Question

I have a problem trying to load a 3d object from a wavefront file and saving it into a structure that I predefined. I get a segmentation fault error, I`m assuming that I have gone wrong with my pointers somewhere. Here are the sructures:

struct vertex
{
    float x,y,z;
};

// the idea is to split each object into groups in order to animate
// them seperately
struct group
{
    float xpos, ypos, zpos;
    float xrot, yrot, zrot;

    int numVertices;
    int numFaces;

    //all verteces
    vertex vertices[MAX_NUM];

    //faces-- a face is made up of one vertex from each of these arrays
    vertex vertexOne[MAX_NUM];
    vertex vertexTwo[MAX_NUM];
    vertex vertexThree[MAX_NUM];
};

struct object
{
    float xpos, ypos, zpos;
    float xrot, yrot, zrot;

    int numGroups;
    group * groups[MAX_NUM];
};

This is the function that should load the object:

object * loadObject(char * path)
{
object * obj;
obj = new object;

char str[1];
obj->numGroups = 0;
FILE * file = fopen(path, "r");

while(fscanf(file, "%s", str) != EOF)
{
    if(str[0] == 'o')
    {
        obj->numGroups++;
        obj->groups[obj->numGroups-1] = new group;

        obj->groups[obj->numGroups-1]->numVertices = 0;
        obj->groups[obj->numGroups-1]->numFaces = 0;
    }
    else if(str[0] == 'v' && str[1] == ' ')
    {
        obj->groups[obj->numGroups-1]->numVertices++;
        sscanf(str, "v %f %f %f",
        &(obj->groups[obj->numGroups-1]->vertices[obj->groups[obj->numGroups-1]->numVertices-1].x),
        &(obj->groups[obj->numGroups-1]->vertices[obj->groups[obj->numGroups-1]->numVertices-1].y),
        &(obj->groups[obj->numGroups-1]->vertices[obj->groups[obj->numGroups-1]->numVertices-1].z));
    }
    else if(str[0] == 'f')
    {
        int one, two, three;
        obj->groups[obj->numGroups-1]->numFaces++;
        sscanf(str, "f %i/%*i/%*i %i/%*i/%*i %i/%*i/%*i", &one, &two, &three);

        obj->groups[obj->numGroups-1]->vertexOne[obj->groups[obj->numGroups-1]->numFaces-1] = obj->groups[obj->numGroups-1]->vertices[one-1];
        obj->groups[obj->numGroups-1]->vertexTwo[obj->groups[obj->numGroups-1]->numFaces-1] = obj->groups[obj->numGroups-1]->vertices[two-1];
        obj->groups[obj->numGroups-1]->vertexThree[obj->groups[obj->numGroups-1]->numFaces-1] = obj->groups[obj->numGroups-1]->vertices[three-1];
    }
}

fclose(file);
return obj;
}

I have tried for quite a while to find the problem but i get no information on the error at all. I know the way I`m assigning the vertices and faces is a bit tedious, any suggesstions for neater code are very welcome. Please tell me if any additional information is needed, thank you very much in advance.

Answer 1

First you do

obj->numGroups = 0;

Then in some cases you do

obj->groups[obj->numGroups-1]

without increasing obj->numGroups, meaning you access the array way out of bounds. You also don’t check if the array entry been allocated or not.

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Another problem is that str is only one character, and your fscanf scans for a string which will put at least two character into the string: The actual character you want and then the string terminator character. You also access str[1] later in the loop, which is out of bounds. All of this is undefined behaviour and can lead to crashes.