How to implement dynamic memory allocation in C language

How to implement dynamic memory allocation in c language

We often open up memory space for the program in advance and then operate it.

int arr[5] ;

We must open up space in advance when defining this array, and during the execution of the program, this opened memory space will always exist, unless the function is executed, it will be Space freed up. One problem is that this array cannot be modified in the program.

These problems have caused us some inconveniences in use, so the malloc() function is provided in C.

Regarding the malloc() function, this function accepts one parameter: the number of bytes of memory required. Then malloc() finds a suitably sized block in available memory. In this process, malloc() can return the address of the first byte of that memory. So, it means that we can use pointers to operate. malloc() can be used to return array pointers, structure pointers, etc., so we need to assign the type of the return value to the appropriate type. When malloc() cannot find the required space, it returns a null pointer.

Example:

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double *p;
p=(double*)malloc(30*sizeof(double));

In this program, first open up 30 double type spaces, and then Point p to the position in this space. The pointer here points to the first double value. Not all of the 30 double spaces we have opened up. This is just like an array. The pointer to the array points to the address of the first element of the array, not the elements of the entire array. Therefore, our operations here are the same as for arrays,

p[0] is the first element, and p[1] is the second element.

At this point, we can master a method of declaring dynamic array.

int arr[n];
p=(int *)malloc(n*sizeof(int));
//我们在这里使用的时候要元素个数乘类型字节长度，这样就达到了动态开辟内存空间。

After we use malloc() to open up the memory space, all we have to consider is to release the memory space. Here, C provides us with the free() function. The parameter of free() is the address returned by the malloc() function, which releases the space opened by the previous malloc() function.

Example:

To release the space we opened above, then we can do this

free(p);

The program also calls the exit() function, which is in the memory End the program when allocation fails.

Program example:

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<malloc.h>//malloc()函数被包含在malloc.h里面
#include<stdlib.h>
int main(void)
{
    char*a = NULL;//声明一个指向a的char*类型的指针
    a = (char*)malloc(100 * sizeof(char));//使用malloc分配内存的首地址，然后赋值给a
    if (!a)//如果malloc失败，可以得到一些log
    {
        perror("malloc");
        return-1;
    }
    sprintf(a, "%s", "HelloWorld\n");//"HelloWorld\n"写入a指向的地址
    printf("%s\n", a);//输出用户输入的数据
    free(a);//释放掉使用的内存地址
    system("pause");
    return 0;//例2有无内存泄露？
}

This program is mainly used to detect incorrect malloc return value conditions.

Here we need to note that in C, type assignment (char *) is optional, but in C this is required, so using type assignment will make the C program more portable to C easy.

Use dynamic arrays mainly to gain program flexibility. We can create as many arrays as we need. No need to waste space.

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