#include<stdio.h>
int main()
{
    //fopen
    FILE *p = fopen("filename","r");
    char str[100] = {0};
    fscanf(p,"%s",str);
    
    //freopen
    freopen("filename","r",stdin);
    scanf("%s",str);
}

#define
#include

#define PI 3.1415926//不含参数的构造,语句结束不需要分号,#include亦然
//编译器会直接将所有的PI替换为3.1415926
#define R 3.0
#define L 2*PI*R
//以上的两行也是可以的,宏定义会展开.
printf("L");//格式控制输出时不置反

#define S(a,b) a*b//含参构造,遇到S(数字,数字),替换为a*b

#undefine PI

#include<file name>
#include"file name"//偏向于用户自定义部分

#ifdef //flag
#define LETTER 1

#if LETTER
//code
#else
//code

int a=3;
int *ptr = &a;//ptr is a variable that points to the address of a
*ptr;//use * to access the data at the address which ptr points to.
*(ptr++);//returns *ptr,and then ptr++

int a[]={1,2,3,5,2,6,2,5}
int *ptr = &a;//the address of an array is just same as the address of the first element of the array.
printf("%d",*ptr++); //the ptr++ refers the pointer is going to point to the next element of the array

int *p[4];//a define of a pointer array
char *p[4]={"Brazil","Russia","India","China"};//a array of pointer,each points to a string.

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main()
{
    char *p[]={"Aa","Bb","Cc","Dd"};
    for(int i=0;i<4;i++)
    {
        int pos =i;
        char max = *p[i];
        for(int j=i;j<4;j++)
        {
            if(*p[j]>max)
            {
                max=*p[j];
                pos=j;
            }
        }
        char* t= p[i];
        p[i]=p[pos];
        p[pos]=t;
    }

    
    return 0;
}

int *p = (int*)malloc(4);; //set a space whose size is 4.
int *p[4] = (int*)calloc(n,sizeof(int));//set n*size array.
realloc(p,8);//resize.
free(p);//delete.

int **p;
**p=2;
*p;//an address
**p;//a value

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main(int argc,char* argv[])//argc refers string numbers and argv refers string that put in.
    //argv[0]=your program name
{
    for(int i=1;argv[i]!=NULL;i++)
    printf("%s\n",argv[i]);

    
    return 0;
}

int main()
{
    struct Student *head;
    head = (struct Student *)malloc(sizeof(struct Student));
}

int main()
{
    head = (struct Student*)calloc(100,sizeof(struct Student))
}

int main()
{
    realloc(*p,size);
}

int main()
{
    free(*p);
}

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

struct Student 
{
    int age=0;
    struct Student *next=NULL;
};

int main()
{
    struct Student *head,*current;
    int i=0;
    head = (struct Student*)malloc(sizeof(struct Student));
    current=head;
    while(i==0)
    {
        printf("Add student? 0/yes 1/no\n");
        scanf("%d",&i);
        if(i==0)
        {
            printf("input age\n");
            scanf("%d",&(*current).age);
            (*current).next=(struct Student*)malloc(sizeof(struct Student));
            current = (*current).next;
        }else
        {
            (*current).age=0;
            (*current).next=NULL;
        }
        
    }
    return 0;
}

FILE *fp;//create a pointer point to the head of the file.
fp = fopen("file.txt","a+");//open the file.
fclose(fp);

//freopen
freopen("File","a+",stdin);
freopen("File","w",stdout);
fclose(ptr);//or the file will not be write.

fgetc(fp);
fputc(ch,fp);
//read and write character in a file.

fgets(str,n,fp);//See carefully, there's n in parameter list, n refers the string's length is n-1 .
fputs(str,fp);

fscanf(fp,"Format",parameter list);
fprintf(fp,"Format",parameter list);

//read and write in bit
fread(buffer,size,count,fp);
fwrite(buffer,size,count,fp);

//examples:
fread(&stu_list[i],sizeof(struct student),1,fp);
fwrite(&stu_list[i],sizeof(struct student),1,fp);//you can see it as using disk as memory.

rewind(fp);//move the file flag back to the head.
fseek(fp,100L,0);//move the flag to the position 100 byte away from the head.
fseek(fp,100L,1);//move foward 100 byte from current position
fseek(fp,-50L,3);//move back 50 byte from the end.