Mapping the Growth Hint

In a ZIP archive, the growth hint is used to reserve additional bytes that can be used to allow an item to grow in-place. The padding is stored in the Extra field, as defined in the ZIP file format specification. If a growth hint is used for an interleaved part, the package implementer should store the Extra field containing the growth hint padding with the item that represents the first piece of the part. [S3.2]

The format of the ZIP item's Extra field, when used for growth hints, is shown in Table 9–3, Structure of the Extra field for growth hints below.

Table 1. 9–3. Structure of the Extra field for growth hints
Field	Size	Value
Header ID	2 bytes	A220
Length of Extra field	2 bytes	The signature length (2 bytes) + the padding initial value length (2 bytes) + Length of the padding (variable)
Signature (for verification)	2 bytes	A028
Padding Initial Value	2 bytes	Hex number value is set by the producer when the item is created
<padding>	[Padding Length]	Should be filled with NULL characters