-bash-4.2$ pstack 42752
#0  0x00007f8efbd53d16 in longest_match () from /lib64/libz.so.1
#1  0x00007f8efbd54690 in deflate_slow () from /lib64/libz.so.1
#2  0x00007f8efbd55624 in deflate () from /lib64/libz.so.1
#3  0x00007f8efd48a6fd in R__zipMultipleAlgorithm () from /data/oshadura/root/builds/lib/libCore.so
#4  0x00007f8eff82c16d in TBasket::WriteBuffer() () from /data/oshadura/root/builds/lib/libTree.so
#5  0x00007f8eff83880d in TBranch::WriteBasketImpl(TBasket*, int, ROOT::Internal::TBranchIMTHelper*) () from /data/oshadura/root/builds/lib/libTree.so
#6  0x00007f8eff8393c2 in TBranch::FillImpl(ROOT::Internal::TBranchIMTHelper*) () from /data/oshadura/root/builds/lib/libTree.so
#7  0x00007f8eff89d7e0 in TTree::Fill() () from /data/oshadura/root/builds/lib/libTree.so
#8  0x00007f8eff89d161 in TTree::CopyEntries(TTree*, long long, char const*) () from /data/oshadura/root/builds/lib/libTree.so
#9  0x00007f8eff89910e in TTree::CloneTree(long long, char const*) () from /data/oshadura/root/builds/lib/libTree.so
#10 0x00007f8eff89294a in TTree::Merge(TCollection*, TFileMergeInfo*) () from /data/oshadura/root/builds/lib/libTree.so
#11 0x00007f8efdc52114 in TFileMerger::MergeRecursive(TDirectory*, TList*, int) () from /data/oshadura/root/builds/lib/libRIO.so
#12 0x00007f8efdc528db in TFileMerger::PartialMerge(int) () from /data/oshadura/root/builds/lib/libRIO.so
#13 0x00000000004058e6 in main ()