[Reprint] stack, stack and stack differences

SELF: http://www.cppblog.com/oosky/archive/2006/01/21/2958.html
I. prerequisites-ProgramMemory Allocation The memory occupied by a C/C ++ compiled program is divided into the following parts: 1. the stack zone (stack) is automatically allocated and released by the compiler to store the function parameter values, the value of a local variable. The operation method is similar to the stack in the data structure. 2. Heap-generally 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 together, and the initialized global variables and static variables are in one area, uninitialized global variables and uninitialized static variables are in another adjacent area. -Release the system after the program ends. 4. In the text constant area, the constant string is placed here. After the program ends, the system releases the program. Code Zone-stores the binary code of the function body.

 

Ii. Example Program This is written by a senior. It is 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 \ 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 to the "123456" pointed to by P3.
}

 

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 a space heap for B in the stack: the programmer needs to apply for it and specify the size, in C, malloc functions such as P1 = (char *) malloc (10); in C ++, use the new operator such as P2 = (char *) malloc (10 ); but note that P1 and P2 are in the stack. 2.2 System Response stack after application: as long as the remaining space of the stack is larger than the requested space, the system will provide memory for the program. 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 apply for a size limit Stack: in windows, the stack is a data structure extended to the low address and is 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 in the address space of the process, although it is the most inconvenient to use. However, it is fast and flexible. 2.5 heap and the storage content stack in the stack: During function calling, the first entry to the stack is the address of the next instruction (the next executable statement of the function call statement) 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 comparison of access efficiency Char S1 [] =   " Aaaaaaaaaaaaaaa " ; // Value assignment during runtime
Char   * S2 =   " Bbbbbbbbbbbbbbbbbbb " ; // Determined during 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: 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. 2.7 conclusion: the difference between stack and stack can be seen in the following metaphor: using Stack is like eating at a restaurant, just ordering food (issuing an Application) pay, and eat (use). When you are full, you don't have to worry about cooking, washing, and other preparation work and cleaning, such as washing dishes, flushing, etc. His advantage is fast, but the 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.