Reading a binary file bit by bit

Question

I know the function below:

size_t fread(void *ptr, size_t size_of_elements, size_t number_of_elements, FILE *a_file);

It only reads byte by

Answer 1

Read the first two bytes from your a_file file pointer and check the bits in the least or greatest byte — depending on the endianness of your platform (x86 is little-endian) — using bitshift operators.

You can't really put bits into an array, as there isn't a datatype for bits. Rather than keeping 1's and 0's in an array, which is inefficient, it seems cheaper just to keep the two bytes in a two-element array (say, of type unsigned char *) and write functions to map those two bytes to one of 4096 (2^12) values-of-interest.

As a complication, on subsequent reads, if you want to fread through the pointer every 12 bits, you would read only one byte, using the left-over bits from the previous read to build a new 12-bit value. If there are no leftovers, you would need to read two bytes.

Your mapping functions would need to address the second case where bits were used from previous read, because the two bytes would need different mapping. To do this efficiently, a modulus on a read-counter could be used to swap between two mappings.