How to use bitmask

Question

How do i use it in C     when is it useful to use   Please give me an example of a problem where bitmask is used   how it actually works   Thanks

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Let s say I have 32-bit ARGB value with 8-bits per channel  I want to replace the alpha component with another alpha value  such as 0x45  unsigned long alpha   0x45 unsigned long pixel   0x12345678  pixel     pixel  amp  0x00FFFFFF     alpha  lt  lt  24      The mask turns the top 8 bits to 0  where the old alpha value was  The alpha value is shifted up to the final bit positions it will take  then it is OR-ed into the masked pixel value  The final result is 0x45345678 which is stored into pixel

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Briefly bitmask helps to manipulate position of multiple values  There is a good example here     Bitflags are a method of storing multiple values  which are not mutually exclusive  in one variable  You ve probably seen them before  Each flag is a bit position which can be set on or off  You then have a bunch of bitmasks  defined for each bit position so you can easily manipulate it        define LOG ERRORS            1     2 0  bit 0      define LOG WARNINGS          2     2 1  bit 1      define LOG NOTICES           4     2 2  bit 2      define LOG INCOMING          8     2 3  bit 3      define LOG OUTGOING         16     2 4  bit 4      define LOG LOOPBACK         32     and so on        Only 6 flags bits used  so a char is fine unsigned char flags      initialising the flags    note that assigning a value will clobber any other flags  so you    should generally only use the   operator when initialising vars  flags   LOG ERRORS     sets to 1 i e  bit 0    initialising to multiple values with OR     flags   LOG ERRORS   LOG WARNINGS   LOG INCOMING     sets to 1   2   8 i e  bits 0  1 and 3     setting one flag on  leaving the rest untouched    OR bitmask with the current value flags    LOG INCOMING      testing for a flag    AND with the bitmask before testing with    if   flags  amp  LOG WARNINGS     LOG WARNINGS             testing for multiple flags    as above  OR the bitmasks if   flags  amp   LOG INCOMING   LOG OUTGOING                LOG INCOMING   LOG OUTGOING              removing a flag  leaving the rest untouched    AND with the inverse  NOT  of the bitmask flags  amp    LOG OUTGOING      toggling a flag  leaving the rest untouched flags    LOG LOOPBACK             WARNING  DO NOT use the equality operator  i e  bitflags    bitmask    for testing if a flag is set - that expression will only be true if   that flag is set and all others are unset  To test for a single flag   you need to use  amp  and            if  flags    LOG WARNINGS    DON T DO THIS        if   flags  amp  LOG WARNINGS     LOG WARNINGS     The right way        if   flags  amp   LOG INCOMING   LOG OUTGOING      Test for multiple flags set              LOG INCOMING   LOG OUTGOING            You can also search C   Triks

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Bitmasks are used when you want to encode multiple layers of information in a single number   So  assuming unix file permissions  if you want to store 3 levels of access restriction  read  write  execute  you could check for each level by checking the corresponding bit   rwx --- 110   110 in base 2 translates to 6 in base 10   So you can easily check if someone is allowed to e g  read the file by and ing the permission field with the wanted permission   Pseudocode   PERM READ   4 PERM WRITE   2 PERM EXEC   1  user permissions   6  if  user permissions  amp  PERM READ    TRUE  then      this will be reached  as 6  amp  4 is true fi   You need a working understanding of binary representation of numbers and logical operators to understand bit fields

User · Answer

Bit masking is  useful  to use when you want to store  and subsequently extract  different data within a single data value   An example application I ve used before is imagine you were storing colour RGB values in a 16 bit value  So something that looks like this   RRRR RGGG GGGB BBBB   You could then use bit masking to retrieve the colour components as follows     const unsigned short redMask     0xF800    const unsigned short greenMask   0x07E0    const unsigned short blueMask    0x001F     unsigned short lightGray   0x7BEF     unsigned short redComponent      lightGray  amp  redMask   gt  gt  11    unsigned short greenComponent    lightGray  amp  greenMask   gt  gt  5    unsigned short blueComponent     lightGray  amp  blueMask

[c++] How to use bitmask?

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