How can I store value in bit in C language

Question

I wish to store either 0 or 1 in each bit of an allocated memory. For example I have char *block_of_memory = (char *)malloc(125000 * sizeof(char)) , here I have 125000 * 8 bits = 1000,000 bits of memory. How can I access each bit and give it a value as 0 or 1. Say , I want to make the 20th bit to 1 and 21st bit as 0.

First you find out which byte you need to change, then which bit in that byte. The first is done by dividing the bit number by 8, the second by looking at the remainder of that operation. — Peter - Reinstate Monica, May 02 '15 at 21:27
@KenWhite And there, specifically answer http://stackoverflow.com/a/3234773/3150802. — Peter - Reinstate Monica, May 02 '15 at 21:31
Is this right way to set the 20th bit to 1? (block_of_memory[20/8] >> (20 % 8)) & 1 — Arnold, May 02 '15 at 21:31

SzG · Accepted Answer · 2015-05-05T06:30:27.520

You need to calculate a byte offset and a bitmask within that byte.

Set the bit: bitwise OR with mask
Clear the bit: bitwise AND with complement of mask
Read the bit: return bitwise AND of byte and mask

The code:

void set_bit(char *buf, int bit, int val)
{
    int byte = bit / 8;
    char mask = 1 << (bit % 8);
    if (val)
        buf[byte] |= mask;
    else
        buf[byte] &= ~mask;
}

int get_bit(char *buf, int bit)
{
    int byte = bit / 8;
    char mask = 1 << (bit % 8);
    return buf[byte] & mask ? 1 : 0; 
}

Example: Set bit 17 to 1. Byte offset is 17/8 = 2. Bit offset is 17%8 = 1. The bitmask is generated by left-shifting 1 by the bit offset: results in 00000010 binary. Bitwise OR byte[2] with it: all bits remain the same, except where the mask bit is 1.

Thanks! Can you explain the mask please? Why and How are we using it here? — Arnold, May 03 '15 at 19:35

How can I store value in bit in C language

1 Answers1