C/C ++ Memory Allocation

I. prerequisites-program memory allocation

The memory occupied by a C/C ++ compiled program is divided into the following parts:

1. STACK: the stack is automatically allocated and released by the compiler, storing the parameter values of the function and the values of local variables. The operation method is similar to the stack in the data structure.
2. Heap (HEAP)-generally, it is assigned and released by programmers. If the programmer does not release it, it may be recycled by the OS at the end of the program. Note that it is different from the heap in the data structure. The allocation method is similar to the linked list.
3. Global (static)-the storage of global and static variables is put in one area, and the initialized global and static variables are in one area, uninitialized global variables and uninitialized static variables are in another adjacent area. -The program is released by the system after it ends.
4. Constant area-the constant string is placed here. The program is released by the System
5. Code area-stores the binary code of a program.

This is an example program written by a senior.

// Main. cpp
Int A = 0; // global initialization Zone
Char * P1; // not initialized globally
Main ()
{
Int B; // Stack
Char s [] = "ABC"; // Stack
Char * P2; // Stack
Char * P3 = "123456"; // 123456 \ 0 is in the constant zone, and P3 is in the stack.
Static int C = 0; // global (static) initialization Zone
P1 = (char *) malloc (10 );
P2 = (char *) malloc (20); // The allocated 10-byte and 20-byte areas are in the heap area.
Strcpy (P1, "123456"); // 123456 \ 0 is placed in the constant area, and the compiler may optimize it into a place with the "123456" pointed to by p3
Free (P1 );
Free (P2 );
}

Ii. Theoretical knowledge of heap and stack 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: the programmer must apply for it and specify the size. In C, the malloc function, for example, P1 = (char *) malloc (10); in C ++, the new operator is used, for example, P2 = (char *) malloc (10 );

But note that P1 and P2 are in the stack.

2.2 system response after application

STACK: as long as the remaining space of the stack exceeds the applied space, the system will provide the program with memory. Otherwise, an exception will be reported, prompting stack overflow.

Heap: First, you should know that the operating system has a linked list that records idle memory addresses. When the system receives a program application, it will traverse the linked list, find the heap node with the first space greater than the requested space, delete the node from the idle node linked list, and allocate the space of the node to the program. In addition, for most systems, the size of the allocation will be recorded at the first address in the memory space, so that the delete statement in the code can correctly release the memory space. In addition, because the size of the heap node is not necessarily equal to the applied size, the system automatically places the excess part in the idle linked list.

2.3 Application size limit

STACK: in windows, a stack is a data structure extended to a low address and a continuous memory area. This statement indicates that the stack top address and the maximum stack capacity are pre-defined by the system. In Windows, the stack size is 2 MB (OR 1 MB, in short, it is a constant determined during compilation. If the requested space exceeds the remaining space of the stack, overflow will be prompted. Therefore, the space available from the stack is small.

Heap: the heap is a data structure extended to the high address and a non-sequential memory area. This is because the system uses the linked list to store the idle memory address, which is naturally discontinuous, And the traversal direction of the linked list is from the low address to the high address. The heap size is limited by the valid virtual memory in the computer system. It can be seen that the space obtained by the heap is flexible and large.

2.4 comparison of Application Efficiency

The stack is automatically allocated by the system, which is faster. But programmers cannot control it.

The heap is the memory allocated by new, which is generally slow and prone to memory fragments. However, it is most convenient to use.

In addition, in windows, the best way is to use virtualalloc to allocate memory. Instead of heap or stack, it is to reserve a fast memory directly in the address space of the process, although it is the most inconvenient to use. However, it is fast and flexible.

Storage content in 2.5 heap and stack

STACK: when calling a function, the first entry to the stack is the address of the next instruction in the main function (the next executable statement in the function call statement), and then the parameters of the function, in most C compilers, parameters are written from right to left into the stack, followed by local variables in the function. Note that static variables are not included in the stack. When the function call ends, the local variable first goes out of the stack, then the parameter, and the top pointer of the stack points to the address of the initial storage, that is, the next instruction in the main function, where the program continues to run.

Heap: Generally, the heap size is stored in one byte in the heap header. The specific content in the heap is arranged by the programmer.

