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//myuc.h

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<fcntl.h>
#include<string.h>
#include<sys/types.h>
#include<sys/mman.h>
#include<sys/stat.h>

code1

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
//#include<sys/types.h>
//#include<sys/stat.h>
#include<string.h>
#include<fcntl.h>
int main(){
//系统会减去other组的写权限
int fd=open("open4.txt",O_RDWR|O_CREAT|O_TRUNC,0777);
if(fd==-1)
perror("open"),exit(-1);//连接两个语句
char buf[100]={};
strcpy(buf,"mybuff1\n");
write(fd,buf,strlen(buf));
printf("openok,fd=%d\n",fd);
close(fd);
int fd2=open("open4.txt",O_RDONLY);
if(fd2==-1) perror("open read"),exit(-1);
memset(buf,0,sizeof(buf));
int readcount=read(fd,buf,sizeof(buf));
printf("fd2:%d,readed(%d):%s\n",fd2,readcount,buf);
close(fd2);
fd2=open("a.out",O_RDONLY);
if(fd2==-1) perror("open src"),exit(-1);
fd=open("a.out1",O_RDWR|O_CREAT|O_TRUNC,0777);
if(fd==-1) perror("write dest"),exit(-1);
//readcount=0;
while(1){
readcount=read(fd2,buf,sizeof(buf));
if(readcount==0) break;
else if(readcount==-1){
perror("read src");break;
}
   write(fd,buf,readcount);
}
//while(readcount>0);
close(fd);
close(fd2);
printf("文件复制成功！\n");
return 0;
}

code2

#include "myuc.h"
int main(){
int fd=open("b.txt",O_RDWR);
if(fd==-1) perror("open"),exit(-1);
char ch;
read(fd,&ch,1);
printf("%c\n",ch);
read(fd,&ch,1);
printf("%c\n",ch);
lseek(fd,7,SEEK_SET);
read(fd,&ch,1);
printf("%c\n",ch);
lseek(fd,0,SEEK_SET);//回到开始
write(fd,"1",1);//替换a
lseek(fd,3,SEEK_SET);
write(fd,"2",1);//替换d
lseek(fd,-2,SEEK_END);//注意最后的\n,也是一个字节
write(fd,"3",1);//替换到数第二个
int len=lseek(fd,0,SEEK_END);//文件大小
close(fd);
return 0;
}

code3

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
//#include<sys/types.h>
//#include<sys/stat.h>
#include<string.h>
#include<fcntl.h>
typedef struct EMP{
int id;
char name[100];
double salary;
}emp;
int main(){
int fd=open("write4.txt",O_CREAT|O_RDWR|O_APPEND,0777);
if(fd==-1) perror("open"),exit(-1);
printf("请输入编号，姓名，薪水：\n");
emp em;
scanf("%d%s%lf",&em.id,em.name,&em.salary);
int res=write(fd,&em,sizeof(emp));
if(res==-1) printf("失败\n");
else printf("成功！\n");
close(fd);
fd=open("write4.txt",O_RDONLY);
while(1)
{
res=read(fd,&em,sizeof(em));
if(res==0) break;
else if(res==-1) {
perror("read");break;
}
printf("%d,%s,%g\n",em.id,em.name,em.salary);
}
close(fd);
return 0;
}

code4

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<fcntl.h>
void w1()//系统调用多，花时间多
{
int fd=open("a.txt",O_CREAT|O_TRUNC,0777);
if(fd==-1) perror("openwrite1"),exit(-1);
int i;
for(i=0;i<1000000;i++){
write(fd,&i,4);
}
}
void w2(){//系统调用少，省时间
int fd=open("a.txt",O_RDWR|O_TRUNC);
if(fd==-1) perror("openwrite2"),exit(-1);
int arr[10000];
int i;
for(i=0;i<1000000;i++){
arr[i%10000]=i;
if(i%10000==9999) write(fd,arr,sizeof(arr));//10000个数字才写入一次
}
close(fd);
}
int main()
{
//w1();
w2();
}