这个“>> =”运算符在C中意味着什么？

Question

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

I found this in a kernel system call but I don't understand, how does it work?

Answer 1

The expression set >>= 1; means set = set >> 1; that is right shift bits of set by 1 (self assigned form of >> bitwise right shift operator check Bitwise Shift Operators ).

Suppose if set is:

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

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

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

Notice the bits number shifted.

Note a interesting point: Because set is unsigned long so this >> operation should be logical shift ( unsigned shift ) a logical shift does not preserve a number's sign bit.

Additionally, because you are shifting all bits to right (towards lower significant number) so one right shift is = divide number by two.

check this code (just to demonstrate 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 output:

 set :134152192 
 set :134152192

(note: its doesn't means >> and / are both same)

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

Answer 2

This "right-shift"s the value by one bit. If you move all the bits of an integer to the right by 1 then you effectively "divide by 2" because binary is a base-2 numbering system.

Imagine you have the number 12 in binary:

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

Just like if you moved all of the digits in a base-10 number right by one you would be dividing by 10.

Answer 3

这会将位向右移1，相当于除以2.有关位移的更多信息，请参阅http://msdn.microsoft.com/en-us/library/f96c63ed(v=vs.80）。 ASPX

Answer 4

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

dest op= expression

is equivalent to

dest = dest op expression

(except if dest has any side effects, they only take place once).

So this means that

set>>=1;

is equivalent to:

set = set >> 1;

Since >> is the binary right-shift operator, it means to shift the value in set right by 1 bit.

Answer 5

上面的命令执行右移一位。从此链接中查看 c中的位操作http://www.cprogramming.com/tutorial/bitwise_operators.html