C语言——动态内存管理

1. int val = 20;//在栈空间上开辟四个字节
2. char arr[10] = {0};//在栈空间上开辟10个字节的连续空间

void* malloc (size_t size);

void free (void* ptr);

#include <stdio.h>
int main()
{
//代码1
int num = 0;
scanf("%d", &num);
int arr[num] = {0};
//代码2
int* ptr = NULL;
ptr = (int*)malloc(num*sizeof(int));
if(NULL != ptr)//判断ptr指针是否为空
{
int i = 0;
for(i=0; i<num; i++)
{
*(ptr+i) = 0；
}
}
free(ptr);//释放ptr所指向的动态内存
ptr = NULL;//是否有必要？
return 0;
}

void* calloc (size_t num, size_t size);

#include <stdio.h>
#include <stdlib.h>
int main()
{
int *p = calloc(10, sizeof(int));
if(NULL != p)
{
//使用空间
}
free(p);
p = NULL;
return 0;
}

#include <stdio.h>
int main()
{
int *ptr = malloc(100);
if(ptr != NULL)
{
  //业务处理
}
else
{
  exit(EXIT_FAILURE);  
}
//扩展容量
//代码1
ptr = realloc(ptr, 1000);//这样可以吗？(如果申请失败会如何？)
//代码2
int*p = NULL;
p = realloc(ptr, 1000);
if(p != NULL)
{
ptr = p;
}
//业务处理
free(ptr);
return 0;
}

void test()
{
int *p = (int*)malloc(40);
  if (p == NULL)
  {
 
  }
*p = 20;
free(p);
}

int main()
{
  int* p = calloc(10, sizeof(int));
  if (p == NULL)
  {
    perror("calloc");
    return 1;
  }
 
  int i = 0;
  //越界了
  for (i = 0; i <= 10; i++)
  {
    p[i] = i;
  }
  //打印
  for (i = 0; i <= 10; i++)
  {
    printf("%d\n", *(p + i));
  }
  
  //释放
  free(p);
  p = NULL;
 
  return 0;
}

void test()
{
int a = 10;
int *p = &a;
free(p);//对非动态内存释放
}

void test()
{
int *p = (int *)malloc(100);
p++;
free(p);//p不再指向动态内存的起始位置
}

void test()
{
int *p = (int *)malloc(100);
free(p);
free(p);//重复释放
}

void test()
{
  int* p = (int*)malloc(40);
  //...
  if (3)
    return;
 
  free(p);
  p = NULL;
}
 
int main()
{
  test();
  //...
  while (1)
  {
    ;
  }
 
  return 0;
}

int* test()
{
  int* p = (int*)malloc(40);
  //...
  if (3)
    return p;
 
  free(p);
  p = NULL;
}
 
int main()
{
  int* ret = test();
  free(ret);
  ret = NULL;
  //...
  while (1)
  {
    ;
  }
 
  return 0;
}

void GetMemory(char *p)
{
p = (char *)malloc(100);
}
void Test(void)
{
char *str = NULL;
GetMemory(str);
strcpy(str, "hello world");
printf(str);
}

char* GetMemory()
{
  char* p = (char*)malloc(100);
  return p;
}
 
void Test(void)
{
  char* str = NULL;
  str = GetMemory();
  strcpy(str, "hello world");
  printf(str);//ok
 
  free(str);
  str = NULL;
}

char *GetMemory(void)
{
char p[] = "hello world";
return p;
}
void Test(void)
{
char *str = NULL;
str = GetMemory();
printf(str);
}

int* test()
{
  int a = 10;
  return &a;
}
 
int main()
{
  int* p = test();
  printf("haha\n");
  printf("%d\n", *p);
 
  return 0;
}

int test()
{
  int a = 10;
  int b = 20;
  int c = a + b;
  return c;
}
int main()
{
  int d = test();
 
  return 0;
}

void GetMemory(char **p, int num)
{
*p = (char *)malloc(num);
}
void Test(void)
{
char *str = NULL;
GetMemory(&str, 100);
strcpy(str, "hello");
printf(str);
//free(str);
//str = NULL;
}

void Test(void)
{
char *str = (char *) malloc(100);
strcpy(str, "hello");
free(str);
if(str != NULL)
{
strcpy(str, "world");
printf(str);
}
}

void Test(void)
{
  char* str = (char*)malloc(100);
 
  strcpy(str, "hello");
  free(str);
  str = NULL;
 
  if (str != NULL)
  {
    strcpy(str, "world");
    printf(str);
  }
}

1. typedef struct st_type
2. {
3. int i;
4. int a[];//未知大小的数组 - 柔性数组成员
5. }type_a;

1. typedef struct st_type
2. {
3. int i;
4. int a[0];//柔性数组成员
5. }type_a;
6. printf("%d\n", sizeof(type_a));//输出的是4

//代码1
int i = 0;
type_a *p = (type_a*)malloc(sizeof(type_a)+100*sizeof(int));
//业务处理
p->i = 100;
for(i=0; i<100; i++)
{
p->a[i] = i;
}
free(p);

//代码2
typedef struct st_type
{
int i;
int *p_a;
}type_a;
type_a *p = malloc(sizeof(type_a));
p->i = 100;
p->p_a = (int *)malloc(p->i*sizeof(int));
//业务处理
for(i=0; i<100; i++)
{
p->p_a[i] = i;
}
//释放空间
free(p->p_a);
p->p_a = NULL;
free(p);
p = NULL