Figure 1 shows blocked and unblocked variable-length (format-V) records without spanning. A block in a data set containing unblocked records is in the same format as a block in a data set containing blocked records. The only difference is that with blocked records each block can contain multiple records.
The system uses the record or segment length information in blocking and unblocking. The first four bytes of each record, record segment, or block make up a descriptor word containing control information. You must allow for these additional 4 bytes in both your input and output buffers.