Question

I have a buffer with some data which come in different bit sizes (8 bits field, then 4 bits field, then a 9 bits field…).

I need to read it. It would be great if there where some library which allowed reading it using a pointer at bit level, and not a byte level.

Copying the buffer to a struct is not an option, because after researching I would need to use #pragma pack() or something similar, and would not be portable.

Any idea?

EDIT: I will try to explain the magnitude of my problem with an example:

field1: 8 bits --> ok, get first byte
field2: 6 bits --> ok, second byte, and a mask
field3: 4 bits --> gets harder, i have to get 2 bytes, apply 2 different masks, and compose
field4 
... 
field 15: 9 bits ---> No idea of how to do it with a loop to avoid writing manually every single case

And the only solution I can think of, is copying to an struct, pragma pack, and go. But I have been told in previous questions that it is not a good solution, because of portability. But I am willing to hear a different opinion if it rescues me.

Answer 1

Use bit manipulation:

unsigned char[64] byte_data;
size_t pos = 3; //any byte
int value = 0; 
int i = 0;
int bits_to_read = 9;
while (bits_to_read) {
    if (i > 8) {
      ++readPos;
      i = 0;
    }
    value |= byte_data[pos] & ( 255 >> (7-i) );
    ++i;
    --bits_to_read;
}