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

The memory used by a program compiled by C + + is divided into the following sections
1, stack area (stack)-by the compiler automatically assigned to release, store the function of the parameter values, local variables and other values. The operation is similar to the stack in the data structure.
2, heap area (heap)-Generally by the programmer assigned to release, if the programmer does not release, the program at the end may be reclaimed by the OS. Note that it is different from the heap in the data structure, the distribution is similar to the list, hehe.
3, Global area (static)--------------------------------------------------------------- -System release after the program is finished
4, literal constant area-the constant string is here. Released by system after program is finished
5, program code area-the binary code that holds the function body.

Second, the example procedure
This is written by a predecessor, very detailed
Main.cpp
int a = 0; Global initialization Area
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 area
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 is placed in the constant area, the compiler may optimize it to a place with the "123456" that P3 points to.
}

Theory knowledge of heap and stack
2.1 Application Methods
Stack
Automatically assigned by the system. For example, declare a local variable int b in a function; The system automatically opens up space for B in the stack
Heap
Requires programmers to apply themselves, and indicate size, in C malloc function
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 System after application
Stack: As long as the remaining space of the stack is larger than the application space, the system will provide memory for the program, otherwise it will be reported abnormal stack overflow.
Heap: First you should know that the operating system has a record of the free memory address of the list, when the system received the application of the program,
Will traverse the list to find the first heap node that is larger than the requested space, the node is then removed from the list of free nodes, and the space of the node is allocated to the program, and for most systems, the size of this assignment is recorded at the first address in the memory space, so that The DELETE statement in your code can properly free this memory space. In addition, because the size of the found heap node does not necessarily equal the size of the application, the system automatically puts the extra part back into the free list.

2.3 Limit of application size
Stacks: In Windows, stacks are data structures that extend to a low address and are a contiguous area of memory. The address of the top of the stack and the maximum capacity of the stack are predetermined by the system, in Windows, the size of the stack is 2M (also some say 1M, in short, a compile-time constant), if the application space over the stack of remaining space, will prompt overflow. Therefore, the space can be obtained from the stack is small.
Heap: The heap is a data structure that is extended to a high address and is a contiguous area of memory. This is because the system is used to store the free memory address of the list, nature is discontinuous, and the link list of the traversal direction is from the low address to the high address. The size of the heap is limited by the virtual memory available in the computer system. This shows that the heap to obtain a more flexible space, but also relatively large.

2.4 Comparison of the efficiency of the application:
The stack is automatically allocated by the system, faster. But programmers are out of control.
A heap is a memory that is allocated by new, typically slower, and prone to memory fragmentation, but is most convenient to use.
In addition, in Windows, the best way is to allocate memory with VirtualAlloc, he is not in the heap, nor in the stack is directly in the process of the address space to keep a fast memory, although the most inconvenient to use. But fast, and most flexible.

2.5 stacks and stacks of stored content
Stacks: When a function is called, the first stack is the address of the next instruction in the main function (the next executable statement of the function call statement). Then there are the parameters of the function, in most C compilers, the parameters are pushed from right to left, 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 first goes out of the stack, then the argument, and the last stack pointer points to the address that was first saved, the next instruction in the main function, where the program continues to run.
Heap: The size of the heap is usually stored in a byte at the head of the heap. The specifics of the heap are arranged by the programmer.

2.6 Comparison of access efficiency

Char s1[] = "AAAAAAAAAAAAAAA";
Char *s2 = "BBBBBBBBBBBBBBBBB";
The AAAAAAAAAAA is assigned at run time;
And the BBBBBBBBBBB is determined at compile time;
However, in future 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 reads the pointer value into the edx and reads the characters according to EdX, which is obviously slow.

2.7 Summary:
The difference between stacks and stacks can be seen in the following analogy:
Use stacks just like we eat in restaurants, just order (apply), pay, and eat (use), go after full, do not have to pay attention to cutting vegetables, vegetables, such as preparation and washing dishes, such as cleaning the pot, his advantage is fast, but the degree of freedom is small.
The use of the heap is like a do-it-yourself like to eat dishes, more trouble, but more in line with their own taste, and greater freedom.

1. Memory allocation:

Heap: The programmer is typically assigned to release, and if the programmer does not release, the program may end up being reclaimed by the OS. Note that it is different from the heap in the data structure and is distributed in a similar way to a linked list. The following keywords may be used: New, malloc, delete, free, and so on.

Stacks: The compiler (Compiler) automatically allocates the release, storing the function's parameter values, local variables, and so on. The operation is similar to the stack in the data structure.

2. Application Method:

Heap: Requires the programmer to apply and indicate the size. In c malloc functions such as P1 = (char *) malloc (10) and the new operator in C + +, but note that P1, p2 itself is on the stack. Because they can still be considered as local variables.

Stacks: Automatically allocated by the system. For example, declare a local variable int b in a 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 free memory addresses, when the system receives the application, it traverses the list, looks for the first heap node that is larger than the requested space, deletes the node from the Free node list, and assigns the node space to the program, and for most systems, The size of this assignment is recorded at the first address in this memory space, so that the DELETE statement in the code can properly release the memory space. In addition, because the size of the found heap node does not necessarily equal the size of the application, the system automatically puts the extra part back into the free list.

Stack: As long as the remaining space of the stack is larger than the application space, the system will provide memory for the program, otherwise it will be reported abnormal stack overflow.

4, size limit:

Heap: A data structure that is extended to a high address and is a contiguous area of memory. This is because the system is used to store the free memory address of the list, nature is discontinuous, and the link list of the traversal direction is from the low address to the high address. The size of the heap is limited by the virtual memory available in the computer system. This shows that the heap to obtain a more flexible space, but also relatively large.

Stacks: In Windows, stacks are data structures that extend to a low address and are a contiguous area of memory. This means that the top of the stack address and the maximum capacity of the stack is the system in advance, in Windows, the size of the stack is fixed (is a compile-time constant), if the requested space over the stack of the remaining space, will prompt overflow. Therefore, the space can be obtained from the stack is small.

5. Efficiency:

Heap: Is the memory allocated by new, generally slower, and easy to produce memory fragments, 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 in the stack is directly in the process of the address space to keep a fast memory, Although it is the most inconvenient to use. But fast, and most flexible.

Stack: Automatically distributed by the system, faster. But programmers are out of control.

6, storage content aspects:

Heap: The size of the heap is usually stored in a byte at the head of the heap. The specifics of the heap are arranged by the programmer.

Stacks: 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, parameters are pushed from the right to the left into the stack, and then the local variables in the function. Note: Static variables are not in the stack. When the function call is finished, the local variable first goes out of the stack, then the argument, and the last stack pointer points to the address that was first saved, the next instruction in the main function, where the program continues to run.

7, Access efficiency aspects:

Heap: Char *s1 = "Hellow Word", which is determined at compile time;

Stack: char s1[] = "Hellow Word"; it is assigned at run time; it is faster to use an array than to use a pointer, because the pointer needs to be relayed to the EDX register in the underlying assembly, and the array is read directly on the stack.