I search for free("LGPL" or something similar), multi platform archive library (c/c++)
Wanted features:
1) Multi platform (gcc/msvc)
2)read and write
3)Unicode (not 100% necessary )
4)encryption (not 100% necessary )
5)compression (not 100% necessary )
6) Good documentation
I found some libraries but they are read only or not really free or something else
zlib (compress only i think)
zziplib (read only i think)
PhysicalFS (read only)
LZO (GPL 2+)
LZMA SDK(can't build on linux)
did you look at http://libarchive.github.com/
Take a look at https://github.com/libarchive/libarchive/wiki/LibarchiveFormats for supported formats. Works on posix and windows.
But I would normaly go with zlib. The most portabel lib in the world.