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

int copy_from_back(FILE * srcfd, FILE * dstfd);

int main(int argc, char *argv[])
{
 FILE *srcfd = NULL;
 FILE *dstfd = NULL;
 
 if (argc != 3)
 {
  printf("usage: FileStudy srcFiel dstFile\n");
 }

 srcfd = fopen(argv[1], "r");
 dstfd = fopen(argv[2], "w+");

 if (copy_from_back(srcfd, dstfd) == -1)
 {
  printf("copy error: ");
  exit(EXIT_FAILURE);
 }

 return 0; 
}

int copy_from_back(FILE *srcfd, FILE *dstfd)
{
 char ch;
 int num_read = 0;
 
 if (srcfd == NULL || dstfd == NULL)
 {
  perror("fopen error: ");
  return -1;
 }

 fseek(dstfd, 0, SEEK_SET);
 
 /*指向文件末尾，如果继续读，返回EOF*/
 if (fseek(srcfd, 1, SEEK_END) != 0)
 {
  perror("fseek error");
 } 
 else
 {
  
  printf("begin to read file\n");
  while (ftell(srcfd) > 0)
  {
   char ch = fgetc(srcfd);
   if (ch == EOF)
   {
    printf("file end!\n");
    fseek(srcfd, -2, SEEK_CUR);
    continue;
   }
   putchar(ch);
   fputc(ch, dstfd);
   num_read++;
   fseek(srcfd, -2, SEEK_CUR);
  }

  char start = fgetc(srcfd);
  putchar(start);
  printf("\nread file end: %d \n", num_read);
 }

 return 0;
}

/*
int copy_from_back(FILE *srcfd, FILE *dstfd)
{
 char ch;
 
 if (srcfd == NULL || dstfd == NULL)
 {
  perror("fopen error: ");
  return -1;
 }
 
 fseek(srcfd, 0, SEEK_SET);
 
 while ((ch = fgetc(srcfd)) != EOF)
 { 
  putchar(ch);
  
  fputc(ch, dstfd); 
  fseek(dstfd, 1, SEEK_SET);
 }

 return 0;
}
*/