libpng16-16 needs ZLIB_1.2.9 but package does not install it

[Yann Salmon]

Since upgrading from 16.04 to 18.04, some programs that depend on libpng16.so.16 do not work with the error

ImportError: /opt/yann/pyzo-4.5.1/lib/python3.5/../libz.so.1: version `ZLIB_1.2.9' not found (required by /usr/lib/x86_64-linux-gnu/libpng16.so.16)

Reinstalling the libpng16-16 package does not improve things.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: libpng16-16 1.6.34-1
ProcVersionSignature: Ubuntu 4.15.0-24.26-generic 4.15.18
Uname: Linux 4.15.0-24-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CurrentDesktop: XFCE
Date: Thu Jul 5 13:19:44 2018
Dependencies:
 gcc-8-base 8-20180414-1ubuntu2
 libc6 2.27-3ubuntu1
 libgcc1 1:8-20180414-1ubuntu2
 zlib1g 1:1.2.11.dfsg-0ubuntu2
InstallationDate: Installed on 2016-09-07 (665 days ago)
InstallationMedia: Xubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: libpng1.6
UpgradeStatus: Upgraded to bionic on 2018-07-01 (3 days ago)

[Gianfranco Costamagna (costamagnagianfranco)]

Hello,
ldd /usr/lib/x86_64-linux-gnu/libpng16.so.16.34.0
 linux-vdso.so.1 (0x00007fff3e164000)
 libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f97a4da5000)
 libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f97a4a07000)
 libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f97a4616000)
 /lib64/ld-linux-x86-64.so.2 (0x00007f97a51f4000)

I see zlib is correctly linked... can you please double check?

[Andreas Nicolai (ghorwin)]

Bug appears to be related to software build and linked against different version of libz:

Analysis:

Output of ld when trying to build an application that uses libpng16:

/usr/lib/x86_64-linux-gnu/libpng16.so.16: undefined reference to `inflateReset2@ZLIB_1.2.3.4'
/usr/lib/x86_64-linux-gnu/libpng16.so.16: undefined reference to `inflateValidate@ZLIB_1.2.9'

Output of ldd (same output as above):

> ldd /usr/lib/x86_64-linux-gnu/libpng16.so.16
 linux-vdso.so.1 (0x00007ffeda752000)
 libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f0dd35dd000)
 libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f0dd323f000)
 libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f0dd2e4e000)
 /lib64/ld-linux-x86-64.so.2 (0x00007f0dd3a2c000)

checking for missing symbols:

> readelf --syms /usr/lib/x86_64-linux-gnu/libpng16.so.16 | grep inflateReset2
     9: 0000000000000000 0 FUNC GLOBAL DEFAULT UND inflateReset2@ZLIB_1.2.3.4 (4)

Ok, symbol appears to be there. Why does ld pop up the error?

--> linker command line references an older z-lib earlier on in the library dependencies -> hence, zlib from system is not used (though required by libpng16).

Solution: remove linker dependency to custom libz (remove "-lz") and try linking against system libz.