Blosc Chunk Format

The chunk is composed by a header and a blocks / splits section:

+---------+--------+---------+
|  header | blocks / splits  |
+---------+--------+---------+

These are described below.

The header section

Blosc (as of Version 1.0.0) has the following 16 byte header that stores information about the compressed buffer:

|-0-|-1-|-2-|-3-|-4-|-5-|-6-|-7-|-8-|-9-|-A-|-B-|-C-|-D-|-E-|-F-|
  ^   ^   ^   ^ |     nbytes    |   blocksize   |    cbytes     |
  |   |   |   |
  |   |   |   +--typesize
  |   |   +------flags
  |   +----------versionlz
  +--------------version

Datatypes of the header entries

All entries are little endian.

The blocks / splits section

After the header, there come the blocks / splits section. Blocks are equal-sized parts of the chunk, except for the last block that can be shorter or equal than the rest.

At the beginning of the blocks section, there come a list of int32_t bstarts to indicate where the different encoded blocks starts (counting from the end of this bstarts section):

+=========+=========+========+=========+
| bstart0 | bstart1 |   ...  | bstartN |
+=========+=========+========+=========+

Finally, it comes the actual list of compressed blocks / splits data streams. It turns out that a block may optionally (see bit 4 in flags above) be further split in so-called splits which are the actual data streams that are transmitted to codecs for compression. If a block is not split, then the split is equivalent to a whole block. Before each split in the list, there is the compressed size of it, expressed as an int32_t:

+========+========+========+========+========+========+========+
| csize0 | split0 | csize1 | split1 |   ...  | csizeN | splitN |
+========+========+========+========+========+========+========+

Note: all the integers are stored in little endian.