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

#include <stdio.h>

int main(void)
{
 int fd=open("file.txt",O_RDWR);
 struct stat infos;
 if(!fstat(fd,&infos))
 {
  off_t sz=infos.st_size, i;
  char * p=(char *)mmap((void *)0,sz,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);
  close(fd);
  if(p!=(char *)MAP_FAILED)
  {
   for(i=0;i<sz/2;i++) { char c=p[i]; p[i]=p[sz-i-1]; p[sz-i-1]=c; }
   munmap(p,sz);
  }
 }

 return(0);
}