c++ - What is most efficient way to read a file of integer in each line without opening it?

What is most efficient way to read a file of integer in each line without opening it?

I have a file with ONLY integer in each line, ie: num.txt

100
231 
312
...

In my program, I use while loop to read it;

int input = 0;
while(cin >> input)
{        
// Assignment
}

I use time a.out to read it in Linux
It turns out that it will take about 15 second (user time) to read 100 million numbers. So I was wondering is there any better way to decrease user time?

Thank you in advance!

Answer

int input = 0;
ios_base::sync_with_stdio(false); 
//Add this statement and see the magic!
while(cin >> input)
{        
// Assignment
}

To make it ultra fast (Not recommended for assignments!), use getchar_unlocked():

int read_int() {
  char c = getchar_unlocked();
  while(c<'0' || c>'9') c = gc();
  int ret = 0;
  while(c>='0' && c<='9') {
    ret = 10 * ret + c - 48;
    c = getchar_unlocked();

  }
  return ret;
}

int input = 0;
while((input = read_int()) != EOF)
{        
// Assignment
}

Vaughn Cato's answer explains it beautifully.