c - Setting bits in a bit stream -

i have encountered following c function while working on legacy code , compeletely baffled, way code organized. can see function trying set bits @ given position in bit stream can't head around individual statements , expressions. can please explain why developer used divison 8 (/8) , modulus 8 (%8) expressions here , there. there easy way read these kinds of bit manipulation functions in c?

static void setbits(u8 *input, u16 *bpos, u8 len, u8 val) {   u16 pos;   if (bpos==0)   {     pos=0;   }   else   {     pos = *bpos;     *bpos += len;   }   input[pos/8] = (input[pos/8]&(0xff-((0xff>>(pos%8))&(0xff<<(pos%8+len>=8?0:8-(pos+len)%8)))))                 |((((0xff>>(8-len)) & val)<<(8-len))>>(pos%8));   if ((pos/8 == (pos+len)/8)|(!((pos+len)%8)))     return;   input[(pos+len)/8] = (input[(pos+len)/8]                         &(0xff-(0xff<<(8-(pos+len)%8))))                        |((0xff>>(8-len)) & val)<<(8-(pos+len)%8); } 

please explain why developer used divison 8 (/8) , modulus 8 (%8) expressions here , there

first of all, note individual bits of byte numbered 0 7, bit 0 least significant one. there 8 bits in byte, hence "magic number" 8.

generally speaking: if have raw data, consists of n bytes , can therefore treated array of bytes uint8_t data[n]. access bit x in byte array, can example this:

given x = 17, bit x found in byte number 17/8 = 2. note integer division "floors" value, instead of 2.125 2.

the remainder of integer division gives bit position in byte, 17%8 = 1.

so bit number 17 located in byte 2, bit 1. data[2] gives byte.

to mask out bit byte in c, bitwise , operator & used. , in order use that, bit mask needed. such bit masks best obtained shifting value 1 desired amount of bits. bit masks perhaps expressed in hex , possible bit masks byte (1<<0) == 0x01 , (1<<1) == 0x02, (1<<3) == 0x04, (1<<4) == 0x08 , on.

in case (1<<1) == 0x02.

c code:

uint8_t data[n]; ... size_t  byte_index = x / 8; size_t  bit_index  = x % 8; bool    is_bit_set;  is_bit_set = ( data[byte_index] & (1<<bit_index) ) != 0;