Shift operator and bitwise or

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?

Please explain the output you get and why you think it's wrong. Or at least ask a more specific question. — Carl Norum, Nov 22 '10 at 07:16
Please read up on [bitwise operations](http://en.wikipedia.org/wiki/Bitwise_operation). — wkl, Nov 22 '10 at 07:16
See http://stackoverflow.com/questions/276706/what-are-bitwise-operators — kennytm, Nov 22 '10 at 07:18

score 4 · Answer 1 · edited Nov 22 '10 at 08:19

4

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.

edited Nov 22 '10 at 08:19

Martin York

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answered Nov 22 '10 at 07:18

Simone

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score 2 · Answer 2 · answered Nov 22 '10 at 07:23

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

score 2 · Answer 3 · answered Nov 22 '10 at 07:24

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.

score -1 · Answer 4 · answered Nov 22 '10 at 07:20

-1

Break it down by steps:

It reads 4 8 bit bytes from the file pointed to by hTTF or returns EOF;
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.

answered Nov 22 '10 at 07:20

the wolf

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Shift operator and bitwise or

4 Answers4