Value Size Description
----- ---- -----------
(Stream) 0x6C78 2 bytes Tag for this extra block type ("xl")
Size 2 bytes Data size for this block
Bitmap m bytes Determines which fields below this
point are included
Version Made By 2 bytes As in Central Directory File Header
Int File Attrs 2 bytes As in Central Directory File Header
Ext File Attrs 4 bytes As in Central Directory File Header
This extra block is used to include information in the local file
header that previously has only been included in fields in the
central directory file header for this entry. This extra field
is intended for use only in the local header, not the central
directory.
The information in this local extra field must match the information
in the central directory header for this entry, or this extra field
should be considered invalid. The purpose of this local extra field
is to provide in the local header a copy of central directory
information required for the proper extraction of entries so that
entries can be extracted as they appear in the stream.
If this streaming extra field is present in the central directory
header, the information must match the information already in the
central directory header as well as information in the local header
streaming extra field, if present.
The bitmap identifies which fields actually appear in this block.
Fields always appear in the exact order listed in the bitmap,
starting from byte zero/bit zero. If fields are added to this extra
block in the future, they will appear at the location (in the order)
indicated by the respective bit in the bitmap. A 1 at a bit position
in the bitmap indicates presence of the corresponding field block,
while a 0 indicates the field block is absent. This allows for
removing obsolete fields in the future. A map bit may map to
multiple fields, where a 1 means those fields are present. The size
of each field block must be determined, either as a fixed size field
or by including a length count at the start of the field block.
The bitmap consists of as many bytes as needed to define the
contents of this extra block. For each byte in the bitmap, bit 7 is
1 if and only if there is a following byte in the bitmap. Bit 7
will be 0 in the last byte of the bitmap. Currently only one map
byte is used, so bit 7 of that byte is 0. Bitmap bytes are stored
in order, starting with Byte 0 immediately following Size. For
example, for a three byte BitMap:
+--+--+--+--+--+--+--+--+
| Tag |
+--+--+--+--+--+--+--+--+
| Tag |
+--+--+--+--+--+--+--+--+
| Size |
+--+--+--+--+--+--+--+--+
| Size |
+--+--+--+--+--+--+--+--+
Bitmap (showing locations of bit numbers 0, 1, and 2):
+--+--+--+--+--+--+--+--+
| 1| | | | | 2| 1| 0| Byte 0
+--+--+--+--+--+--+--+--+
| 1| | | | | | | | Byte 1
+--+--+--+--+--+--+--+--+
| 0| | | | | | | | Byte 2
+--+--+--+--+--+--+--+--+
^
|
+-------- Bit 7
+--+--+--+--+--+--+--+--+
| Version Made By |
+--+--+--+--+--+--+--+--+
...
The current field blocks and bitmap mappings are shown below.
(MB = Map Byte (numbered starting from 0), BN = Bit Number,
MV = Mask Value and FBS = Field Block Size = number of bytes in
this field block when present.) Currently only one byte is
needed for the Bitmap. This one byte map consists of the
following bits ORed together:
MB BN MV FBS Description
-- -- -- --- ------------
0 0 1 2 "version made by" field is included
0 1 2 2 "internal file attributes" field is included
0 2 4 4 "external file attributes" field is included
The bitmap to include all these fields would be the one byte (bit 0
on the right):
00000111
A user of this extra block should verify that this is a correctly
formatted block by summing the expected sizes (FBS) of each present
field block, adding to this the size of the bitmap in bytes, and
comparing to Size. If they do not match, this extra block should
not be used.
-Info-ZIP Placeholder Extra Field:
================================
This extra field holds no data. It is only used to reserve space
in the local header extra field block for possible use by another
extra field. Currently this is only used to reserve space for the
Zip64 local extra field. No data should ever be stored in this
extra field and it should always be ignored.
(Last Revision 20150605)
Value Size Description
----- ---- -----------
(PHold) 0x4850 Short tag for this extra block type ("PH")
TSize Short total data size for this block
Data Variable see below
The size of this extra field is selected to match the size of the
extra field it is reserving space for. The purpose of this extra
field is to reserve space in the extra field block so that another
extra field can use the space later.
The main use currently is to reserve space for the Zip64 local
header extra field when the size of the input data is not known
(for instance, when the input is a stream). If, once the data is
read, Zip64 is needed, this Placeholder extra field can be replaced
by the Zip64 local extra field. If not, the Placeholder remains
and should be ignored. This allows rewriting the local header
after data is read without it changing size.
Though the contents of this extra field should never be used and so
does not need to be specified, it is recommended that the Data field
be written with all zero (character code 0x00) bytes.
