Characteristics of pointer

• He's an address in memory
• Operation of pointer itself
• What the pointer points to is actionable

How the operating system manages memory

Stack space

• 4M~8m
• When entering the function, stack data will be pressed

Heap space

• 4g size 1g is the operating system
• global variable

Memory mapping

• You can modify the contents of the hard disk by modifying the contents of the memory
• Commonly used in database

Allocation and release of memory

• The method of allocating memory in c language

  //  Malloc (the size to be allocated); the allocated size here needs the exponent of 2 to be aligned
  void *mem = malloc(size);

• Free memory

  // Generally, the allocated content is in the heap space
  // If it is not released after use, memory leak and wild pointer will appear
  free(men);

• What is a memory leak:
  • Constantly apply memory to the system
  • The applied memory is not used or released
  • Never allow memory leaks
• What is the wild pointer
  • This pointer has been released
  • Someone else created this pointer again
  • In the past, I used this pointer again

Function pointer

int func(int x); // Declare a function
int (*f)(int x); // Declare a function pointer
f = func; // Assign the first address of func function to pointer f

#include <stdio.h>

int sum (int a, int b)
{
    return (a+b);
}

int main(int argc, int *argv[])
{
    // Define a function pointer 
    int (*f) (int, int);
    // f points to sum function
    f = sum;
    // Executing the f function is equivalent to executing the sum function
  int f_sum = f(2,3);
    printf("f_sum: %d\n", f_sum);
  
  return 0;
}

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    // Define two pointers of type int
    int *a, *b;
    // Making room in the heap through malloc
    a = (int*)malloc(sizeof(int));
    b = (int*)malloc(sizeof(int));
    *a = 1;
    *b = 2;
    // The address of pointer A is & A, a is the address pointing to space, * a is the value pointing to space
    printf("addr of a:%p, %p, %d\n", &a, a, *a);
    printf("addr of b:%p, %p, %d\n", &b, b, *b);
    return 0;
}

#include<stdio.h>
#include<stdlib.h>

int main(int argc, char *argv[])
{
    // Create an array c with 3 data, an array of int type takes up 4 bytes, and the address related space difference is 1 byte
    int c[3] = {1,2,3};
    printf("c Address:%p\t%p\tc[0]:%p\tc[1]:%p\tc[2]:%p\t\n",c, &c,  &c[0], &c[1], &c[2]);
    printf("%d, %d, %d\n", c[0], c[1], c[2]);
}