程序代码：

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define MAXLEN 100
void charge(char argv[],int n,int key)
{
     int i;
     for(i=0;i<n;i++)
     argv[i]=argv[i]+key;
}
void xxx(int fd,int fd1,char buff[],int maxlen,int key)
{
    int n;
while((n=read(fd1,buff,maxlen))>0){
                   if(n!=maxlen){
                   charge(buff,n,key);
                   if(write(fd,buff,n)!=n){
                   printf("xie ru shuju shibai");
                   exit(0);
                  }
          }
          else{
                   charge(buff,maxlen,key);
                   if(write(fd,buff,maxlen)!=maxlen){
                   printf("xie ru shuju shibai");
                          exit(0);
          }
         }
          if(n<0)
                  printf("du shibai");
    }
}
int main()
{
      int fd;
      int fd1;
      int fd2;
      char buff[MAXLEN];
      int n,keyback=-16,key=16;
      if((fd=open("kle.txt",O_RDWR))==-1)
            printf("open fail\n");
      if((fd1=open("readme.txt",O_RDONLY))==-1)
           printf("readme.txt open faile!!\n");
      if((fd2=open("mmt.txt",O_RDWR))==-1)
           printf("open fail\n");
      xxx(fd,fd1,buff,MAXLEN,key);
      lseek(fd,0,SEEK_SET);
      xxx(fd2,fd,buff,MAXLEN,keyback);
      printf("xieruchenggong\n");
      exit(0);
}