Dynamic memory allocation

Catalog

One 、malloc

Two 、free

3、 ... and 、calloc

Four 、realloc

---

 

The stack area	local variable 、 The formal parameter of the function
Heap area	Dynamic memory allocation （malloc,free,calloc,realloc）
Static zone	Global variables 、 Static variables

One 、malloc

form ：    void* malloc(size_t size)

//C99 Yes, variable length arrays can be created , Not all compilers support .gcc test_std=c99
#include<stdio.h>  //visual Compiler does not support , but gcc Compiler support 
#include<string.h>
int mian()
{
int n=0;
scanf("%d",&n);
int arr[n];
int i=0;
for(i=0;i<n;i++)
{arr[i]=i;
for(i=0;i<n;i++)
{
printf("%d ",arr[i]);
}
return 0;
}

#include<stdlib.h>
#include<string.h>
#include<errno.h>
#include<stdio.h>
int mian()
{
int *p=(int* )malloc(10*sizeof(int));// Integer pointer maintenance . Apply to memory 10 An integer space 
if(p==NULL)
{
printf("%s\n",strerror(errno));// One way to print the cause of the error , The requested space exceeds the memory space 
}
else
{
int i=0;
for(i=0;i<10;i++)
{
*(p+i)=i;
}
for(i=0;i<10;i++)
{
printf("%d ",*(p+i);
}
}
free(p)
return 0;
}

Explain the above code ： This function applies to memory for a contiguous block of free space , And return the pointer to this space

• If the development is successful , Then return a pointer to open a good space
• If the development fails , Returns a NULL Pointer therefore malloc The return value of must be checked
• The return value type is void*, therefore malloc Function doesn't know the type of open space , It is up to the user to decide when using
• If parameters size by 0,malloc Behavior is defined by standards , Depends on the compiler ,free Function is used to release memory opened dynamically
• If parameters ptr The pointed space is not opened dynamically , that free The behavior of a function is undefined
• If parameters ptr yes NULL The pointer , Then the function does nothing  

---

 

Two 、free

When the dynamically requested space is no longer used , It should be returned to the operating system .

free(p);

p=NULL;//p When the pointer is set to null ,p You can't find that storage space

malloc And free Use in pairs , Otherwise, it will cause memory leakage

---

3、 ... and 、calloc

form ： void* calloc(size_t num, size_t size))  Used for dynamic memory allocation

int *p=calloc(10,sizeof(int));

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

And functions malloc The only difference is calloc It initializes each byte of the requested space to... Before returning the address 0.

---

Four 、realloc

form ：  void *realloc(void* ptr,size_t size);   realloc Function can be used to adjust the size of dynamic development memory ;

ptr Is the memory address to be adjusted ,size Is the new size after adjustment , The return value is the starting memory position after adjustment , This function adjusts the size of the original memory space .

 realloc Precautions for use ：

1. If p There is enough space to add after the space pointed to , Then... Is added directly , After the return p;

2. If p There is not enough space to add after the space pointed to , be realloc The function will find a new memory area again , Open up a memory space to meet your needs , And copy the data in the original memory , Free up old memory space , Finally, return the newly opened memory space address ;

3. You have to accept... With a new variable realloc The return value of the function .

int main()
{
int *p=(int*)malloc(20);
if(p==NULL)
{
printf("%s\n",strerror(errno));
}
else
{
int i=o;
for(i=0;i<5;i++)
{
*(p+i)=i;
}
}
}
int *p2=realloc(p,40);
if(p2!=NULL) // Here we need to judge whether the newly opened space overflows 
{
p=p2;
}
int i=0;
for(i=5;i<10;i++)
{
*(p2+i)=i;
}
for(i=0;i<10;i++)
{
printf("%d ",*(p2+i));
}
free(p);
p=NULL;
return 0;
}