What does this ">> =" operator mean in C?

Question

unsigned long set; /*set is after modified*/ set >>= 1; 

I found this in a kernel system call, but I don’t understand how it works?

Answer 1

The expression set >>= 1; means set = set >> 1; , i.e. right shift bits set by 1 (self-tuning form >> bitwise shift shift operator Bitwise shift operators ).

Assume set :

 BIT NUMBER 31 n=27 m=17 0 ▼ ▼ ▼ ▼ set = 0000 1111 1111 1110 0000 0000 0000 0000 

Then after set >> = 1; the set variable becomes:

 BIT NUMBER 31 n=26 m=16 0 ▼ ▼ ▼ ▼ set = 0000 0111 1111 1111 0000 0000 0000 0000 

Note that the number of bits is shifted.

Pay attention to an interesting point: since set is unsigned long , this operation >> must be a logical shift ( unsigned shift ), a logical shift does not save the bit of the sign of a number.

Also, since you are shifting all the bits to the right (towards a lower significant number), therefore, one right shift = division by two.

check this code (just to demonstrate the last point):

 int main(){ unsigned long set = 268304384UL; set >>= 1; printf(" set :%lu \n", set); set = 268304384UL; set /= 2; printf(" set :%lu \n", set); return 1; } 

And the conclusion:

  set :134152192 set :134152192 

(note: it does not mean >> and / both)

Similarly, you have the <<= operator for left shift, check the other available Bitwise operators and Compound assignment operators , also check the section: bit expressions and the difference between: signed / arithmetic shift and unsigned shift .

Answer 2

This “right shift” means a value of one bit. If you move all the bits of an integer to the right by 1, you actually "divide by 2" because the binary code is a numbering system for base 2.

Imagine you have number 12 in binary format:

 1100 = 12 in binary 110 = 6 in binary (1100 right-shifted) 

Just as if you moved all the numbers in base-10 to the right by one, you would divide by 10.

Answer 3

This shifts the bit to the right by 1, which is equivalent to dividing by 2. For more information on bit offsets, see http://msdn.microsoft.com/en-us/library/f96c63ed(v=vs.80).aspx

Answer 4

Each binary operator can be combined with = . In all cases

 dest op= expression 

equivalently

 dest = dest op expression 

(except when dest has any side effects, they occur only once).

So that means that

 set>>=1; 

is equivalent to:

 set = set >> 1; 

Since >> is a binary operator with a right shift, this means shifting the value in set right by 1 bit.

Answer 5

The above command performs a right shift of one bit. Repeat bit wise operations in c from this link http://www.cprogramming.com/tutorial/bitwise_operators.html