Reading input using getchar_unlocked()

Question

I have learnt that using getchar_unlocked is fast way of reading input. I have seen the code to read at many places but was unable to understand. Can anyone please help me understand how to read using getchar_unlocked ? Thanks in Advance.

void scanint(int &x)
{
 register int c = getchar_unlocked();
 x = 0;
 for(;(c<48 || c>57);c = getchar_unlocked())
  ;
 for(;c>47 && c<58;c = getchar_unlocked()) 
 {
   x = (x<<1) + (x<<3) + c - 48;
 }
}

I have seen many other codes as well. I dont particularly understand the purpose of shifting the number. Any help regarding that is appreciated

Answer 1

getch_lock reads a character at a time. here in the given code we are trying to read an integer. the purpose of first for loop is to read digit character if any present and neglect it. The second for loop reads a char which must be digit and performs
n=n*10+c
As C is in Ascii we have subtracted 48 ie Ascii code of '0' . To make code faster instead of using multiplication shift is used.
n*10=n*(8+2)=n*8+n*2=n<<3+n<<1

Answer 2

getchar_unlocked() is like getchar() except that it does not check for multi-thread locks.

So, it is faster, but it is not thread-safe.