Stack and stack

 

Zone 1 heap and stack

1.1 Memory Allocation

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

1. STACK: the stack zone is automatically allocated and released by the compiler, and stores function parameter values and local variable values. The operation method is similar to the stack in the data structure.

2. Heap: the heap is assigned and released by the programmer. If the programmer does not release the heap, 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 variables and static variables is put in one area, and the initialized global variables 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.

4. In the text constant area, the constant string is placed here, and is released by the system after the program ends.

5. program code area-stores the binary code of the function body.

1.2 example Program

// Main. cpp

Int A = 0; global initialization Zone

Char * P1; uninitialized globally

Main ()

{Int B; stack char s [] = "ABC"; stack char * P2; stack char * P3 = "123456"; 123456 \ 0 in the constant zone, and P3 in the stack. Static int C = 0; Global (static) initialization zone p1 = (char *) malloc (10); [1] P2 = (char *) malloc (20 );}

The allocated 10-byte and 20-byte areas are located 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" that P3 points.

2. Theoretical knowledge

2.1 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 is like p1 = (char *) malloc (10 ); in C ++, use the new operator such as P2 = new char [20]; // (char *) malloc (10); but note that P1 and P2 are in the stack. They store the address on the stack.

2.2 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. (use the algorithm for the first time) 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 (when the remaining part is large ).

2.3 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 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 directly retains a piece of memory in the address space of the process, although it is the most inconvenient to use. However, it is fast and flexible.

2.5 5. Storage content in heap and stack

STACK: When a function is called, the first entry to the stack is the address of the next instruction (the next executable statement of the function call statement) after the function is called in the main function, then there are various parameters of the function. In most C compilers, the parameters are from right to left and then the 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 6. comparison of access efficiency: Char S1 [] = "aaaaaaaaaaaaa"; char * S2 = "bbbbbbbbbbbbbbbbbbb"; aaaaaaaaaaa is assigned a value at the 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 and then reads the characters according to edX, which is obviously slow.

2.7 7. Summary

The difference between stack and stack can be seen from the following metaphor: using Stack is like eating at a restaurant, just ordering food (sending an application), paying for it, and eating (using it ), when you are full, you don't have to worry about the preparation work, such as cutting and washing dishes, washing dishes, and so on. His advantage is that it 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.

3. Stack is a storage component, that is, the address is not required for data writing and reading. Instead, the stack is a Pipeline Based on the sequential image of Data Reading, the main program to be processed in the pipeline is the method. When the stack is allocated, the program is executed from top to bottom in order, and the program instruction is pushed into the stack one by one, just like the pipeline. What stands on the stack is the staff who process the commodities in the assembly line, allocated by the programmers: When to process and how to process. Instead, we usually use the new operator to allocate memory (C #) to the object on the stack. The task of searching for the object on the stack is handed over to the handle, and the stack is managed by the stack pointer.

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