Suddenly think of the stack of processes and the stack of threads, by the way, the thread's stack is automatically assigned to the process's memory space
Processes and threads are the basic units that the operating system realizes, and the system uses this basic unit to realize the concurrency of the system to the application. The difference between a process and a thread is:
In short, a program has at least one process, and a process has at least one thread.
The thread's dividing scale is smaller than the process, which makes the multi-thread procedure high concurrency.
In addition, the process has a separate memory unit during execution, and multiple threads share memory, which greatly improves the efficiency of the program operation.
Threads are still different from the process during execution. Each separate thread has a program run entry, sequence of sequence execution, and exit of the program. However, threads cannot be executed independently, and must be dependent on the application, which provides multiple threads of execution control.
From a logical point of view, the meaning of multithreading is that in an application, multiple execution parts can be executed concurrently. However, the operating system does not consider multiple threads as separate applications to implement scheduling and management of processes and resource allocation. This is the important difference between processes and threads.
A process is a program with a certain independent function about a single run activity on a data set, a process that is an independent unit of the system's resource allocation and scheduling.
A thread is an entity of a process that is the basic unit of CPU dispatch and dispatch, which is a smaller unit that can run independently than a process. The thread itself basically does not own the system resources, only has a point in the operation of the necessary resources (such as program counters, a set of registers and stacks), However, it can share all of the resources owned by the process with other threads that belong to one process.
One thread can create and revoke another thread, and can execute concurrently between multiple threads in the same process.
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Heap: is a common space for everyone, divided into global heap and local heap. The global heap is all unallocated space, and the local heap is the space allocated by the user. The heap is allocated when the operating system initializes the process, and it can be added to the system during the run, but remember to return it to the operating system, or memory leaks.
Stack:is a thread-unique, saving its running state and local automatic variables. Stack is initialized at the beginning of the thread, and the stack is independent of each other, so the stack is thread safe. The data members of each C + + object are also present in the stack, each with its own stack, which is used to pass parameters between functions. The operating system automatically switches the stack when switching threads, or switches the SS/ESP register. Stack space does not require explicit allocation and deallocation in high-level languages.
the difference between heaps and stacks
first, the preparation of knowledge
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memory allocation of the program
The memory used by a program compiled by C + + is divided into the following sections:
1. Stack (stack)-Automatically allocated by the compiler to release, store the function parameter value, local variable value and so on. It operates in a manner similar to a stack in a data structure.
2, heap area (heap)-Generally by the programmer assigned to 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)-, 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. -released by the system after the program is finished.
4, literal constant area-the constant string is put here. Released by the system after the program is finished.
5. Program code area-binary code that holds the function body.
Second, the example procedure//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 in the constant area, p3 on the stack.
static int c = 0; global (static) initialization zone
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 in the constant area, the compiler might optimize it to a place with the "123456" that P3 points to.
}
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 the programmer to apply himself and indicate the size of the malloc function in C
such as P1 = (char *) malloc (10);
Using the new operator in C + +
such as P2 = (char *) malloc (10);
But note that P1, p2 itself is in the stack.
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, look for 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, , the size of this allocation is recorded at the first address in this memory space, so that theDelete statement to properly dispose of this 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 Windows, 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 Windows, the size of the stack is 2M (also may be 1M, it is a compile-time determination of the constant), if the request for more space than the stack of space, will be prompted 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, the general speed is relatively slow, and prone to memory fragmentation, but the most convenient to use.
In addition, under Windows, the best way is to use VirtualAlloc to allocate memory, he is not in the heap, nor in the stack is directly in the process's address space to keep a fast memory, although the 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
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).
For example: 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 into the register cl, while the second first reads the pointer value into edx, based on
EdX reads the characters, apparently slowly.
2.7 Summary:
the difference between heaps and stacksCan be seen in the following analogy:
Use the stack like we go to a restaurant to eat, just order (send application), pay, and eat (use), eat enough to go, do not bother to cut vegetables, wash vegetables and other preparation work and washing dishes, brush pots and other finishing work, his advantage is fast, but the freedom is small.
The use of the heap is like a DIY dish that you like to eat, more trouble, but more in line with their own tastes, and great freedom.
http://blog.csdn.net/echoisland/article/details/6403763
Summary of process threads and stack relationships (go)
