storing a bit in a bit of character array in C lin

问题:

How can i store a bit in a bit? I need to extract various no. of bits from different variables and put them in the buffer as shown.

unsigned char a;
a=5;

Now I want to take LSB and store it in a bit of a buffer of type unsigned char.

unsigned char buffer[5];

For extraction I am using

a & 00000001

Now how to store it and further bits after this?

回答1:

I'm not sure of what you want to do but here is an exemple of bitwise shifting.

unsigned char b;
unsigned char c;

b = a & (1 << 0); /* Will store the least significant bit of a in b*/
c = a & (1 << 1); /* Will store the 2nd least significant bit of a in c*/

You can use this to create variables that only have a limited amount of bits.

typedef struct bit_s
{
    unsigned int    a : 1; /* only 1 bit available */
    unsigned int    b : 12; /* only 12 bits available */
}              bit_t;

bit_t var;

var.a = 0;
var.a = 1;
var.a = 2; /* Will overflow the variable and create a warning with -Woverflow */

回答2:

use shift operator >>

buffer[0] = a & 0x01; // 1 will be stores to buffer[0] if LSB of a is one
buffer[1] = (a >> 1) & 0x01; // 1 will be stored to buffer[1] if 2nd bit of a is one.

回答3:

Something like:

(apologies for syntax errors etc. this is untested code! ) 


unsigned char getbit;
unsigned char storeBit[5];

for (x = 0; x < inBufferLength ; x++) {
  // get least significant bit
  getbit = inBuffer[x] & x00000001;
  int ByteNum = x / 8;
  int BitNum = x % 8;
  if (getBit) {
    switch (BitNum) {
     case 0;
        storeBit[ByteNum] = storeBit[ByteNum] | x10000000;
        break; 
     case 1;
        storeBit[ByteNum] = storeBit[ByteNum] | x01000000;
        break; 
     case 2;
        storeBit[ByteNum] = storeBit[ByteNum] | x00100000;
        break;
     case 3;
        storeBit[ByteNum] = storeBit[ByteNum] | x00010000;
        break; 
     case 4;
        storeBit[ByteNum] = storeBit[ByteNum] | x00001000;
        break; 
     case 5;
        storeBit[ByteNum] = storeBit[ByteNum] | x00000100;
        break; 
     case 6;
        storeBit[ByteNum] = storeBit[ByteNum] | x00000010;
        break; 
     case 7;
        storeBit[ByteNum] = storeBit[ByteNum] | x00000001;
        break;
     }
  }

回答4:

Setting bits...

/** Set bit in any sized bit block.
 *
 * @return   none
 *
 * @param   bit    - Bit number.
 * @param   bitmap - Pointer to bitmap.
 *
 * @note    Please note that this function does not know the size of the
 *          bitmap and it cannot range check the specified bit number.
 */
void SetBit( int bit, unsigned char *bitmap)
{
    int n, x;

    x = bit / 8;                        // Index to byte.
    n = bit % 8;                        // Specific bit in byte.

    bitmap[x] |= (1 << n);      // Set bit.
}

Resetting bits...

/** Reset bit in any sized mask.
 *
 * @return  None
 *
 * @param   bit    - Bit number.
 * @param   bitmap - Pointer to bitmap.
 *
 * @note    Please note that this function does not know the size of the
 *          bitmap and it cannot range check the specified bit number.
 */
void ResetBit( int bit, unsigned char *bitmap)
{
    int n, x;

    x = bit / 8;                        // Index to byte.
    n = bit % 8;                        // Specific bit in byte.

    bitmap[x] &= (1 << n);              // Reset bit.
}

Toggle bit...

/** Toggle bit in any sized bit mask.
 *
 * @return   none
 *
 * @param   bit    - Bit number.
 * @param   bitmap - Pointer to bitmap.
 *
 * @note    Please note that this function does not know the size of the
 *          bitmap and it cannot range check the specified bit number.
 */
void ToggleBit( int bit, unsigned char *bitmap)
{
    int n, x;

    x = bit / 8;                        // Index to byte.
    n = bit % 8;                        // Specific bit in byte.

    bitmap[x] ^= (1<<n);        // Toggle bit.
}

Checking bit...

/** Checks specified bit.
 *
 * @return  1 if bit set else 0.
 *
 * @param   bit    - Bit number.
 * @param   bitmap - Pointer to bitmap.
 *
 * @note    Please note that this function does not know the size of the
 *          bitmap and it cannot range check the specified bit number.
 */
int IsBitSet( int bit, const unsigned char *bitmap)
{
    int n, x;

    x = bit / 8;            // Index to byte.
    n = bit % 8;            // Specific bit in byte.

    // Test bit (logigal AND).
    if (bitmap[x] & (1<<n))
        return 1;

    return 0;
}

Is bit reset...

/** Checks specified bit.
 *
 * @return  1 if bit reset else 0.
 *
 * @param   bit    - Bit number.
 * @param   bitmap - Pointer to bitmap.
 *
 * @note    Please note that this function does not know the size of the
 *          bitmap and it cannot range check the specified bit number.
 */
int IsBitReset( int bit, const unsigned char *bitmap)
{
    return IsBitSet( bit, bitmap) ^ 1;
}

Hope this helps...

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