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 ： This issue focuses on ： Knowledge of dynamic memory

   I hope everyone can study and work happily every day

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  What is dynamic memory ？

malloc Use

calloc

realloc

C Program address space

Why dynamic memory development ？

Error accessing memory

Pointer legitimacy problem

Memory out of bounds

  Memory leak

free How much space ？

free What is the release ？

Next up ：

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  What is dynamic memory ？

Dynamic memory is to use malloc,calloc,realloc and free These dynamic memory functions are used to request space from the heap , To the programmer for management and use , This is dynamic memory management .

malloc Use

malloc Is to apply for a space , You need to initialize manually , And manually free And empty

calloc

calloc Is to open up space num individual sizeof（ type ） And initialize to 0.

realloc

realloc There are two situations ：

1. When the source space is 0, The incoming NULL when , and malloc It works the same .

2. If you expand the space , That is, when the passed in pointer is valid , There are two cases

         2.1. If there is enough space behind the original space , Then open up directly behind the original space , Returns the lowest address of the original space .（ That is, the return value is the same as the incoming value ）

         2.2. If there is not enough space behind the original space , Then the function will be on the heap , Find a space that can meet the space size required by users , Then copy the original spatial data to the space opened by the function , Then release the original space , Return the lowest address of the reopened space .

This is generally rare , So no more examples .

C Program address space

First of all, we need to understand that the stacking area is C Where is the program address space ？ Or say C What is the program address space ？

Today we mainly talk about the pile area .

Why dynamic memory development ？

First of all, we know that the normal application space can be applied directly on the stack area , So why do we need dynamic memory development ？

for instance ：

We need to store some numbers , These numbers can be many , There may be very little , At this time, how can we apply for space in the stack area ？

In the idea of saving everything , We need to open up more space , But the stack space is limited , A little bigger will cause stack overflow , That's why there's a heap area .

Error accessing memory

For example, define a structure , Among them, the structure members are dynamically opened up , If not initialized, use , Then it will lead to an error in accessing memory .

struct stu
{
	char* p;
	int age;
}s1;


int main()
{
	strcpy(s1.p, " Under Populus euphratica ");
	s1.age = 18;
	return 0;
}

there   s1.p  Although it's a variable , But there is no space for relativity , So it will lead to an error in accessing memory .

It says , It should be given to s.p Open up space , Let's see how to open it up , The following development is no problem .

struct stu
{
	char* p;
	int age;
}s1;
void Show(char *name)
{
	printf("%s\n", name);
}
int main()
{
	struct stu s1 = { NULL, 0 };
	s1.p = (char *)malloc(sizeof(char)* 30);
	strcpy(s1.p, " Under Populus euphratica ");
	
	Show(s1.p);
	free(s1.p);
	s1.p = NULL;

	return 0;
}

Pointer legitimacy problem

First , what " legal " The pointer of ？

Generally speaking , Is that the passed pointer can be used normally .

If a pointer has a point （ The wild pointer also has a point , Just don't know where to point ）, Then we can't judge  " legal " sex . So we ask that if the pointer is not used directly , Then it should be assigned to NULL, So we can verify ” legal “ When it comes to sex , It becomes to judge whether the pointer is NULL.

After improving the above code, it will become the following code ：

struct stu
{
	char* p;
	int age;
}s1;
void Show(char *name)
{
	if (name == NULL)
	{
		exit(-1);
	}
	printf("%s\n", name);
}
int main()
{
	struct stu s1 = { NULL, 0 };
	s1.p = (char *)malloc(sizeof(char)* 30);
	if (s1.p == NULL)
	{
		exit(-1);
	}
	strcpy(s1.p, " Under Populus euphratica ");
	Show(s1.p);
	free(s1.p);
	s1.p = NULL;
	return 0;
}

Here we judge whether the pointer is NULL, This avoids null pointers . You can also use assert( macro ) To assert that , Reference header file "assert.h", However, this macro can only be used in the debug version , Cannot be used for release version .

Memory out of bounds

I believe everyone has heard of the topic of memory cross-border , Will the pointer cross the boundary and report an error ？

Sometimes when the pointer is out of bounds, an error will not be reported , such as ：

  perhaps malloc There was no free There is also a probability of not reporting an error .

But no free More serious things will happen , It's called a memory leak . 

  Memory leak

Memory leaks are simply , The program only requests memory , No release, No free.

So if there is no program , Or the program exits , So is there a memory leak problem ？

If the program exits , Then the operating system will forcibly reclaim the requested memory .

The biggest fear of memory leakage is some programs that will never quit voluntarily , Then it's a terrible thing , A process , Only apply for memory , Don't release , Then there is no memory available .

free How much space ？

We only know free From , So how big is the release ？

  Let's take a look at ,free The size of the is much larger than the free space , So you can say , I malloc It's bigger than we're applying for .

because malloc To apply for space, there should be extra space to manage the applied space , So it's bigger than what we normally apply for , So if we malloc If you want to cross the border, you don't have to report an error .

free What is the release ？

free After that, we usually set the pointer to null , Why? ？ Will it still point to the original space if it is not empty ？

  We can see that the direction of space has not changed , In other words, the direction has not changed , So what has changed ？

The change is p Relationship with the heap application , therefore free Is to change the relationship ,free The essence is to change the relationship , in other words p It also points to the space applied for in the heap area , So we should break this relationship , Just put p Set as NULL.

It's like talking about your girlfriend , It's all broken up , You shouldn't leave your contact information , So it's time to break the contact information .

Next up ：

In the next issue, we'll talk about functions

The next issue is more exciting ~！~！~！