Seg Fault In My Texture Loading Functions

This is a discussion on Seg Fault In My Texture Loading Functions within the C++ Programming forums, part of the General Programming Boards category; I have two functions, one takes a file name and a pointer to a GLuint. It appears the textures get ...

  1. #1
    Registered User IdioticCreation's Avatar
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    Seg Fault In My Texture Loading Functions

    I have two functions, one takes a file name and a pointer to a GLuint. It appears the textures get loaded OK, but I get a segmentation fault in a call to glBindTexture();

    Code:
    #include "textureloader.h"
    
    bool loadTexture(const char* fname, GLuint *texid, GLuint internalF, GLuint byteF)
    {
    	SDL_Surface* imgFile;
    	if ((imgFile = IMG_Load(fname)) == NULL) //Load Image to an SDL surface
    	{
    	    return false;
    	}
        glPixelStorei(GL_UNPACK_ROW_LENGTH,0);
        glGenTextures(1,texid);
        glBindTexture(GL_TEXTURE_2D,*texid); //Segementation Fault NOT here
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
        glTexImage2D(GL_TEXTURE_2D,0,internalF,imgFile->w, imgFile->h,0,byteF,GL_UNSIGNED_BYTE,imgFile->pixels);
        SDL_FreeSurface(imgFile);
        return true;
    }
    
    bool loadEmUp(char* fname, GLuint *texsToLoad)
    {
        std::string texpath;
        int texnum;
        GLuint internalformat, byteformat;
        int texcount = 0;
        std::ifstream texfile (fname);
        if (texfile.is_open())
        {
            texfile >> texnum; //Now holds the number of textures
            texsToLoad = new GLuint[texnum]; //Create room for the new textures
            while (texcount < texnum)
            {
                texfile >> texpath;
                texfile >> internalformat;
                texfile >> byteformat;
                if (!loadTexture(texpath.c_str(), &texsToLoad[texcount], internalformat, byteformat))
                {
                    std::cout << "Failed to load image: " << texpath << std::endl;
                    return false;
                }
                else std::cout << "Loaded " << texpath << std::endl;
                texcount++;
            }
            texfile.close();
        }
        else return false;
        std::cout << texnum << " Textures Loaded" << std::endl;
        return true;
    }
    The file with textures names in it is something like this:
    Code:
    2
    media/img1.png 3 3
    media/img2.png 3 3
    Segmentation fault does not occur in the the loadTexture() function, it's in a different one. Also: GLuint *texsToLoad is declared as a global where the loadEmUp() functions is called from, but it's named myTextures.
    Last edited by IdioticCreation; 12-26-2007 at 11:04 PM.

  2. #2
    Registered User IdioticCreation's Avatar
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    OK, I made a very simplified version of the above code.

    Code:
    #include <iostream>
    
    void func_two(int *toChange)    //like loadTexture()
    {
        *toChange = 7;
    }
    
    void func_one(int **myArray)    //like loadEmUp()
    {
        *myArray = new int[2];
        func_two(myArray[0]);
        func_two(myArray[1]);
    }
    
    int main()
    {
        int *myArray;
        func_one(&myArray);
        std::cout << myArray[0] << std::endl;
        std::cout << myArray[1] << std::endl;
    
        return 0;
    }
    I find it very strange that it outputs:
    7
    0

    I would expect it to output
    7
    7

  3. #3
    C++ Witch laserlight's Avatar
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    In func_one(), myArray is a pointer to an array of ints. So, myArray[0] is *(myArray + 0), i.e., *myArray, which is the array itself. So what you passed to func_two() is the array itself.

    In func_two, toChange is now a pointer to the array. *toChange, therefore, is the first element of the array, thus it is changed to 7.

    Back to func_one(). myArray[1] is *(myArray + 1), i.e., the next array. However, myArray only points to one array, thus this is a mistake. What you should write is:
    Code:
    *myArray = new int[2];
    func_two(&(*myArray)[0]);
    func_two(&(*myArray)[1]);
    Better yet, use pass by reference:
    Code:
    #include <iostream>
    
    void func_two(int& toChange)    //like loadTexture()
    {
        toChange = 7;
    }
    
    void func_one(int*& myArray)    //like loadEmUp()
    {
        myArray = new int[2];
        func_two(myArray[0]);
        func_two(myArray[1]);
    }
    
    int main()
    {
        int *myArray = 0; // To safely delete[] if new[] is not done.
        func_one(myArray);
        std::cout << myArray[0] << std::endl;
        std::cout << myArray[1] << std::endl;
        delete[] myArray; // Remember to delete[] what you new[] !
    }
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  4. #4
    Registered User IdioticCreation's Avatar
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    Thanks laser. I never understood well how passing by reference is different from passing a pointer. Like function(int* x) to function(int& x). Do you have a link where it's explained well, or would you mind doing it yourself?

    Thanks!

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