Dynamic memory development (C language)

推荐一个CLanguage Manual website：cplusplus.com

头文件： #include<stdlib.h>

&nbsp;

 #include<malloc.h>

three regions of memory

如下图所示

Why use dynamic memory development

At present, there are two methods for requesting space from memory that we know of：

（1）创建一个变量（如 int a; It applies for four bytes of space in memory）.

（2）创建一个数组.

But like creating one50元素的数组（int a[50]）There will definitely be wasted or insufficient memory,

In order to use as much memory as possible, apply for as much memory space as possible

,We can use dynamic memory development（在堆区）.

动态内存分配的函数

free

函数free专门是用来做动态内存的释放和回收的,That is, when the dynamically requested space is no longer used,Should be returned to the operating system.

To put it simply, if you apply, you have to pay it back.

虽然不用free函数,在程序执行结束后,The system will automatically release the memory you applied for,But if you apply once,There is no more space left,The main program has not been executed yet,Then you should free the memory first,Let others apply for use.

使用方法：

For example, before you defined a pointerp,在结尾用

free(p);
p=NULL;


Q：

为什么还要把pAssign it to a null pointer？I have not been released？

A：

pAlthough back to the operating system,But still have the ability to find the space you just applied for,所以再给pAssign a null pointer.

malloc

Looking through the manual we can seemallocThe use of the function is

void* malloc (size_t size);

直接上案例

int* p = (int*) malloc (10* sizeof(int));

（1）The above code is to apply for memory10个int字节的空间,in the current compilerint一般为4字节大小,10个int就是40字节大小,也可以直接打40.

（2）因为malloc的返回类型为void* ,会报警告,需要强制类型转换.

（3）当没有足够的空间时,malloc会返回一个

空指针

（NULL）.

打印错误原因的一种方式

This is off topic.

When we want to know what went wrong in our program（For example, memory request failed）.可以用下面的函数.

#include<string.h>
#include<errno.h>
#include<stdio.h>
printf(&quot;%s\n&quot;,strerror(errno));


stdio.h对应printf , errno.h对应errno , string.h对应strerror

由malloc和free的实例演示

#include<stdio.h>
#include<malloc.h>
#include<stdlib.h>
#include<string.h>
#include<errno.h>
 
int main()
{
 int* p = (int*)malloc(10 * sizeof(int));
 if (p == NULL)
 printf(&quot;%s\n&quot;, strerror(errno));
 else
 {
 int i = 0;
 for (i = 0; i < 10; i++)
 {
 *(p + i) = i; 
 }
 for (i = 0; i < 10; i++)
 {
 printf(&quot;%d &quot;, *(p + i));
 }
 }
 free(p);
 p = NULL;
 
 return 0;
}


注释：（1）Introducing header files is essential.

（2）I applied here40个字节空间大小（10*sizeof(int) ）,并用一个int *类型的指针pPoints to the first address of this space.

（3）ifThe statement judges that the memory application is unsuccessful,还记得我们上面说的吗？当没有足够的空间时,malloc会返回一个空指针（NULL）.

（4）elsein the sentence will0-9These numbers are put into these memories,i=0时

p=0,i=1时,

（p+1）=1.以此类推

（5）Print out the numbers put into this space.

（6）Free up this space,将p赋为空指针.

打印出的结果如下

注：If using a newer onevs（比如2022版）使用strerrorto be added at the top of the program

#define _CRT_SECURE_NO_WARNINGS

realloc

reallocThe function is to adjust the size of the dynamically allocated memory space.

Assume originally usedmalloc申请20字节空间,不够了,我希望有40字节空间.

Then you can use the following code

int* p = (int*)malloc(20);
int* p2 = realloc(p, 40);


注意事项：

Q：

Why use a new onep2不再用p？

（1）如果p指向的空间之后有足够的内存空间可以追加,则直接追加,后返回p.

（2）如果p指向的空间之后没有足够的空间可以追加,则rallocThe function will find a new memory area to open up a space that meets the needs,

并且把原来内存中的数据拷贝回来

,释放旧的内存空间,最后返回新开辟的内存空间地址.

Q：

If I still want to use the originalp不用p2怎么办？

（3）To accept as a new variablerealloc函数的返回值（The above is usedp2）

看代码如下

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<errno.h>
 
int main()
{
 int* p = (int*)malloc(20);
 
 if (p == NULL)
 {
 printf(&quot;%s\n&quot;, strerror(errno));
 }
 else
 {
 int i = 0;
 for (i = 0; i < 5; i++)
 {
 *(p + i) = i;
 }
 }
 
 int* p2 = realloc(p, 40);
 
 if (p2 != NULL)
 {
 p = p2; //把p2The first address was given againp,Connected
 int i = 0;
 for (i = 5; i < 10; i++)
 {
 *(p + i) = i;
 }
 for (i = 0; i < 10; i++)
 {
 printf(&quot;%d &quot;, *(p + i));
 }
 }
 
 free(p);
 p = NULL;
 
 return 0;
}