String stream producing memory-map

Question

I'm having some trouble with string streams; whenever I try to do use the in-operator (<<) on an non-empty string I get a memory-map printed when I run my program. Here's the code:

for(int i=1 ; i<djikstra_result[0] ; i++)
{
    cout << "DEBUG 5:10 - Round " << i << endl;
    if(i==djikstra_result[0]-1)
    {
        ss << djikstra_result[i] << "km.";
        cout << "DEBUG 5:11" << endl;
    }
    else
    {
        int j = djikstra_result[i];
        cout << "DEBUG 5:13 - j=" << j << endl;
        ss << this->airport[j] << " - ";
        cout << "DEBUG 5:14" << endl;
    }
}

ss is my stringstream, djikstra_result[] is a dynamic int array, this-airport[] is a dynamic string array.

The code compiles just fine, but it only runs until "cout << "DEBUG 5:13 - j=" << j << endl;". The result from that point and onward is:

DEBUG 5:13 - j=0
*** glibc detected *** ./graph: malloc(): memory corruption: 0x0000000000ec10f0 ***
======= Backtrace: =========
/lib/libc.so.6(+0x77806)[0x7f9f8403a806]
/lib/libc.so.6(+0x7b928)[0x7f9f8403e928]
/lib/libc.so.6(__libc_malloc+0x6e)[0x7f9f8403f7de]
/usr/lib/libstdc++.so.6(_Znwm+0x1d)[0x7f9f848ab24d]
/usr/lib/libstdc++.so.6(_ZNSs4_Rep9_S_createEmmRKSaIcE+0x59)[0x7f9f84887d79]
/usr/lib/libstdc++.so.6(_ZNSs4_Rep8_M_cloneERKSaIcEm+0x2b)[0x7f9f8488874b]
/usr/lib/libstdc++.so.6(_ZNSs7reserveEm+0x3c)[0x7f9f8488927c]
/usr/lib/libstdc++.so.6(_ZNSs6appendERKSs+0x4f)[0x7f9f848896ff]
./graph[0x402138]
./graph[0x4032ce]
/lib/libc.so.6(__libc_start_main+0xfd)[0x7f9f83fe1c4d]
./graph[0x401439]
======= Memory map: ========
00400000-00404000 r-xp 00000000 00:15 15341576                           /home/psyberion/Dropbox/algorithms/assign-3b/graph
00604000-00605000 r--p 00004000 00:15 15341576                           /home/psyberion/Dropbox/algorithms/assign-3b/graph
00605000-00606000 rw-p 00005000 00:15 15341576                           /home/psyberion/Dropbox/algorithms/assign-3b/graph
00ec1000-00ee2000 rw-p 00000000 00:00 0                                  [heap]
7f9f7c000000-7f9f7c021000 rw-p 00000000 00:00 0 
7f9f7c021000-7f9f80000000 ---p 00000000 00:00 0 
7f9f83fc3000-7f9f8413d000 r-xp 00000000 08:05 2887651                    /lib/libc-2.11.1.so
7f9f8413d000-7f9f8433c000 ---p 0017a000 08:05 2887651                    /lib/libc-2.11.1.so
7f9f8433c000-7f9f84340000 r--p 00179000 08:05 2887651                    /lib/libc-2.11.1.so
7f9f84340000-7f9f84341000 rw-p 0017d000 08:05 2887651                    /lib/libc-2.11.1.so
7f9f84341000-7f9f84346000 rw-p 00000000 00:00 0 
7f9f84346000-7f9f8435c000 r-xp 00000000 08:05 2884391                    /lib/libgcc_s.so.1
7f9f8435c000-7f9f8455b000 ---p 00016000 08:05 2884391                    /lib/libgcc_s.so.1
7f9f8455b000-7f9f8455c000 r--p 00015000 08:05 2884391                    /lib/libgcc_s.so.1
7f9f8455c000-7f9f8455d000 rw-p 00016000 08:05 2884391                    /lib/libgcc_s.so.1
7f9f8455d000-7f9f845df000 r-xp 00000000 08:05 2887634                    /lib/libm-2.11.1.so
7f9f845df000-7f9f847de000 ---p 00082000 08:05 2887634                    /lib/libm-2.11.1.so
7f9f847de000-7f9f847df000 r--p 00081000 08:05 2887634                    /lib/libm-2.11.1.so
7f9f847df000-7f9f847e0000 rw-p 00082000 08:05 2887634                    /lib/libm-2.11.1.so
7f9f847e0000-7f9f848d6000 r-xp 00000000 08:05 25175504                   /usr/lib/libstdc++.so.6.0.13
7f9f848d6000-7f9f84ad6000 ---p 000f6000 08:05 25175504                   /usr/lib/libstdc++.so.6.0.13
7f9f84ad6000-7f9f84add000 r--p 000f6000 08:05 25175504                   /usr/lib/libstdc++.so.6.0.13
7f9f84add000-7f9f84adf000 rw-p 000fd000 08:05 25175504                   /usr/lib/libstdc++.so.6.0.13
7f9f84adf000-7f9f84af4000 rw-p 00000000 00:00 0 
7f9f84af4000-7f9f84b14000 r-xp 00000000 08:05 2887636                    /lib/ld-2.11.1.so
7f9f84ceb000-7f9f84cef000 rw-p 00000000 00:00 0 
7f9f84d10000-7f9f84d13000 rw-p 00000000 00:00 0 
7f9f84d13000-7f9f84d14000 r--p 0001f000 08:05 2887636                    /lib/ld-2.11.1.so
7f9f84d14000-7f9f84d15000 rw-p 00020000 08:05 2887636                    /lib/ld-2.11.1.so
7f9f84d15000-7f9f84d16000 rw-p 00000000 00:00 0 
7fff9500a000-7fff9501f000 rw-p 00000000 00:00 0                          [stack]
7fff951ff000-7fff95200000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Aborted

