memory block from the middle of the stack pop, before it pops up, the contents of the upper stack above it has been ejected.

2.4 direction of growth
For the heap, the direction of growth is upward, that is to think of the direction of memory address increase.
For the stack, it grows in a downward direction, increasing in the direction of memory address reduction.

2.5 allocation Method
The heap is dynamically allocated and there is no statically allocated heap.
Stacks are allocated in two ways: static allocation and dynamic allocation. Static allocations are done by the compiler, such as the allocation of local variables. The dynamic allocation is assigned by the function Alloca function, but the dynamic allocation of the stack is different from that of the heap, and its dynamic allocation is released by the compiler without the need for us to manually be wanted.

2.6 Allocation Efficiency
The stack is the data structure provided by the machine system, the computer will provide support on the underlying stack: allocate the address of the special register storage stack, the stack stack has a special instruction execution, which determines the efficiency of the stack is high.
The heap is provided by the C/s + + function library, and its mechanism is complex, for example, in order to allocate a piece of memory, the library function searches the heap memory for a sufficient amount of space available, if there is not enough space (possibly due to too many fragments), according to a certain algorithm. It is possible to invoke the system function to increase the memory space of the program data segment, so that there is a chance to divide the memory in sufficient size and then return. Obviously, the heap is much less efficient than the stack.

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.

1, memory allocation aspects:
Heap: Typically released by programmers, if the programmer does not release, the program may end up being recycled by the OS. Note that it is different from the heap in the data structure, and the distribution is similar to a linked list. The following keywords may be used: New, malloc, delete, free, and so on.
Stack: The compiler (Compiler) automatically allocates the release, storing the function's parameter value, the value of the local variable, and so on. It operates in a manner similar to a stack in a data structure.

2. Application Method:
Heap: Requires programmers to apply themselves and indicate size. In C, the malloc function such as P1 = (char *) malloc (10); Use the new operator in C + +, but note that P1, p2 itself is in the stack. Because they can still be considered as local variables.
Stack: Automatically assigned by the system. For example, declare a local variable int b in the function, and the system automatically opens up space for B in the stack.

3. System response:
Heap: The operating system has a linked list that records the free memory address, when the system receives the application, it iterates through the list, finds the first heap node that is larger than the requested space, and then removes the node from the list of idle nodes and assigns the node's space to the program, in addition, for most systems, 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. In addition, because the size of the found heap node does not necessarily equal the size of the application, the system will automatically re-put the extra portion into the idle list.
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.

4. Size limitation:
Heap: Is the data structure that extends to the high address, which is a discontinuous memory area. 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.
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 address and the maximum capacity of the stack is the system pre-defined, under Windows, the size of the stack is fixed (is a compile-time determination of the constant), if the requested space exceeds the stack's remaining space, will prompt overflow. Therefore, the space available from the stack is small.

5, Efficiency aspects:
Heap: Is the memory allocated by new, generally slow, and easy to produce memory fragmentation, but the most convenient to use, in addition, in Windows, the best way is to use VirtualAlloc to allocate memory, he is not in the heap, nor is the stack is directly in the process of the address space to retain a fast memory, Although it is most inconvenient to use. But the speed is fast, also the most flexible.
Stack: Automatically allocated by the system, faster. But programmers can't control it.

6, storage content aspects:
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.
Stack: The first stack in a function call 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 local variables in the function. Note: 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.

7, Access efficiency aspects:
Heap: Char *s1 = "Hellow Word"; it is determined at compile time;
Stack: char s1[] = "Hellow Word"; it is assigned at run time; it is faster to use an array than a pointer, because the pointer needs to be brokered in the underlying assembly with the EDX register, and the array is read directly on the stack.

Go C + + heap, stack, free storage, global/static storage, and constant storage