What is Bit Masking

Question

I am fairly new to C programming  and I encountered bit masking  Can someone explain to me the general concept and function of bit masking  Examples are much appreciated

User · Answer

Masking means to keep change remove a desired part of information  Lets see an image-masking operation  like- this masking operation is removing any thing that is not skin-    We are doing AND operation in this example  There are also other masking operators- OR  XOR     Bit-Masking means imposing mask over bits  Here is a bit-masking with AND-        1 1 1 0 1 1 0 1    input    amp    0 0 1 1 1 1 0 0     mask  ------------------------------      0 0 1 0 1 1 0 0   output     So  only the middle 4 bits  as these bits are 1 in this mask  remain   Lets see this with XOR-        1 1 1 0 1 1 0 1    input       0 0 1 1 1 1 0 0     mask  ------------------------------      1 1 0 1 0 0 0 1   output     Now  the middle 4 bits are flipped  1 became 0  0 became 1      So  using bit-mask we can access individual bits  examples   Sometimes  this technique may also be used for improving performance  Take this for example-  bool isOdd int i        return i 2      This function tells if an integer is odd even  We can achieve the same result with more efficiency using bit-mask-  bool isOdd int i        return i amp 1      Short Explanation  If the least significant bit of a binary number is 1 then it is odd  for 0 it will be even  So  by doing AND with 1 we are removing all other bits except for the least significant bit i e          55  - gt   0 0 1 1 0 1 1 1    input    amp     1  - gt   0 0 0 0 0 0 0 1     mask  ---------------------------------------       1   lt -  0 0 0 0 0 0 0 1   output

User · Answer

A mask defines which bits you want to keep  and which bits you want to clear   Masking is the act of applying a mask to a value   This is accomplished by doing    Bitwise ANDing in order to extract a subset of the bits in the value Bitwise ORing in order to set a subset of the bits in the value Bitwise XORing in order to toggle a subset of the bits in the value   Below is an example of extracting a subset of the bits in the value   Mask    00001111b Value   01010101b   Applying the mask to the value means that we want to clear the first  higher  4 bits  and keep the last  lower  4 bits  Thus we have extracted the lower 4 bits  The result is   Mask    00001111b Value   01010101b Result  00000101b   Masking is implemented using AND  so in C we get   uint8 t stuff          uint8 t mask   0x0f       00001111b   uint8 t value   0x55      01010101b   return mask  amp  value      Here is a fairly common use-case  Extracting individual bytes from a larger word   We define the high-order bits in the word as the first byte  We use two operators for this   amp   and  gt  gt   shift right    This is how we can extract the four bytes from a 32-bit integer   void more stuff uint32 t value                   Example value  0x01020304     uint32 t byte1    value  gt  gt  24                0x01020304  gt  gt  24 is 0x01 so                                                  no masking is necessary     uint32 t byte2    value  gt  gt  16   amp  0xff        0x01020304  gt  gt  16 is 0x0102 so                                                  we must mask to get 0x02     uint32 t byte3    value  gt  gt  8    amp  0xff        0x01020304  gt  gt  8 is 0x010203 so                                                  we must mask to get 0x03     uint32 t byte4   value  amp  0xff                here we only mask  no shifting                                                  is necessary             Notice that you could switch the order of the operators above  you could first do the mask  then the shift   The results are the same  but now you would have to use a different mask   uint32 t byte3    value  amp  0xff00   gt  gt  8

[c] What is Bit Masking?

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