Segmentation fault when declaring std::vector

[Kosmos]

Hello,

When I run my program, I get a segmentation fault. This is what the gdb debugger outputs:

Program received signal SIGSEGV, Segmentation fault.
0x0804d048 in std::vector<int, std::allocator<int> >::emplace_back<int>(int&&)
    (this=0x0) at /usr/include/c++/4.6/bits/vector.tcc:95
95		if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)

I believe this is what is causing the segmentation fault:

#ifndef WORLD_HPP
#define WORLD_HPP

#include <tmx_loader.hpp>
#include <SFML/Graphics.hpp>

const int SCR_WIDTH=1024;
const int SCR_HEIGHT=768;

class World
{
private:
  TMXLoader loader;
  std::vector<std::vector<int>> map;  // Right here
  std::vector<int> meta_layer;
  sf::Image tileset;
  sf::Sprite tile;
  sf::IntRect subRect[160];
  sf::RenderWindow *App;
  int tx;
  int ty;
public:
  bool equalView;
  World(const char *fileName,sf::RenderWindow *dest,int xoff,int yoff);
  void Draw(int x,int y);
  std::vector<int> getMetaLayer() const { return meta_layer; }
};

#endif // world.hpp

When I create an instance of the above object, there is a segmentation fault.

Do you guys have any ideas on why is the OS throwing a segmentation fault?

Thanks in advance and cheers,
Kosmos.

[Catfish3]

A segmentation fault happens when you try to access memory you aren't allowed to. In your case, this may happen if you try to read/write outside the vector.

Hint: vector::at(), unlike vector::operator[], will at least throw an exception with a slightly better error message, when you're going out of range.

As for your posted code, it looks fine to me. I suggest you post the functions too.

[Kosmos]

Thanks a lot Catfish,

I changed all vector::operator[] to vector::at(), and the program threw an exception, here is the function that does it, I localized the line that throws the exception, the line is marked with a comment:

std::vector<std::vector<int>> TMXLoader::Parse()
{
  std::vector<std::vector<int>> output;
  std::string line;
  std::vector<std::string> tmp;
  int yt=1;
  int pos=0;
  int l_pos=0;

  int tilewidth=0;
  int tileheight=0;
  int tw_pos;
  int tw_pos_end;
  int th_pos;
  int th_pos_end;

  int layer_i=0;

  while(getline(map,line))
    {
      if(line.find("<map")!=std::string::npos)
	{
	  tw_pos=line.find("width=\"");
	  tw_pos+=std::string("width=\"").size();
	  for(tw_pos_end=tw_pos;line[tw_pos_end]!='"';++tw_pos_end);
	  tilewidth=atoi(line.substr(tw_pos,tw_pos_end-tw_pos).c_str());
	  map_width=tilewidth;
	  th_pos=line.find("height=\"");
	  th_pos+=std::string("height=\"").size();
	  for(th_pos_end=th_pos;line[th_pos_end]!='"';++th_pos_end);
	  tileheight=atoi(line.substr(th_pos,th_pos_end-th_pos).c_str());
	  map_height=tileheight;
	}
      else if(line.find("<data encoding=\"csv\">")!=std::string::npos)
	{
	  while(yt<=tileheight)
	    {
	      getline(map,line);       
	      tmp=split(line,',');
	      for(int i=0;i<tilewidth;++i)
		{
		  output.at(layer_i).push_back(atoi(tmp[i].c_str())); // This is what is throwing the exception
		}
	      yt++;
	    }
	  layer_i++;
	}
    }
    return output;
}

Here is the exception in more detail:

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check

I don't know why it is doing that though.

Cheers,
Kosmos.

[Athar]

It's doing that because you're trying to access an element that doesn't exist.
Since output is empty and you never add any elements, that's not particularly surprising.

[vlad from moscow]

Vector output is empty but you are trying to use method at().

output.at(layer_i).push_back(atoi(tmp[i].c_str())); // This is what is throwing the exception

What are you awaiting has to be?

[Kosmos]

Athar wrote:

It's doing that because you're trying to access an element that doesn't exist.
Since output is empty and you never add any elements, that's not particularly surprising.

vlad from moscow wrote:

Vector output is empty but you are trying to use method at().

output.at(layer_i).push_back(atoi(tmp[i].c_str())); // This is what is throwing the exception

What are you awaiting has to be?

I thought that doing output.at(1), would create an element. I was wrong; it also makes no sense.

I fixed the code, perhaps it will be of particular use to future readers:

for(int i=0;i<tilewidth;++i)
		{
                 /* Because the the line below us is trying to access an element of the array that has not been
                     created yet, there is an exception, 'output' is of type std::vector<std::vector<int>>, I
                     changed it's type to std::vector<int>, and rewrote that line as:
                     output.push_back(atoi(tmp[i].c_str())); */
		  output.at(layer_i).push_back(atoi(tmp[i].c_str())); // This is what is throwing the exception 
		}
	      yt++;
	    }
          /* The 'layer_i' variable was removed, and a std::vector<std::vector<int>> object was created, called
               'f_output'. I rewrite this line as:
               f_output.push_back(output); */
	  layer_i++; // This variable was removed
	}
    }
// Note: Of course I return f_output instead of output. 

Thanks to Athar and vlad from moscow.

Cheers,
Kosmos.