#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

int main(void)
{
 int fd=open("file.txt",O_RDWR);
 off_t i, halfSize=lseek(fd,0,SEEK_END)/2;
 lseek(fd,0,SEEK_SET);

 for(i=0;i<halfSize;i++)
 {
  char c1,c2;
  read(fd,&c1,sizeof(char));
  lseek(fd,-(i+1),SEEK_END);
  read(fd,&c2,sizeof(char));
  lseek(fd,-sizeof(char),SEEK_CUR);
  write(fd,&c1,sizeof(char));
  lseek(fd,i,SEEK_SET);
  write(fd,&c2,sizeof(char));
 }

 close(fd);

 return(0);
}