How to define and work with an array of bits in C?

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If I am not too late, this page gives awesome explanation with examples.

An array of int can be used to deal with array of bits. Assuming size of int to be 4 bytes, when we talk about an int, we are dealing with 32 bits. Say we have int A[10], means we are working on 10*4*8 = 320 bits and following figure shows it: (each element of array has 4 big blocks, each of which represent a byte and each of the smaller blocks represent a bit)

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So, to set the kth bit in array A:

// NOTE: if using "uint8_t A[]" instead of "int A[]" then divide by 8, not 32
void  SetBit( int A[],  int k )
{
    int i = k/32;        //gives the corresponding index in the array A
    int pos = k%32;      //gives the corresponding bit position in A[i]

    unsigned int flag = 1;   // flag = 0000.....00001

    flag = flag << pos;      // flag = 0000...010...000   (shifted k positions)

    A[i] = A[i] | flag;      // Set the bit at the k-th position in A[i]
}

or in the shortened version

void  SetBit( int A[],  int k )
{
    A[k/32] |= 1 << (k%32);  // Set the bit at the k-th position in A[i]
}

similarly to clear kth bit:

void  ClearBit( int A[],  int k )                
{
    A[k/32] &= ~(1 << (k%32));
}

and to test if the kth bit:

int TestBit( int A[],  int k )
{
    return ( (A[k/32] & (1 << (k%32) )) != 0 ) ;     
}

As said above, these manipulations can be written as macros too:

// Due order of operation wrap 'k' in parentheses in case it
// is passed as an equation, e.g. i + 1, otherwise the first
// part evaluates to "A[i + (1/32)]" not "A[(i + 1)/32]"
#define SetBit(A,k)     ( A[(k)/32] |= (1 << ((k)%32)) )
#define ClearBit(A,k)   ( A[(k)/32] &= ~(1 << ((k)%32)) )
#define TestBit(A,k)    ( A[(k)/32] & (1 << ((k)%32)) )

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