Difference Between Stack and stack (1)

Label: Pointer in C
Last Update Time: 2010-01-10 14:09:19
Excerpted from: Http://up2pain.bokee.com/3919242.html. The original author of this article isLaomai.

1. prerequisites-program 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-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 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. Text Constant Area-constant strings are placed here. The program is released by the system.

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

1.1. Example Program

// Main. cpp, which was written by a senior, very detailed
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 to "123456" as directed by P3.
}

 

2. 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 and specify the size. In C, this is done using the malloc function. For example, P1 = (char *) malloc (10); P2 = (char *) malloc (10); but note that P1 and P2 are in the stack. Use the new operator in C ++.

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. In this way, 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

STACK: the stack is automatically allocated by the system, and the speed is fast. But programmers cannot control it.

Heap: Memory allocated by new. It 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 virtual alloc to allocate memory. Instead of heap or stack, the virtual alloc directly keeps 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.

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

Compare the following two statements:

Char S1 [] = "aaaaaaaaaaaaaaa"; // aaaaaaaaaaaaa is assigned at runtime;

Char * S2 = "bbbbbbbbbbbbbbbbbbb"; // bbbbbbbbbbbbbbb 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
Voidmain ()
{
Char A = 1;
Char C [] = "1234567890 ";
Char * P = "1234567890 ";
A = C [1];
A = P [1];
Return;
}

Corresponding assembly code
10: A = C [1];
004010678a4df1movcl, byteptr [ebp-0Fh]
0040da-a884dfcmovbyteptr [ebp-4], Cl
11: A = P [1];
00401_d8b55ecmovedx, dwordptr [ebp-14h]
004010708a4201moval, byteptr [edX + 1]
004010738845 fcmovbyteptr [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.

3. 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.

There is also the heap and stack in terms of data structure. In this case, the heap actually refers to a Data Structure (meeting the heap nature) of the priority queue, with 1st elements having the highest priority; stack is actually a mathematical or data structure that meets the needs of the advanced and later stages. Note that this is totally different from the operating system stack discussed in this article.

Appendix 1: Interesting thread 2

"A heap actually refers to a Data Structure of a priority queue (satisfying the heap nature), with 1st elements having the highest priority"-it seems wrong, the heap in the data structure is such a sequence:
{.... Ki .....} this element meets the following requirements: Ki> = k2 * I and Ki> = k2 * I + 1 or ki <= k2 * I and Ki <= k2 * I + 1.

Appendix 2: Interesting thread 2

"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. But it is fast and flexible ." -- Do not use it blindly. virtualalloc is allocated by PAGE (> = 4 kb), which is a huge waste of small items.