Thread: Segmentation Fault with Vectors~

I'm attempting to create a Hash Table using vectors for open hashing. The issue is the program runs just fine in NetBeans IDE, however when run on the older unix based test server, it seg faults. I've ran through GDB numerous times, and here's what I've found:

Program received signal SIGSEGV, Segmentation fault.0x001c538b in std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::string const&) () from /usr/lib/i386-linux-gnu/libstdc++.so.6
(gdb) where
#0 0x001c538b in std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::string const&) () from /usr/lib/i386-linux-gnu/libstdc++.so.6
#1 0x08049841 in __gnu_cxx::new_allocator<std::string>::construct (this=0x804fffc, __p=0x4, __val=...)
at /usr/include/c++/4.6/ext/new_allocator.h:108
#2 0x080493f8 in std::vector<std::string, std::allocator<std::string> >:ush_back (this=0x804fffc,
__x=...) at /usr/include/c++/4.6/bits/stl_vector.h:830
#3 0x08048e74 in main () at openhash.cpp:65

Here's the piece of the code that's acting up:

Code:

int main() {



    vector< vector<string> > hashTable; //initialize hash table, 100 buckets, using vectors to chain strings.
    vector<string> buffer;
    hashTable.resize(100);
    for (int j = 0; j < 100; j++)
        hashTable[j].resize(15);


    string buff;
    int key;
    int collisions = 0;
    
    while (cin >> buff) {
        buffer.push_back(buff);
    }
    int placeholder =0;
    while(placeholder < buffer.size())
    {
        key = hashKey(buff);
        
        if(hashTable[key].empty())
        {
            (hashTable[key]).push_back(buffer[placeholder]);
            
            placeholder++;
        }
        else {
            
            (hashTable[key]).push_back(buffer[placeholder]);                   //line 65 in full program//
            
            collisions++;
            placeholder++;
        }
        
        
    }

If anyone can offer some insight into the problem, I would be forever grateful!

Thank You very much.

~ALex~

Originally Posted by MutantJohn

Maybe it's because of placeholder. Try using std::vector::at to check bounds?

I second this. Use .at() to check that you don't do out-of-bounds. If you still can't find it, provide your hash function and a set of inputs that reproduces the issues.

Where's the code for the hashKey() function? How does it calculate its return value? If it's actually generating hashtable keys, they would tend to be random and evenly distributed through the range of the type that contains the hash value, unless you are constraining it to a narrower range. Is your vector big enough to handle the range of values that hashKey() produces?