Segmentation fault disappears when debugging with GDB

Question

I am having a segmentation fault while running this code:

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
using namespace std;

class cipher {
    public:
        char letter;
        int frequency;
};

bool comp(const cipher &A, const cipher &B) {
    if(A.frequency<B.frequency)
        return true;
    else
        return false;
}

vector<cipher> cipherF;

int posOf(char c)
{
    int i;
    for(i=0; i<26; ++i)
        if(cipherF[i].letter == c)
            return i;
    return -1;
}

int main()
{
    int t;
    cin >> t;
    string tmp;
    getline(cin, tmp);
    while(t--) {
        cipherF.clear();
        for(int i=97; i<=122; ++i)
        {
            cipher cip;
            cip.letter = i;
            cip.frequency = 0;
            cipherF.push_back(cip);
        }

        string txtF, cipherText;
        getline(cin, txtF);
        getline(cin, cipherText);

        for(int i=0; i<cipherText.size(); ++i)
        {
            char c = tolower(cipherText[i]);
            ++(cipherF[c-97].frequency);
        }

        stable_sort(cipherF.begin(), cipherF.end(), comp);
        for(int i=0; i<cipherText.size(); ++i)
        {
            int pos = posOf(cipherText[i]);
            if(pos == -1)
                continue;
            else if(isupper(cipherText[i]))
                cipherText[i] = toupper(txtF[pos]);
            else
                cipherText[i] = txtF[pos];
        }
        cout << cipherText << endl;
    }
}

The problem is when I run it in GDB, the code runs fine and excellent. Why is this running in GDB without any segmentation fault, but running at every other place with a segmentation fault?

Here is the problem I am trying to solve: http://www.codechef.com/DEC13/problems/CHODE

Answer 1

The problem is that your input includes characters that are not in the range [a-Z]. For example: ! That causes the vector to be accessed at invalid indexes.

You can check these things running your program with valgrind.

valgrind ./ideone < stdin
...
==2830== Invalid read of size 4
==2830==    at 0x40111A: main (ideone.cpp:53)
...
==2830== Invalid write of size 4
==2830==    at 0x401120: main (ideone.cpp:53)

The problem is in these lines:

    for(int i=0;i<cipherText.size();++i)
    {
        char c = tolower(cipherText[i]);
        ++(cipherF[c-97].frequency);
    }

c - 97 may be lower than 0.

You can check, for example:

    for(int i=0;i<cipherText.size();++i)
    {
        char c = tolower(cipherText[i]);
        if (c < 'a' || c > 'z') continue;
        ++(cipherF[c-97].frequency);
    }