Reading a buffer bitwise

bit-manipulation, c++

Solution

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;
}

Problem

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.

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