I have to write individual Bits to a file (for huffman code). To do so I send bits to a function which buffers them until a byte is filled and then returns the byte. I can´t figure out why it doesn’t work, the function outputs wrong bytes (but no errors).
Here is the bitsBuffer function:

// Buffers bits until one byte is filled, the returns it and return code turns positive
int bitsBuffer(char inByte, char nBits, char* outByte, char finish)
{
    int i;
    static short unsigned buffer = 0;
    static char unsigned offset = 0;

    // Very readable way to add nBits Bits to buffer
    buffer |= (inByte & ~(~0 << nBits)) << offset;
    offset += nBits;
    if (offset >= 8) {
        *outByte = (char)buffer;
        buffer >>= 8;
        offset -= 8;
        return 1;
    }
    if (finish) {
        buffer = 0;
        if (offset) {
            *outByte = (char)buffer;
            offset = 0;
            return 1;
        }
        offset = 0;
        return 0;
    }
    return 0;
}

I use this program to test the bitbuffer (I pipe the output to xxd -b to view the bits):

#include "bitsHandler.h"
#include <stdio.h>

int main()
{
    char a[] = { 0b0110, 0b1, 0b100010, 0b111, 0b100110, 0b0 };
    char b[] = { 4, 1, 6, 3, 6, 1 };
    char c[100];
    int counter = 0;
    for (int i = 0; i < 6; i++) {
        if (bitsBuffer(a[i], b[i], &c[counter], 0)) {
            counter++;
        }
    }
    if (bitsBuffer(0, 0, &c[counter], 1)) {
        counter++;
    }
    fwrite(c, sizeof(char), counter, stdout);
}

I replicated the function on paper (went through every step by hand) and I can’t find my mistake. Help is appreciated.