Uninitialised value was created by a heap allocation: Unordered_map - c++

Question

SOLUTION: I can have huge strings, then I have to reserve memory for them. Instead of use string I use char pointer in hash table and therefore I reserve the appropriate memory for my hash table keys.
PROBLEM:
I'm sorry if the question already made, but I could not find any answer that helped me.
I've the following code:
EDIT (the main loop of the problematic function for Valgrind)
i = 0;
wordPos = 0;
for (; it != end; ++it,i++){
// I want to ignore this element on purpose
if (i == 1) continue;
bool isscript;
string tag(it->tagName());
convertToLower(tag);
if (it->isTag()==1){
if (tag=="script") isscript = true;
else isscript = false;
}
if (it->isComment()==0 && it->isTag()==0 && isscript==0){
wordlist.clear();
tokenize(it->text(),wordlist);
int ii = 0;
vector<string>::iterator it_palavras = wordlist.begin();
vector<string>::iterator it_words = wordlist.begin();
int ii = 0;
while(ii<wordlist.size()){
string word(wordlist[ii]);
convertToLower(word);
wordsPos++;
if (voc.find(word) == voc.end()){
voc[word] = countwords;
voc_inv[countwords] = words;
term_pos[countwords] = new vector<int>();
term_pos[countwords]->push_back(wordpos);
countwords++;
}else{
if (term_pos.find(voc[word]) == term_pos.end())
term_pos[voc[word]] = new vector<int>();
term_pos[voc[word]]->push_back(wordpos);
}
ii++;
}
}
The type of voc is unordered_map, but when I run valgrind in my code there is the following message:
EDIT Now I'm pasting the complete error with the flag --track-origins=yes.
EDIT 2 Now I'm pasting the complete error with the flag --—dsymutil=yes.
==21036== Use of uninitialised value of size 8
==21036== at 0x4201FF: _platform_memcmp (in /usr/lib/system/libsystem_platform.dylib)
==21036== by 0x10001F10D: std::__1::__hash_iterator<std::__1::__hash_node<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, int>, void*>*> std::__1::__hash_table<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, int>, std::__1::__unordered_map_hasher<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, int, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, true>, std::__1::__unordered_map_equal<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, int, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, true>, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, int> > >::find<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) (string:642)
==21036== by 0x10000358F: Colecao::ler_arvore_dom(tree<htmlcxx::HTML::Node, std::__1::allocator<tree_node_<htmlcxx::HTML::Node> > >, int, std::__1::unordered_map<int, std::__1::vector<int, std::__1::allocator<int> >, std::__1::hash<int>, std::__1::equal_to<int>, std::__1::allocator<std::__1::pair<int const, std::__1::vector<int, std::__1::allocator<int> > > > >&) (colecao.cpp:135)
==21036== by 0x100002A19: Colecao::ler(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) (colecao.cpp:73)
==21036== by 0x100001781: main (index.cpp:47)
==21036== Uninitialised value was created by a heap allocation
==21036== at 0x70AB: malloc (in /usr/local/Cellar/valgrind/HEAD/lib/valgrind/vgpreload_memcheck-amd64-darwin.so)
==21036== by 0x7528D: operator new(unsigned long) (in /usr/lib/libc++.1.dylib)
==21036== by 0x77E12: std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__init(char const*, unsigned long) (in /usr/lib/libc++.1.dylib)
==21036== by 0x10001A0FF: std::__1::unordered_map<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, int, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const, int> > >::__construct_node(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) (memory:1505)
==21036== by 0x10000838D: std::__1::unordered_map<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, int, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const, int> > >::operator[](std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) (unordered_map:1209)
==21036== by 0x100003835: Colecao::ler_arvore_dom(tree<htmlcxx::HTML::Node, std::__1::allocator<tree_node_<htmlcxx::HTML::Node> > >, int, std::__1::unordered_map<int, std::__1::vector<int, std::__1::allocator<int> >, std::__1::hash<int>, std::__1::equal_to<int>, std::__1::allocator<std::__1::pair<int const, std::__1::vector<int, std::__1::allocator<int> > > > >&) (colecao.cpp:139)
==21036== by 0x100002A19: Colecao::ler(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) (colecao.cpp:73)
==21036== by 0x100001781: main (index.cpp:47)
When I run the code in a huge amount of data I get segmentation fault, and I think it is because of this Valgrind error.
I don't think I need to reserve space for string in unordered_map, then I figured out that is something in word variable constructor. When I initialize word with a static string (for instance, word("test")), Valgrind stops to complain.
I don't know how to fix this string/unordered_map/memory issue.
EDIT: GDB didn't help me. The segmentation fault is only when I use huge amount of data and then huge amount of memory. The only thing GDB give me is Segmentation Fault and memory address nothing more. Valgrind gave me a more complete message.

Answer 1

This might actually be a problem between Valgrind and the memcmp() implementation of your platform (Mac OS X I suppose?).
The uninitialized value in your application supposedly comes from a malloc() call in std::string constructor, the latter of which is unlikely to "create" uninitialized memory on its own. So my guess would be that malloc() allocates a bit more memory than necessary (aligned to 8 bytes maybe), and _platform_memcmp() also takes these bytes into account. System libraries often have highly-optimized implementations of such functions (memcpy, memcmp, strcpy...). As Valgrind often has trouble with these optimizations, it provides own replacement functions (in mc_replace_strmem.c).
Maybe Valgrind lacks these replacements for OS X memcmp(), or your Valgrind version is too old? Also, there might be a setup problem with your system which prevents Valgrind from detecting the memcmp() function at runtime (I'm not familiar with OS X, but maybe you need some kind of debug info for your system libraries).
So, some questions:
are you running the latest Valgrind version? If not, upgrade it.
what OS X version are you using exactly?
does the problem disappear if you disable optimizations when compiling your application?
If this doesn't help, you might want to ask at the Valgrind users mailing list (http://valgrind.org/support/mailing_lists.html) for this specific problem.
Btw. it's pretty difficult to analyze the Valgrind backtraces without any line numbers. See Debugging Symbols Lost When Linking? for a suggestion to get line number info in the backtraces (in short: add "--dsymutil=yes" to Valgrind command line - but check out the notes for this option in http://valgrind.org/docs/manual/manual-core.html#manual-core.erropts first).