Bit operations query

Hi all,
I am new to programming, and learning bit operations. I have written a code which is supposed to set selected bits in a character array to 1 from 0.
When I run this code, I get segmentation fault as output.
I tried to find similar examples on internet, but all explain bit operations but none explain with any sample codes.
Any help would be greatly appreciated.

Code:

main(){
	unsigned char *bitmap;         //bitmap array
	int i = 0;

	for (i=0; i<3; i++){           //for 3 chars = 3 bytes = 24 bits
		bitmap[i]=0;
	}

	int base_bit = 3,         //trying to convert 00000000 00000000 00000000
	    bits_from_base = 15,                // to 00111111 11111111 11000000
	    bit_index=0;

	for (i=base_bit/8; i<(base_bit+bits_from_base)/8; i++){
		if( i==base_bit/8 ){ //First byte:change the bit_index and all last bytes after it to 1
			bit_index = i%8;
			while(bit_index<8){
				bitmap[i] |= (0x10>>bit_index);
				bit_index++;
			}
		}
		else if( i==(base_bit+bits_from_base)/8 ){//Last byte:change the first bytes till bit index to 1
			bit_index = i%8;
			while(bit_index>=0){
				bitmap[i] |= (0x10>>bit_index);
				bit_index--;
			}
		}		
		else{ //all the bytes between first and last
			bitmap[i] |= 0x11;
		}
	}

	cout << "The final bitmap is: " << bitmap << " \n";
	
}

In the code below you are never allocating any memory for bitmap.

You either need to us new to allocate memory

Code:

    
    unsigned char *bitmap;         //bitmap array
    int i = 0;

    for (i=0; i<3; i++){           //for 3 chars = 3 bytes = 24 bits
        bitmap[i]=0;
    }

Or allocate memory when creating the variable.

Code:

    unsigned char bitmap[3];

Jim

Thanks for the reply. I though that when I declare a pointer the memory would get dynamically allocated in the stack. Please correct me if I am wrong.
If I make the change, I still not getting any output for

Code:

cout << "The final bitmap is: " << bitmap << " \n";

Expression: (bitmap) returns a pointer to unsigned char. You are printing pointer, not the value it points to (placing array's name is placing address of its first element - here, pointer to unsigned char).

Your code should look like this:

Code:

cout << (int)bitmap[0] << (int)bitmap[1] << (int)bitmap[2] << endl;

(int) type cast is required to print it as a number, not as a character.

nice! thanks for the reply.
Can I ask why do I need to convert it to an int instead of displaying it as a char?
Also, what can I do to see that I have achieved the desired bitmap?

Originally Posted by rvgsd

Thanks for the reply. I though that when I declare a pointer the memory would get dynamically allocated in the stack. Please correct me if I am wrong.

No a declaring a pointer does not allocate any memory.

How much would it allocate?

You, the programmer should know how much memory to allocate.

You must either use new in C++ or malloc for C to allocate memory dynamically. Or use static allocation of the memory.

Jim

oh..okay. thanks for explaining. I am reading on these things, but its easy to get confused, at least at this moment for me.! Thanks.