[C + +] memory allocation

Reprinted from: http://www.cnblogs.com/pengrui/archive/2011/04/02/2002841.html

First, the basic knowledge of program memory allocation

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

1, the Stack area: The compiler automatically allocates the release, the stored function parameter value, the local variable value and so on. It operates in a manner similar to a stack in a data structure.

2, heap area: Generally by the programmer allocation 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, 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 to the program after the end of the system release.

4, text constant area: the constant string is placed here, the program is released after the end of the system.

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

Example Program
It was written by a predecessor, very detailed
Main.cpp
int a = 0; Global initialization Zone
Char *p1; Global uninitialized Zone
int 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 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\0 is placed in a constant area, the compiler may optimize it with the "123456" pointed to by P3 as a place

　 return 0;

}

Stacks are separate spaces, continuously allocated, from high addresses to low addresses , and other areas are from low to high.

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 programmers to apply themselves and indicate size, in C malloc function, C + + with new.

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, 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 space allocated to the program, in addition, for large 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 Linux, the stack is the data structure to the low address extension, which is a contiguous area of memory. This sentence means that the stack top address and the maximum capacity of the stack is the system pre-defined, under Linux, using ulimit-a know that the size of the stack is 8192K (is a compile-time determined constant), if the requested space exceeds the remaining space on the stack, 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, generally slower, and prone to memory fragmentation, but the most convenient to use. In addition, under Windows, the best way is to allocate memory with VirtualAlloc , he is not in the heap, Nor is it that the stack is directly retaining a fast memory in the process's address space, although it is 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

This is an example of a programmer's interview, to analyze:

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.

2.6 Stack vs. heap comparison

Stack is a system-provided function, characterized by fast and efficient, the disadvantage is limited, the data is not flexible, and the heap is a function library provides the function, the characteristic is flexible and convenient, the data adapts widely, but the efficiency has a certain reduction. A stack is a system data structure that is unique to a process/thread, and a heap is a library internal data structure that is not necessarily unique. The memory allocated by different heaps cannot be manipulated logically on each other. The stack space is divided into two kinds: static allocation and dynamic allocation. Static allocations are done by the compiler, such as the automatic variable (auto) assignment. Dynamic allocation is done by the Alloca function. The dynamic allocation of the stack does not have to be released (it is automatic), and there is no release function. For portable programs, the dynamic allocation of stacks is not encouraged! The allocation of heap space is always dynamic, although all data spaces are released back to the system at the end of the program, but the exact application of memory/release memory matching is the basic element of a good program

Summary:

Heap and Stack (stack) are the two basic concepts that are inevitably encountered in C/s + + programming.

First of all, these two concepts can be found in the Book of data structure, they are the basic structure, although the stack is simpler.

These two concepts are not parallel in a specific C + + programming framework. The study of the underlying machine code reveals that the stack is the data structure provided by the machine system, while the heap is provided by the C/S function library.

Specifically, the modern computer (serial execution mechanism) directly supports the data structure of the stack at the bottom of the code. This is reflected in, there is a dedicated register to the address of the stack, there is a dedicated machine instruction to complete the data into the stack out of the stack operation. This mechanism is characterized by high efficiency, limited data support, generally integer, pointer, floating point and other systems directly supported by the data type, does not directly support other data structures. Because of this feature of the stack, the use of stacks is very frequent in the program. The call to the child program is done directly using the stack. The call instruction of the machine implies that the return address is pushed into the stack and then jumps to the subroutine address, while the RET instruction in the subroutine implicitly pops the return address from the stack and jumps to the operation. The automatic variable in C + + is an example of using the stack directly, which is why the automatic variable of the function is automatically invalidated when the function returns (because the stack recovers the state before the call).

Unlike stacks, the data structure of a heap is not supported by the system (whether it is a machine system or an operating system), but is provided by a library of functions. The basic Malloc/realloc/free function maintains a set of internal heap data structures. When the program uses these functions to obtain new memory space, the function first tries to find the available memory space from the internal heap, if there is no memory space available, then attempts to use the system to dynamically increase the size of the program data segment memory, the newly allocated space is first organized into the internal heap, It is then returned to the caller in the appropriate form. When the program frees the allocated memory space, this memory space is returned to the internal heap structure and may be appropriately processed (such as merging additional free space into a larger free space) to better fit the next memory allocation request. This complex allocation mechanism is actually equivalent to a memory allocation buffer pool (cache), which has several reasons for using this set of mechanisms:

1. System calls may not support memory allocations of any size. Some system calls only support a fixed size and multiple memory requests (per page allocation), which can be wasteful for a large number of small memory classifications.

2. System calls to request memory can be costly. System calls may involve conversion of the user state and kernel mentality.

3. Memory allocations that are not managed can easily cause memory fragmentation under the allocation release operation of a large amount of complex memory.

[C + +] memory allocation