【C语言】10-动态内存分配
此笔记由个人整理
塞上苍鹰_fly
课程来自：尚观C
一、动态内存
void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb,size_t size);
void *realloc(void *ptr,size_t size);
案例：申请空间与释放空间
#include "stdio.h"
#include "stdlib.h"
int main()
{
int *p = NULL;
p = (int *)malloc(sizeof(int));//测试环境为Windows
//p = malloc(sizeof(int));//测试环境为Linux
if(p == NULL)
{
printf("malloc() error!!\n");
exit(1);
}
*p = 10;
printf("%p-->%d\n",*p,*p);
free(p);
*p = 123;
printf("%p-->%d\n",*p,*p);
exit(0);
}
free是通过之前使用的指针不能再去到之前所指向的地址
free后不要在继续使用之前定义的空间，会产生野指针，容易出现不必要的错误
- 结果（windows）
案例：动态变长数组
#include "stdio.h"
#include "stdlib.h"
int main()
{
int *p = NULL;
int num = 5;
int i;
p = malloc(sizeof(int) * num);
for(i = 0;i < num;i++)
scanf("%d",&p[i]);
for(i = 0;i < num;i++)
printf("%d",p[i]);
printf("\n");
exit(0);
}
- 结果
案例：传参避免内存泄露
#include "stdio.h"
#include "stdlib.h"
void func(int **p,int n)
{
*p = malloc(n);
if(*p == NULL)
exit(1);
return ;
}
int main()
{
int *p = NULL;
int num = 100;
func(&p,num);
free(p);//谁申请谁释放
exit(0);
}
#include "stdio.h"
#include "stdlib.h"
void *func(int *p,int n)
{
p = malloc(n);
if(p == NULL)
exit(1);
return p;
}
int main()
{
int *p = NULL;
int num = 100;
p = func(&p,num);
free(p);//谁申请谁释放
exit(0);
}
二、优化学生信息管理系统
- 替换结构体成员变量name为*name
#include "stdio.h"
#include "stdlib.h"
#include "string.h"
#define NAMEMAX 1024
struct student_st
{
int id;
char *name;
int math;
int chinese;
};
void menu(void)
{
printf("********************************\n");
printf("1 set\n2 change name\n3 show\n");
printf("********************************\n");
}
void stu_set(struct student_st *p,const struct student_st *q)
{
p->id = q->id;
p->name = malloc(strlen(q->name)+1);
if(p->name == NULL)
exit(1);
strcpy(p->name,q->name);
p->math = q->math;
p->chinese = q->chinese;
}
void stu_show(struct student_st *p)
{
printf("%d %s %d %d\n",p->id,p->name,p->math,p->chinese);
}
void stu_changename(struct student_st *p,const char *newname)
{
free(p->name);
p->name = malloc(strlen(newname)+1);
strcpy(p->name,newname);
}
int main()
{
int choice;
int ret;
char newname[NAMEMAX];
struct student_st stu;
struct student_st tmp;
menu();
do
{
printf("\nPlease enter num(q for quit):");
ret = scanf("%d",&choice);
if(ret != 1)
break;
switch(choice)
{
case 1:
tmp.name = malloc(NAMEMAX);
printf("Please enter for the stu[id name math chinese]:");
scanf("%d %s %d %d",&tmp.id,&tmp.name,&tmp.math,&tmp.chinese);
stu_set(&stu,&tmp);
break;
case 2:
printf("Please enter the newname:");
scanf("%s",newname);
stu_changename(&stu,newname);
break;
case 3:
stu_show(&stu);
break;
default:
exit(1);
}
}while(1);
exit(0);
}
三、typedef
- 为已有的数据类型改名
typedef 已有的数据类型 新名字
案例
#include "stdio.h"
#include "stdlib.h"
typedef int INT;
int main()
{
INT i = 100;
printf("%d\n",i);
exit(0);
}
- 结果
typedef用法
#define INT int
typedef int INT;
INT i; --> int i;
---------------------------------------
#define IP int *
typedef int IP;
IP p,q; --> int *p,q;
IP p,q; --> int *p,*q;
----------------------------------------
typedef int ARR[6]; --> int [6] ->ARR;
ARR a; --> int a[6];
----------------------------------------
struct node_st{
};
typedef struct node_st NODE;
NODE a; -> struct node_st a;
NODE *P;-> struct node_st *p;
typedef struct node_st *NODEP;
NODEP p; -> struct node_st *p;
----------------------------------------
typedef struct
{
int i;
float f;
}NODE,*NODEP;
----------------------------------------
typedef int FUNC(int); --> int(int) FUNC;
FUNC F; -> int f(int);
typedef int *FUNCP(int);
FUNC p; ---> int *p(int);
typedef int *(*FUNCP)(int);
FUNC p; --> int *(*p)(int);
案例
#include "stdio.h"
#include "stdlib.h"
#include "string.h"
#define NAMEMAX 1024
typedef struct student_st
{
int id;
char *name;
int math;
int chinese;
}STU;
void menu(void)
{
printf("********************************\n");
printf("1 set\n2 change name\n3 show\n");
printf("********************************\n");
}
void stu_set(STU *p,const STU *q)
{
p->id = q->id;
p->name = malloc(strlen(q->name)+1);
if(p->name == NULL)
exit(1);
strcpy(p->name,q->name);
p->math = q->math;
p->chinese = q->chinese;
}
void stu_show(STU *p)
{
printf("%d %s %d %d\n",p->id,p->name,p->math,p->chinese);
}
void stu_changename(STU *p,const char *newname)
{
free(p->name);
p->name = malloc(strlen(newname)+1);
strcpy(p->name,newname);
}
int main()
{
int choice;
int ret;
char newname[NAMEMAX];
STU stu;
STU tmp;
menu();
do
{
printf("\nPlease enter num(q for quit):");
ret = scanf("%d",&choice);
if(ret != 1)
break;
switch(choice)
{
case 1:
tmp.name = malloc(NAMEMAX);
printf("Please enter for the stu[id name math chinese]:");
scanf("%d %s %d %d",&tmp.id,&tmp.name,&tmp.math,&tmp.chinese);
stu_set(&stu,&tmp);
break;
case 2:
printf("Please enter the newname:");
scanf("%s",newname);
stu_changename(&stu,newname);
break;
case 3:
stu_show(&stu);
break;
default:
exit(1);
}
}while(1);
exit(0);
}