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Today's content is Dynamic memory allocation .

Here is the following knowledge content 🥳🥳🥳

One 、 Preface ： Why is there dynamic memory allocation

     Why is there dynamic memory allocation ？
int val = 20;// Open up four bytes in the stack space
char arr[10]={0};// Open up ten bytes of continuous space on the stack space
But these two development methods have two characteristics
1. The size of the space opening is fixed
2. When an array is declared , You must specify the length of the array , The memory it needs is allocated at compile time .

Two 、 Common area division in memory

    In memory , Common areas are
     The stack area 、 Heap area 、 Static zone 、 Constant area, etc
Local variables are stored on the stack , The usage habit of stack area is to use high address first and then low address
Static variables are stored in the static area
     And the space opened up by dynamic memory , It was opened up on the stacking area ,
for instance malloc,realloc,calloc

3、 ... and 、 Dynamic memory exploit function

1、malloc

malloc The use of requires a header file stdlib.h perhaps malloc.h

malloc The development of will find a space on the heap , If the development fails, a null pointer will be returned

You can see ,malloc The return type of is void*, therefore , When receiving the return value , need Cast

malloc Usage of ：

int main()
{
    
	//int arr1[10];//40 Bytes 
	//char arr2[40];//40 Bytes 
	void*malloc(size_t);
	//int* arr3 = (int*)malloc(sizeof(int) * 10);// Apply in the stacking area 
	//char* arr4 = (char*)malloc(sizeof(char) * 40);

	int* p = (int*)malloc(sizeof(int) * 10);
	int* ptr = p;
	if (p==NULL)
	{
    
		perror("malloc");
		return 1;
	}

	int i = 0;
	for (int i = 0; i < 10; i++)
	{
    
		*p = i;
		p++;
		printf("%d ",*p);
	}
	free(ptr);
	ptr = NULL;
	//free After that, this space no longer belongs to us , however ptr Or point to this space , It's dangerous ,
	// therefore , Usually free After falling , Set the pointer pointing to this space to NULL Null pointer 
	if (ptr != NULL)
	{
    
		*ptr = 100;
	}
	return 0;
}

It is worth noting that ：
1. The dynamic space should be actively released , And the operating system , Otherwise, it may lead to memory leakage .
2.malloc(0) It's undefined , The consequences depend on the compiler .
3. If parameters ptr The pointed space is not opened dynamically , that free The behavior of a function is undefined .
4.free(NULL); The result is that nothing happens .
5. There will also be problems if the released space is released again ., because free What is needed is the starting address of the free space , If released , This space does not belong to us , There will be the problem of wild pointer

2、calloc

calloc And malloc It's not that different , It is initialized to 0

3、realloc

realloc Used to adjust the size of the opening space
1. The space you want to expand is large enough in the back
2. Not big enough ( Already occupied ),realloc A space of sufficient size will be selected in a suitable area behind to expand
meanwhile , Will copy the previous space into this space , At the same time, the original space free fall （ At the same time, doing so will also cause some problems , For example, too much data will affect performance , The original space may be wasted ）
3. meanwhile ,realloc Failure also returns null pointer NULL

When dynamic memory development fails , It's time to end the program , Because there is not enough space behind , Unable to continue program logic .

3、 ... and 、 Memory leaks

Program ( process ) Normal end , Even in the program malloc No, free, It will also be released automatically
If it doesn't end normally , The memory is not returned to the operating system , Memory may become less and less in the future
and , Some data needs long-term operation , For example, service background
There is also embedded program , There is little memory , A little leakage makes it smaller

Four 、 A brain burning problem

5、 ... and 、 5、 ... and The color spot Beautiful Of One some Scrap word

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