How to write single bits to a file in C

问题:

I am programming an entropy coding algorithm and I want to write single bits like an encoded character to a file. For example I want to write 011 to a file but if you would store it as character it'd take up 3 Bytes instead of 3 Bits. So my final question is: How can I write single bits to a file?

Thanks in advance!

回答1:

You can't write individual bits to a file, the resolution is a single byte.

If you want to write bits in sequence, you have to batch them up until you have a full byte, then write that. Psuedo-code (though C-like) for that would be along the lines of:

currbyte = 0
bitcount = 0
def writeBit (bit):
    currbyte = currbyte << 1 | bit
    bitcount++
    if bitcount == BITS_PER_BYTE:
        write currbyte to file
        currbyte = 0
        bitcount = 0

Of you want to change individual bits, you have to read in a byte, use bitwise operations to manipulate it, then write it back.