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.
Figure 1. Nonspanned, Format-V 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.