Passing a triple pointer to allocate memory in another function, sscanf exception

问题

Im allocating memory to a double pointer in another function, therefore I need to use a pointer to the pointer to the pointer. Im getting an exception thrown when i use sscanf, im not sure exactly why. Heres a snippet of the code. This was working earlier when it was all in the same function and i only needed to use double pointers, but now that im refactoring the code and using triple pointers im having this issue.

typedef float vector[3]

int mainLoaderFunc() {

    char* memory = NULL;
    size_t size = loadFile(fileName, &memory); // load model file into memory, this works, tested and true

    // create vector arrays
    vector *vertexArray = NULL;         
    vector *normalArray = NULL;         
    vector *textureArray = NULL;        

    loadArrays(size, memory, &vertexArray, &normalArray, &textureArray);

    // do other stuff with arrays

}

void loadArrays(size_t size, char *memory, vector **vertexArray, vector **normalArray, vector **textureArray) {

    int numVerts = 0; 
    int numNormals = 0;  
    int numTextures = 0;  

    char* p = memory;           // pointer to start of memory
    char* e = memory + size;    // pointer to end of memory

    // count verts, normals, textures for memory allocation
    while (p != e) {
        if (memcmp(p, "vn", 2) == 0) {
            numNormals++;
        } else if (memcmp(p, "vt", 2) == 0) { 
            numTextures++;
        } else if (memcmp(p, "v",  1) == 0) {
            numVerts++;
        } 
        while (*p++ != (char) 0x0A);
    }

    // allocate memory for vector arrays
    *vertexArray        = new vector[numVerts];
    *normalArray        = new vector[numNormals];
    *textureArray       = new vector[numTextures];

    p = memory;

    int vertexIndex = 0;
    int normalIndex = 0;
    int textureIndex = 0;  //*** IF BREAK POINT HERE: NO EXCEPTION

    // load data from memory into arrays
    while (p != e) {

        if (memcmp(p, "vn", 2) == 0) {
            sscanf(p, "vn %f %f %f", &normalArray[normalIndex][0], &normalArray[normalIndex][1], &normalArray[normalIndex][2]);
            normalIndex++;
        } else if (memcmp(p, "vt", 2) == 0) {
            sscanf(p, "vt %f %f", &textureArray[textureIndex][0], &textureArray[textureIndex][1]);
            textureIndex++;
        } else if (memcmp(p, "v", 1) == 0) {
            sscanf(p, "v %f %f %f", &vertexArray[vertexIndex][0], &vertexArray[vertexIndex][1], &vertexArray[vertexIndex][2]);
            vertexIndex++;
        } 
        while (*p++ != (char) 0x0A);
    }

}

Once the code hits the sscanf part, i get the exception:

Unhandled exception at 0x5e9cf2dc (msvcr100d.dll) in derp.exe: 0xC0000005: Access violation writing location 0xcccccccc.

回答1:

As someone pointed out, you really shouldn't be using triple pointers here.

Additionally, you might want to do your file scanning as something other than chained if-blocks (or at least split it up into functions).

This should fix your issue, since you only have 2 dereferences where you should have 3:

    if (memcmp(p, "vn", 2) == 0) {
        sscanf(p, "vn %f %f %f", &(*normalArray)[normalIndex][0], &(*normalArray)[normalIndex][1], &(*normalArray)[normalIndex][2]);
        normalIndex++;
    } else if (memcmp(p, "vt", 2) == 0) {
        sscanf(p, "vt %f %f", &(*textureArray)[textureIndex][0], &(*textureArray)[textureIndex][1]);
        textureIndex++;
    } else if (memcmp(p, "v", 1) == 0) {
        sscanf(p, "v %f %f %f", &(*vertexArray)[vertexIndex][0], &(*vertexArray)[vertexIndex][1], &(*vertexArray)[vertexIndex][2]);
        vertexIndex++;
    } 

回答2:

You're passing invalid addresses to sscanf. Look at one of your function's arguments:

vector **vertexArray

vertexArray points to the address of a pointer passed from mainLoaderFunc. *vertexArray refers to a pointer to an actual vector array, as you realise when you allocate memory for it:

*vertexArray = new vector[numVerts];

Therefore a valid element in the array would be:

(*vertexArray)[vertexIndex][0]

Note the * which your code is missing; *vertexArray refers to the actual array. Now sscanf wants an address to this variable, so now get the address of it (extra brackets added for the sake of clarity):

&( (*vertexArray)[vertexIndex][0] )

Change all your arguments to sscanf to be like this one.

回答3:

You are not dereferencing the vector pointers correctly when passing them to sscanf(). Try this instead:

if (memcmp(p, "vn", 2) == 0) { 
    sscanf(p, "vn %f %f %f", &((*normalArray)[normalIndex][0]), &((*normalArray)[normalIndex][1]), &((*normalArray)[normalIndex][2])); 
    ++normalIndex; 
} else if (memcmp(p, "vt", 2) == 0) { 
    sscanf(p, "vt %f %f", &((*textureArray)[textureIndex][0]), &((*textureArray)[textureIndex][1])); 
    ++textureIndex; 
} else if (memcmp(p, "v", 1) == 0) { 
    sscanf(p, "v %f %f %f", &((*vertexArray)[vertexIndex][0]), &((*vertexArray)[vertexIndex][1]), &((*vertexArray)[vertexIndex][2])); 
    ++vertexIndex; 
}  

来源：https://stackoverflow.com/questions/8596563/passing-a-triple-pointer-to-allocate-memory-in-another-function-sscanf-exceptio