I have encountered this problem before, but I haven't been able to work out what's causing it, other than some kind of memory problem. Could someone please help? Here's the code for the whole function:

string graph::get_shortest(string name1, string name2)
{
int* djikstra_result;
int result[this->nr_airport];
int source=-1;
int goal=-1;
bool source_found = false;
bool goal_found = false;
string final="";
string space=" - ";
stringstream ss;
cout << "DEBUG 5:1" << endl;
for(int i=0 ; i<this->nr_airport ; i++)
{
    cout << "DEBUG 5:2 - Round " << i << endl;
    if(name1==this->airport[i])
    {
        cout << "DEBUG 5:3" << endl;
        source = i;
        source_found = true;
    }
    else if(name2==this->airport[i])
    {
        cout << "DEBUG 5:4" << endl;
        goal = i;
        goal_found = true;
    }
}
cout << "DEBUG 5:5" << endl;
if(!source_found || !goal_found)
{
    throw "Starting point or destination not found";
}
cout << "DEBUG 5:6" << endl;
for(int i=0 ; i<this->nr_airport ; i++)
{
    result[i] = -1;
    cout << "DEBUG 5:7 - Round " << i << endl;
}
cout << "DEBUG 5:8" << endl;
djikstra_result = djikstra(source, goal);
cout << "DEBUG 5:9" << endl;
for(int i=1 ; i<djikstra_result[0] ; i++)
{
    cout << "DEBUG 5:10 - Round " << i << endl;
    if(i==djikstra_result[0]-1)
    {
        ss << djikstra_result[i] << "km";
        cout << "DEBUG 5:11" << endl;
    }
    else
    {
        int j = djikstra_result[i];
        cout << "DEBUG 5:13 - j=" << j << endl;
        //string val = this->airport[j];
        cout << "DEBUG 5:14" << endl;
        ss << this->airport[j];
        cout << "DEBUG 5:15" << endl;
    }
}
ss >> final;
return final;   
}

EDIT:

The array this->airport[] is declared like this:

this->cap_airport = 1;
this->airport = new string[this->cap_airport];

When I add something to the array I expand it with the following function:

void graph::expand_list()
{
if(this->nr_airport==this->cap_airport)
{
    this->cap_airport++;
    string* temp = new string[this->cap_airport];
    for(int i=0 ; i<this->nr_airport ; i++)
    {
        temp[i] = "";
        if(this->airport[i]!="")
        {
            temp[i] = this->airport[i];
        }
    }
    delete [] this->airport;
    this->airport = temp;
}
}

I do this to avoid allocating more space than needed, and I only add info to the array when reading from file.

EDIT:

After removing the << " - "; from the string, it seems to make some progress. I'm calling the function get_shortest(string name1, string name2) in my main-function, like this:

graph g1;
cout << g1.get_shortest("A","E") << endl;

I can now see some more output when I run the program:

DEBUG 5:15
DEBUG 5:10 - Round 5
DEBUG 5:11
AAAA0km
*** glibc detected *** ./graph: free(): invalid pointer: 0x0000000001ba7070 ***
======= Backtrace: =========
/lib/libc.so.6(+0x77806)[0x7f400d9cf806]
/lib/libc.so.6(cfree+0x73)[0x7f400d9d60d3]
//And so on as before

So the problem doesn't seem to be connected to the string stream itself. Any ideas?

j may be going beyond the bounds of array: this->airport[j]? — hmjd
Unrelated to your problem, but still good practice: pass your variables by const reference. stackoverflow.com/questions/2582797/… — luke
@hmjd, I don't think that's the case, since j is equal to 0 when the program crashes. I'll add this in the question though, so thanks for pointing it out! — Psyberion
@Psyberion, can you post declaration and population of airport[] ? — hmjd
Also, how are you expanding this->airport everytime you add something? I would suggest to use vector<string> if your array is dynamic. — deebee

Psyberion Psyberion · Accepted Answer · 2012-05-02T10:19:12

I found the problem in a whole other part of the program; the memory map is the result of an invalid memory operation, but it's not reading from neither djikstra_result[] or this->airport[], as suggested. Just trying to read from an invalid position in an array would result in a segmentation fault. The problem is that I in the function djikstra(int source, goal) tried to add data to an invalid position in an array. Here's the code:

result = new int[nr_visited];
for(int i=0 ; i<nr_visited+2 ; i++)
{
    if(i==0)
    {
        result[i] = nr_visited;
    }
    else if(i==nr_visited+1)
    {
        result[i] = distance;
    }
    else
    {
        result[i] = route[i-1];
    }
}
return result;

I create a dynamic int array with the length nr_visited, but I try to put data in the position nr_visited+1, which is invalid. This causes the memory map. Thus, the fix is to declare the int array with the length nr_visited+2:

result = new int[nr_visited+2];

So, now, it all works.

String stream producing memory-map

1 Answers