全局 和 静态变量会自动初始化为零值
局部变量需要显示初始化，否则值不确定

定义时初始化 


int    i  = 0;
float  f = 0.00f;
double d = 0.00;
char ch = '\0'; 
char str[10] = "";

char str[10];
memset(str, 0, sizeof(str));

memset 按字节进行填充 注意是按字节填充

int num;
memset(&num, 0, sizeof(int));
printf("step1=%d\n", num);
memset(&num, 1, sizeof(int));
printf("step2=%d\n", num);

step1 = 0
step2 = 16843009

int  4个字节
00000000 00000000 00000000 00000000
00000001 00000001 00000001 00000001

char year[4+1];
memset(year, 0, sizeof(year));
strcpy(year,"2021");

指针一般初始化为 NULL 

int *pnum = NULL;
int num = 0;
pnum = &num ;

typedef struct student
{
 int id;
 char name[20];
}Student;
Student student1;
memset((char *)&student1, 0, sizeof(student1));




https://gitee.com/dyyx/hellocode/blob/master/web/tech/c/demo/init.c


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

typedef struct student
{
 int id;
 char name[20];
}Student;

int main(int argc,char *argv[]){

  int    i  = 0;
  float  f = 0.00f;
  double d = 0.00;
  char ch = '\0'; 
  char str[3] = "";
  char str2[] = "hello";
  
  char str3[7];
  memset(str3, 0, sizeof(str3));
  
  char str4[7]={0};
  
  int num;
  memset(&num, 0, sizeof(int));
  printf("step1=%d\n", num);
  memset(&num, 1, sizeof(int));
  // 16843009
  // memset 按字节填充
  // 00000001 00000001 00000001 00000001
  printf("step2=%d\n", num);

  char year[4+1];
  memset(year, 0, sizeof(year));
  strcpy(year,"2021");
  
  printf("year=%s\n",year);
  
  int *pnum = NULL;
  pnum = &num  ;
  
  printf("%p\n",pnum);
  
  
  Student student1;
  memset((char *)&student1, 0, sizeof(student1));
  
  printf("id=%d, name=%s\n", student1.id,student1.name);
  
  Student student2={0};
  printf("id=%d, name=%s\n", student2.id,student2.name);
  
}