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    <p>This is what I ended up doing. (<strong>EDIT</strong> Changed on 8/21/2014 for a single bit copy bug.)</p> <pre><code>#include &lt;limits.h&gt; #include &lt;string.h&gt; #include &lt;stddef.h&gt; #define PREPARE_FIRST_COPY() \ do { \ if (src_len &gt;= (CHAR_BIT - dst_offset_modulo)) { \ *dst &amp;= reverse_mask[dst_offset_modulo]; \ src_len -= CHAR_BIT - dst_offset_modulo; \ } else { \ *dst &amp;= reverse_mask[dst_offset_modulo] \ | reverse_mask_xor[dst_offset_modulo + src_len]; \ c &amp;= reverse_mask[dst_offset_modulo + src_len]; \ src_len = 0; \ } } while (0) static void bitarray_copy(const unsigned char *src_org, int src_offset, int src_len, unsigned char *dst_org, int dst_offset) { static const unsigned char mask[] = { 0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff }; static const unsigned char reverse_mask[] = { 0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe, 0xff }; static const unsigned char reverse_mask_xor[] = { 0xff, 0x7f, 0x3f, 0x1f, 0x0f, 0x07, 0x03, 0x01, 0x00 }; if (src_len) { const unsigned char *src; unsigned char *dst; int src_offset_modulo, dst_offset_modulo; src = src_org + (src_offset / CHAR_BIT); dst = dst_org + (dst_offset / CHAR_BIT); src_offset_modulo = src_offset % CHAR_BIT; dst_offset_modulo = dst_offset % CHAR_BIT; if (src_offset_modulo == dst_offset_modulo) { int byte_len; int src_len_modulo; if (src_offset_modulo) { unsigned char c; c = reverse_mask_xor[dst_offset_modulo] &amp; *src++; PREPARE_FIRST_COPY(); *dst++ |= c; } byte_len = src_len / CHAR_BIT; src_len_modulo = src_len % CHAR_BIT; if (byte_len) { memcpy(dst, src, byte_len); src += byte_len; dst += byte_len; } if (src_len_modulo) { *dst &amp;= reverse_mask_xor[src_len_modulo]; *dst |= reverse_mask[src_len_modulo] &amp; *src; } } else { int bit_diff_ls, bit_diff_rs; int byte_len; int src_len_modulo; unsigned char c; /* * Begin: Line things up on destination. */ if (src_offset_modulo &gt; dst_offset_modulo) { bit_diff_ls = src_offset_modulo - dst_offset_modulo; bit_diff_rs = CHAR_BIT - bit_diff_ls; c = *src++ &lt;&lt; bit_diff_ls; c |= *src &gt;&gt; bit_diff_rs; c &amp;= reverse_mask_xor[dst_offset_modulo]; } else { bit_diff_rs = dst_offset_modulo - src_offset_modulo; bit_diff_ls = CHAR_BIT - bit_diff_rs; c = *src &gt;&gt; bit_diff_rs &amp; reverse_mask_xor[dst_offset_modulo]; } PREPARE_FIRST_COPY(); *dst++ |= c; /* * Middle: copy with only shifting the source. */ byte_len = src_len / CHAR_BIT; while (--byte_len &gt;= 0) { c = *src++ &lt;&lt; bit_diff_ls; c |= *src &gt;&gt; bit_diff_rs; *dst++ = c; } /* * End: copy the remaing bits; */ src_len_modulo = src_len % CHAR_BIT; if (src_len_modulo) { c = *src++ &lt;&lt; bit_diff_ls; c |= *src &gt;&gt; bit_diff_rs; c &amp;= reverse_mask[src_len_modulo]; *dst &amp;= reverse_mask_xor[src_len_modulo]; *dst |= c; } } } } </code></pre>
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    1. POWhat's a time efficient algorithm to copy unaligned bit arrays?
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    1. USJamie
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    1. CO+1 nice post! I was looking for this: will your solution work on both 32-bit and 64-bit OSes? I haven't combed through your code yet, but the memcpy() in the middle certainly makes sense to me.
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    2. COIt should work for any architecture that has a c compiler. They're just c pointers.
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    3. COGreat! I will try it out - tyvm.
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