Advanced C language (learn malloc & calloc & realloc & free in simple dynamic memory management)

Catalog

One 、 Why is there dynamic memory allocation

Two 、 Use of dynamic memory functions

1.malloc function

（1）malloc The definition of

（2）malloc Function considerations

（3）malloc Use of functions

2.calloc function

（1）calloc Definition of function

（3）calloc Use of functions

3.realloc function

（1）realloc Definition of function

 （2）realloc Function considerations

（3）realloc Use of functions

summary

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One 、 Why is there dynamic memory allocation

stay c The memory development methods we currently master in the language are ：

int val = 20;// Open up four bytes in the stack space 
char arr[10] = {0};// Open up on the stack space 10 Bytes of contiguous space 
 
 Or create a variable , Or create an array .
 The space created in this way sometimes appears when we use it, or we feel that the space is too large , Or I feel the space is too small , inflexible .
 So we need a way to make space bigger and smaller , At this time, dynamic memory development appears .

But there are two characteristics of the way to open up space ：

1. The size of the space opening is fixed .

2. Arrays are declared , You must specify the length of the array , The memory it needs is allocated at compile time . But the need for space , It's not just that . Sometimes the amount of space we need is known when the program is running , The way to open up space when compiling arrays is not enough . At this time, you can only try dynamic storage development .

Two 、 Use of dynamic memory functions

First of all, we need to understand the data , Variable , How functions are stored in memory , As shown in the figure below ：

1.malloc function

（1）malloc The definition of

 

  This function applies to memory for a contiguous block of free space , And return the pointer to this space .

（2）malloc Function considerations

1. If the development is successful , Then return a pointer to open a good space .

2. If the development fails , Returns a NULL The pointer , therefore malloc The return value of must be checked .

3. The type of return value is void* , therefore malloc Function doesn't know the type of open space , Specifically, when using, the user himself To decide .

4. If parameters size by 0,malloc The standard is undefined , Depends on the compiler .

（3）malloc Use of functions

The code is as follows ：

#include<stdio.h>

// Dynamic memory development 
int main()
{
	// Suppose you open ten integer spaces  -- 10*sizeof(int)
	int arr[10];// The stack area 
	// Dynamic memory development 
	int* p = (int*)malloc(10 * sizeof(int));
	// When using these spaces 
	if (p == NULL)
	{
		perror("main");//main:xxxxxxxxxxx( error message ）
	}
	// Use 
	int i = 0;
	for (i = 0; i < 10; i++)
	{
		*(p + i) = i;
	}
	for (i = 0; i < 10; i++)
	{
		printf("%d", p[i]);//p[i]-->*(p + i)
	}
	// Recycle space 
	free(p);//malloc Functions should match free Function USES , After use, take the initiative to release this space 
	p = NULL;// After the release, put p Set to null pointer 
	return 0;
}

2.calloc function

（1）calloc Definition of function

C The language also provides a function called calloc , calloc Function is also used for dynamic memory allocation .

（2）calloc Function considerations

1. The function is for num Size is size The elements of open up a space , And initialize each byte of the space to 0.

2. And functions malloc The only difference is calloc Initializes each byte of the requested space to full before returning the address 0.

（3）calloc Use of functions

The code is as follows ：

#include<stdio.h>

// Dynamic memory development 
int main()
{

	int* p = calloc(10,sizeof(int));
	// When using these spaces 
	if (p == NULL)
	{
		return 1;
	}
	int i = 0;
	for (i = 0; i < 10; i++)
	{
		printf("%d", p[i]);//p[i]-->*(p + i)
	}
	// Recycle space 
	free(p);//malloc Functions should match free Function USES , After use, take the initiative to release this space 
	p = NULL;// After the release, put p Set to null pointer 
	return 0;
}

The results are as follows ：

It should be noted that ,calloc Functions and malloc There are two differences between functions ,

1.calloc The parameters of the function are two ,malloc The argument to the function is a . 

2.malloc If the function is not initialized, all the printed values are random ,calloc Functions do not need to be initialized , By default, each byte of the requested space will be initialized to 0.

3.realloc function

（1）realloc Definition of function

realloc Functions make dynamic memory management more flexible .

Sometimes we find that the application space is too small in the past , Sometimes we think the application space is too large , That's for a reasonable time Waiting memory , We will make flexible adjustments to the size of memory .

that realloc Function can be used to dynamically open up the memory size Adjustment of .

 （2）realloc Function considerations

1.ptr Is the memory address to be adjusted

2.size New size after adjustment The return value is the starting memory position after adjustment .

3. This function adjusts the size of the original memory space , The original data in memory will also be moved to new Space .

4.realloc There are two ways to adjust memory space ：

situation 1 When it's the case 1 When , To expand memory, add space directly after the original memory , The original spatial data does not change .

situation 2 When it's the case 2 When , When there is not enough space after the original space , The way to expand is ： Find another suitable size on the heap space Continuous space to use . This function returns a new memory address . Because of the above two situations ,realloc We should pay attention to the use of functions .

（3）realloc Use of functions

#include<stdio.h>

// Dynamic memory development 
int main()
{

	int* p = calloc(10,sizeof(int));
	// When using these spaces 
	if (p == NULL)
	{
		perror("main");
		return 1;
	}
	int i = 0;
	for (i = 0; i < 10; i++)
	{
		*(p + i) = 5;
	}// Need here P Point to more space , need 20 individual int The size of the space 
	//realloc Resize space 
	int* ptr = realloc(p, 20 * sizeof(int*));
	if (ptr != NULL)
	{
		p = ptr;
	}
	// Recycle space 
	free(p);//malloc Functions should match free Function USES , After use, take the initiative to release this space 
	p = NULL;// After the release, put p Set to null pointer 
	return 0;
}

summary

This article simply introduces the definition of dynamic memory function , Precautions and use , also free Function is used to free the space opened up by dynamic memory , Then set it to empty . If there are any questions in the article , You are welcome to ask questions . I will study and correct with an open mind , The most important thing is to make progress together , Grow up together , Learn programming well .