Why am I getting a segmentation fault? Valgrind? - c++

Question

I have written a program to determine the longest common subsequence between two strings. I have tested my function LCSLength() with a small hard coded test case and it returns the correct value.
Now, I am reading strings from a file to compare, and my program gives me a segmentation fault. Here is the code:
#include <iostream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
int LCSLength(string X,string Y);
int main()
{
ifstream inData("sequences.dat");
vector<string> lines(1);
string line;
int LCS;
while (getline(inData,line))
{
if (line.empty())
lines.push_back("");
else
lines.back() += line;
}
LCS = LCSLength(lines[0],lines[1]);
cout << "The LCS is: " << LCS << endl;
return 0;
}
int LCSLength(string X,string Y)
{
int m = X.size();
int n = Y.size();
int L[m+1][n+1];
for(int i=0; i<=m; i++)
{
for(int j=0; j<=n; j++)
{
if(i==0 || j==0)
L[i][j] = 0;
else if(X[i-1]==Y[j-1])
L[i][j] = L[i-1][j-1]+1;
else
L[i][j] = max(L[i-1][j],L[i][j-1]);
}
}
return L[m][n];
}
When I compile using -pedantic -ansi, I get the following error: In function LCSLength(std::string, std::string): ISO C++ forbids variable size array 'L'.
I compiled using Valgrind, and here are the errors it produced:
==15183== LEAK SUMMARY:
==15183== definitely lost: 8,624 bytes in 14 blocks
==15183== indirectly lost: 1,168 bytes in 5 blocks
==15183== possibly lost: 5,518 bytes in 58 blocks
==15183== still reachable: 44,925 bytes in 278 blocks
==15183== suppressed: 0 bytes in 0 blocks
==15183== Reachable blocks (those to which a pointer was found) are not shown.
==15183== To see them, rerun with: --leak-check=full --show-reachable=yes
==15183==
==15183== ERROR SUMMARY: 23 errors from 23 contexts (suppressed: 0 from 0)
==15183==
==15183== 1 errors in context 1 of 23:
==15183== Invalid read of size 4
==15183== at 0x38326: std::string::_Rep::_M_grab(std::allocator<char> const&, std::allocator<char> const&) (in /usr/lib/libstdc++.6.0.9.dylib)
==15183== by 0x388EF: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::string const&) (in /usr/lib/libstdc++.6.0.9.dylib)
==15183== by 0x100001AAE: main (firstt.cpp:23)
==15183== Address 0xfffffffffffffff8 is not stack'd, malloc'd or (recently) free'd
==15183==
==15183==
==15183== 1 errors in context 2 of 23:
==15183== Invalid read of size 8
==15183== at 0x388DC: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::string const&) (in /usr/lib/libstdc++.6.0.9.dylib)
==15183== by 0x100001AAE: main (firstt.cpp:23)
==15183== Address 0x100023d28 is 0 bytes after a block of size 8 alloc'd
==15183== at 0x5237: malloc (in /usr/local/Cellar/valgrind/3.8.1/lib/valgrind/vgpreload_memcheck-amd64-darwin.so)
==15183== by 0x4B346: operator new(unsigned long) (in /usr/lib/libstdc++.6.0.9.dylib)
==15183== by 0x100002DFE: __gnu_cxx::new_allocator<std::string>::allocate(unsigned long, void const*) (new_allocator.h:91)
==15183== by 0x100002E42: std::_Vector_base<std::string, std::allocator<std::string> >::_M_allocate(unsigned long) (stl_vector.h:131)
==15183== by 0x100002E9D: std::_Vector_base<std::string, std::allocator<std::string> >::_Vector_base(unsigned long, std::allocator<std::string> const&) (stl_vector.h:116)
==15183== by 0x1000030E4: std::vector<std::string, std::allocator<std::string> >::vector(unsigned long, std::string const&, std::allocator<std::string> const&) (stl_vector.h:215)
==15183== by 0x1000017D9: main (firstt.cpp:11)
==15183==
==15183== ERROR SUMMARY: 23 errors from 23 contexts (suppressed: 0 from 0)
Segmentation fault: 11
My test file has two lines of sequences. I check lines.size() and it returns 2. I also cout << lines[0] and cout << lines[1] and the correct strings are printed.
Can anyone help me figure this out? Thanks.

Answer 1

your lines vector always contains one element, therefore accesing it by index 1 causes UB
LCS = LCSLength(lines[0],lines[1]);
here ^

Answer 2

Did you confirm that after exiting the while loop, both lines[0] and lines[1] exist? This will happen only if you have at least one empty line in your input file (and recognized as such).
According to Valgrind (do read its output!) - you are doing something wrong from within main (lines 11 or 23) and not from LCSLength.