Setting correct state of general purpose flag bits (mismatch between local and central GPF bit 11 ("UTF-8"))

[aral-matrix]

When I open an existing archive with my implementation at https://github.com/troldal/OpenXLSX/blob/ktb_zippy/Tests/ziptest.cpp, adding a single new file to the archive and then perform an unzip on the resulting file, unzip complains like so:

file #14 (xl/tables/table1.xml):
         mismatch between local and central GPF bit 11 ("UTF-8"),
         continuing with central flag (IsUTF8 = 0)
  inflating: xl/tables/table1.xml

This mismatch between GPF bits is reported for all pre-existing files in the archive, but not for the one I added myself.

The extracted data is intact, but I am afraid that may no longer be the case when UTF8 contents are contained.

I found the function zip_source_get_file_attributes to get the (presumably central) general_purpose_bit_flags of the archive, however, looking through the library source code, I see no way to set those flags on API level: all functions that call an internal function with attributes pass to them a default attributes set that is not exposed to the API.

Could you enlighten me

1. why this central archive GPF bit could be set differently from the "local" ones?
2. why unzip seems to default to the central flag in case of mismatch
3. how to create a zip archive that unzip extracts without errors?

I am sure I must be doing something wrong in my implementation & I hope it's an easy fix.

[aral-matrix]

closed in favour of #491