The file header is located at offset 0 within the index file. The first 128 bytes are the same as a standard variable structure file header, except for the fields below.
The remainder of the file header contains the following fields:
| Offset | Size | Description |
|---|---|---|
| 128 | 8 | Offset of logical end of the data file. |
| 136 | 1 | Value 2. |
| 137 | 1 | Value 2. |
| 138 | 1 | Value 4. |
| 139 | 1 | Value 4. |
| 140 | 2 | Contains the number of keys defined for the file. |
| 142 | 1 | Value 0, or 1 if duplicate occurrence records are present. See the topic Types of Indexed File for details. |
| 143 | 1 | Value 2 or 4. Number of bytes used for occurrence numbers in indices where duplicates are permitted. |
| 144 | 8 | Offset of the first key information record. |
| 152 | 8 | Offset of the free space record for the data file. For fixed format files, this is a record in the index file of the same format as the index free space record, but the addresses point to free records in the data file. For variable format files, this is the address in the data file of the data free space record. This record has a different structure to the index free space record. |
| 160 | 8 | Offset of the first free space record in the index file. |
| 168 | 4 | Value zeros. |
| 172 | 4 | Index file record length (node size). |
| 176 | 8 | Value zeros. |
| 184 | 328 | Reserved. Value zeros. For node size 512. |
| 840 | Reserved. Value zeros. For node size 1024. | |
| 3912 | Reserved. Value zeros. For node size 4096. |