How do I extract specific n bits of a 32-bit unsigned integer in C

Question

Could anyone tell me as to how to extract  n  specific bits from a 32-bit unsigned integer in C      For example  say I want the first 17 bits of the 32-bit value  what is it that I should do  I presume I am supposed to use the modulus operator and I tried it and was able to get the last 8 bits and last 16 bits as  unsigned last8bitsvalue  32 bit integer    16 unsigned last16bitsvalue  32 bit integer    32   Is this correct  Is there a better and more efficient way to do this

User · Answer

If you need the X last bits of your integer  use a binary mask     unsigned last8bitsvalue  32 bit integer   amp  0xFF unsigned last16bitsvalue  32 bit integer   amp  0xFFFF

User · Answer

There is a single BEXTR  Bit field extract  with register   x86 instruction on Intel and AMD CPUs and UBFX on ARM  There are intrinsic functions such as  bextr u32    link requires sign-in  that allow to invoke this instruction explicitly   They implement  source  gt  gt  offset   amp    1  lt  lt  n  - 1  C code  get n continuous bits from source starting at the offset bit  Here s a complete function definition that handles edge cases    include  lt limits h gt   unsigned getbits unsigned value  unsigned offset  unsigned n      const unsigned max n   CHAR BIT   sizeof unsigned     if  offset  gt   max n      return 0     value is padded with infinite zeros on the left      value  gt  gt   offset     drop offset bits      if  n  gt   max n      return value     all  bits requested      const unsigned mask    1u  lt  lt  n  - 1     n  1 s      return value  amp  mask      For example  to get 3 bits from 2273  0b100011100001  starting at 5-th bit  call getbits 2273  5  3    it extracts 7  0b111       For example  say I want the first 17 bits of the 32-bit value  what is it that I should do    unsigned first bits   value  amp    1u  lt  lt  17  - 1       amp  0x1ffff   Assuming CHAR BIT   sizeof unsigned  is 32 on your system      I presume I am supposed to use the modulus operator and I tried it and was able to get the last 8 bits and last 16 bits   unsigned last8bitsvalue    value  amp    1u  lt  lt   8  - 1       amp  0xff unsigned last16bitsvalue   value  amp    1u  lt  lt  16  - 1       amp  0xffff   If the offset is always zero as in all your examples in the question then you don t need the more general getbits    There is a special cpu instruction BLSMSK that helps to compute the mask   1  lt  lt  n  - 1

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This is a briefer variation of the accepted answer  the function below extracts the bits from-to inclusive by creating a bitmask  After applying an AND logic over the original number the result is shifted so the function returns just the extracted bits  Skipped index integrity checks for clarity   uint16 t extractInt uint16 t orig16BitWord  unsigned from  unsigned to       unsigned mask      1 lt  lt  to-from 1  -1   lt  lt  from    return  orig16BitWord  amp  mask   gt  gt  from

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Bitwise AND your integer with the mask having exactly those bits set that you want to extract  Then shift the result right to reposition the extracted bits if desired   unsigned int lowest 17 bits   myuint32  amp  0x1FFFF  unsigned int highest 17 bits    myuint32  amp   0x1FFFF  lt  lt   32 - 17     gt  gt   32 - 17     Edit  The latter repositions the highest 17 bits as the lowest 17  this can be useful if you need to extract an integer from    within    a larger one  You can omit the right shift   gt  gt   if this is not desired

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define GENERAL  GET BITS FROM U8 source lsb msb          uint8 t   source   amp              uint8 t    uint8 t  0xFF  gt  gt    uint8 t  7-  uint8 t  msb   amp  7       amp                   uint8 t  0xFF  lt  lt    uint8 t  lsb   amp  7          define GENERAL  GET BITS FROM U16 source lsb msb          uint16 t   source   amp              uint16 t    uint16 t  0xFFFF  gt  gt    uint8 t  15-  uint8 t  msb   amp  15       amp                  uint16 t  0xFFFF  lt  lt    uint8 t  lsb   amp  15          define GENERAL  GET BITS FROM U32 source lsb msb          uint32 t   source   amp              uint32 t    uint32 t  0xFFFFFFFF  gt  gt    uint8 t  31-  uint8 t  msb   amp  31       amp                  uint32 t  0xFFFFFFFF  lt  lt    uint8 t  lsb   amp  31

User · Answer

Instead of thinking of it as  extracting   I like to think of it as  isolating    Once the desired bits are isolated  you can do what you will with them   To isolate any set of bits  apply an AND mask   If you want the last X bits of a value  there is a simple trick that can be used   unsigned  mask  mask    1  lt  lt  X  - 1  lastXbits   value  amp  mask    If you want to isolate a run of X bits in the middle of  value  starting at  startBit       unsigned  mask  mask     1  lt  lt  X  - 1   lt  lt  startBit  isolatedXbits   value  amp  mask    Hope this helps

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Modulus works to get bottom bits  only   although I think value  amp  0x1ffff expresses  take the bottom 17 bits  more directly than value   131072  and so is easier to understand as doing that   The top 17 bits of a 32-bit unsigned value would be value  amp  0xffff8000  if you want them still in their positions at the top   or value  gt  gt  15 if you want the top 17 bits of the value in the bottom 17 bits of the result

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If you want n bits specific then you could first create a bitmask and then AND it with your number to take the desired bits   Simple function to create mask from bit a to bit b   unsigned createMask unsigned a  unsigned b       unsigned r   0     for  unsigned i a  i lt  b  i           r    1  lt  lt  i      return r      You should check that a lt  b    If you want bits 12 to 16 call the function and then simply  amp   logical AND  r with your number N  r   createMask 12 16   unsigned result   r  amp  N    If you want you can shift the result  Hope this helps

[c] How do I extract specific 'n' bits of a 32-bit unsigned integer in C?

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