2.6 comparison of access efficiency

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

Aaaaaaaaaaa is assigned at runtime, while bbbbbbbbbbbbb is determined at compilation;

However, in future access, the array on the stack is faster than the string pointed to by the pointer (such as the heap. For example:

#include
void main()
{
char a = 1;
char c[] = "1234567890";
char *p ="1234567890";
    a = c[1];
    a = p[1];
return;
}

Corresponding assembly code

10: a = c[1];
00401067 8A 4D F1 mov cl,byte ptr [ebp-0Fh]
0040106A 88 4D FC mov byte ptr [ebp-4],cl
11: a = p[1];
0040106D 8B 55 EC mov edx,dword ptr [ebp-14h]
00401070 8A 42 01 mov al,byte ptr [edx+1]
00401073 88 45 FC mov byte ptr [ebp-4],al

The first type reads the elements in the string directly into the CL register, while the second type reads the pointer value into EDX. Reading the characters based on edX is obviously slow.

Conclusion 2.7

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

Using Stacks is like eating at a restaurant, just ordering food (sending an application), paying for it, and eating (using it). If you are full, you can leave, without having to worry about the preparation work, such as cutting and washing dishes, as well as the cleaning and tail work such as washing dishes and flushing pots, his advantage is fast, but his degree of freedom is small.

Using heap is like making your favorite dishes. It is troublesome, but it suits your taste and has a high degree of freedom.

 

The differences between stack and stack are as follows:

The heap and stack of the operating system, as mentioned above, will not be mentioned much. There is also the heap and stack in the data structure. These are different concepts.

Here, the heap actually refers to a Data Structure (meeting the heap nature) of the priority queue. The 1st elements have the highest priority; stack is actually a mathematical or data structure that meets the needs of the advanced and later stages.

Although the stack is called a connection, they still have a lot of difference. The connection is only due to historical reasons.

Bytes -----------------------------------------------------------------------------------------------------------------

Heap and stack are two basic concepts that C/C ++ programming will inevitably encounter.

First, both concepts can be found in the data structure book. They are both basic data structures, although the stack is simpler.

In a specific C/C ++ programming framework, these two concepts are not parallel. The Research on the underlying machine code reveals that the stack is the data structure provided by the machine system, while the stack is provided by the C/C ++ function library.

Specifically, modern computers (serial execution mechanisms) directly support the stack data structure at the bottom of the Code. This is reflected in the fact that there are dedicated registers pointing to the address of the stack, and dedicated machine commands to complete the operations of data in and out of the stack. This mechanism is characterized by high efficiency and limited data types supported by systems such as integers, pointers, and floating point numbers. It does not directly support other data structures. Due to the characteristics of stack, the use of stack is very frequent in the program. The call to the subroutine is done directly using the stack. The call command of the machine implicitly pushes the return address into the stack, and then jumps to the subprogram address. The RET command in the subprogram implicitly pops up the return address and jumps from the stack. The automatic variables in C/C ++ use the stack directly, which is why when the function returns, the reason that the automatic variables of this function are automatically invalid (because the stack restores the status before the call ).

Unlike the stack, the stack data structure is not supported by the system (whether a machine system or an operating system), but provided by the function library. The basic malloc/realloc/free function maintains an internal heap data structure. When the program uses these functions to obtain new memory space, this function first tries to find available memory space from the internal heap, if there is no available memory space, the system calls are used to dynamically increase the memory size of the program data segment. The newly allocated space is first organized into the internal heap and then returned to the caller in an appropriate form. When the program releases the allocated memory space, this memory space is returned to the internal Heap Structure and may be processed properly (for example, merged into a larger idle space with other idle space ), it is more suitable for the next memory allocation application. This complex allocation mechanism is actually equivalent to a buffer pool (cache) for memory allocation. There are several reasons for using this mechanism:

1. System calls may not support memory allocation of any size. Some system calls only support fixed memory requests and their multiples (allocated by PAGE). This will cause a waste for a large number of small memory categories.
2. Applying for memory for system calls may be expensive. System calls may involve switching between user and core states.
3. Unmanaged memory allocation can easily cause memory fragmentation when a large amount of complex memory is allocated and released.

Stack and stack comparison

From the above knowledge, stack is a function provided by the system, featuring fast and efficient, with limitations and inflexible data. Stack is a function provided by the function library, featuring flexibility and convenience, data is widely adapted, but the efficiency is reduced. The stack is the system data structure, which is unique for processes/Threads. The heap is the internal data structure of the function library, which is not necessarily unique. The memory allocated by different heaps cannot be operated logically. Stack space is divided into static allocation and dynamic allocation. Static allocation is completed by the compiler, such as automatic variable allocation. Dynamic Allocation is completed by the alloca function. The stack does not need to be released dynamically (automatically), so there is no release function. For the sake of portable programs, dynamic stack allocation is not encouraged! Heap space allocation is always dynamic. Although all data spaces are released back to the system at the end of the program, precise memory application/release matching is the basic element of a good program.

 

 

References: http://www.cnblogs.com/yxnchinahlj/archive/2011/02/09/1950328.html