Description of problem:
I am seeing a strange behavior of gcc when compiling the latest version of libsolv:
/usr/bin/gcc -Wall -Wl,--as-needed CMakeFiles/deltainfoxml2solv.dir/deltainfoxml2solv.o -o deltainfoxml2solv -rdynamic libtoolstuff.a ../ext/libsolvext.so.0 ../src/libsolv.so.0 -ldb -lrpmio -lrpm -lexpat -lz -Wl,-rpath,/home/akozumpl/libsolv/build/ext:/home/akozumpl/libsolv/build/src:
../ext/libsolvext.so.0: undefined reference to `gzclose'
../ext/libsolvext.so.0: undefined reference to `gzdopen'
../ext/libsolvext.so.0: undefined reference to `gzopen64'
../ext/libsolvext.so.0: undefined reference to `gzread'
../ext/libsolvext.so.0: undefined reference to `gzwrite'
There is a source in libsolvext.so that uses the mentioned zlib functions. However, notice that -lz is the last library on the command line and so ld should in any case be able to pick up any unresolved references from there, as stated by 'man ld': "If the archive defines a symbol which was undefined in some object which appeared before the archive on the command line, the linker will include the appropriate file(s) from the archive. "
If I move -lz forward on the command line, the linking passes fine:
/usr/bin/gcc -Wall -Wl,--as-needed CMakeFiles/deltainfoxml2solv.dir/deltainfoxml2solv.o -o deltainfoxml2solv -rdynamic libtoolstuff.a ../ext/libsolvext.so.0 ../src/libsolv.so.0 -ldb -lz -lrpmio -lrpm -lexpat -Wl,-rpath,/bhome/akozumpl/libsolv/build/ext:/home/akozumpl/libsolv/build/src:
gcc (GCC) 4.7.0 20120507 (Red Hat 4.7.0-5)
Also see: https://github.com/openSUSE/libsolv/pull/22#issuecomment-11489652
First of all, this has nothing to do with gcc.
I think --as-needed only adds libraries that are needed by the binary or library being linked, not for any libraries that were incorrectly linked before (from the above it looks like libsolvext.so.0 should have been linked against -lz when it uses it).
Thanks Jakub, you actually pointed us in the right direction: libsolvext.so should had already been linked against zlib but it was not.
I wonder maybe 'ld' should scream when linking libsolvext.so then as it is clearly missing a dependency then. Also, the original problem still stands: combination of a missing shared dependency and using --as-needed can produce different results depending on the exact order of libraries being linked to the final binary.
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