移位运算符和按位或

Question

I have four characters as ch1,ch2,ch3,ch4. I am reading a binary file. Que- What following code indicates?

int GetLong(FILE * hTTF) 
{
    int ch1 = getc(hTTF);
    int ch2 = getc(hTTF);
    int ch3 = getc(hTTF);
    int ch4 = getc(hTTF);

 if ( ch4 == EOF )
  return( EOF );

 return( (ch1<<24)|(ch2<<16)|(ch3<<8)|ch4 );
}

consider ch1='k',ch2='e',ch3='r',ch4='n'; Tell me output and why this is so?. I am not understanding output value. Que-What is this conversion (ch1<<24)|(ch2<<16)|(ch3<<8)|ch4 What we achive by doing this?

Answer 1

The fact that ch[1234] are characters is not relevant: they are just numeric values.

Just think it's something like this:

ch1 = 0x10;
ch2 = 0x20;
ch3 = 0x30;
ch4 = 0x40;

your output value will be hex value 0x10203040.

Answer 2

What is output is a single int that has four chars inside of it. You can think of it like this:

My four chars are: 0x00, 0x02, 0x53, 0xEF

ch1 << 24 = 0x 00 000000

ch2 << 16 = 0x00 02 0000

ch3 << 8 = 0x0000 53 00

ch4 = 0x000000 EF

Next with bitwise ors.

x | 0 = x
1 | x = 1

So:

0x00000000
0x00020000
0x00005300
0x000000EF
----------
0x000253EF

Answer 3

The return will be a 32 bit value where the most significant 8 bits is ch1, next 8 bits is ch2 and so on. The << operator is a shift left operator, so if (in binary)

ch1 = 10101010

then (dots added for readability)

ch1 << 24 = 10101010.00000000.00000000.00000000 

and so on. The | operator is an bitwise OR operator, so it just combines the variously shifted ch values.

Answer 4

Break it down by steps:

1. It reads 4 8 bit bytes from the file pointed to by hTTF or returns EOF;
2. If it can read those 4 bytes, it creates a 4 byte value by bit or'ing the 4 bytes together after rotating each it turn.