Modifying a member in a struct within a struct array

Question

I am trying to solve a problem in an assignment I was given - the problem being is that one of the functions I am currently implementing does not work.

First and foremost, I was told that code provided by my professor (down below) cannot be modified in any way:

#include <crtdbg.h>
#include <stdio.h>
#include <time.h>
#include <string.h>

#define BLOCK_SIZE 64
    typedef enum { FALSE = 0, TRUE } BOOL;

typedef struct {
    char* fileName;         // Frame fileName
}Frame, *pFrame, **ppFrame;

typedef struct {
    unsigned int numFrames;     // number of Frame* 
    ppFrame frames;         // array of Frame*
}Animation, *pAnimation;

// Forward declarations
void initAnimation(pAnimation);
void insertFrame(pAnimation);
void deleteFrames(pAnimation);
void runFrames(pAnimation);

int main(void)
{
    char response;
    BOOL RUNNING = TRUE;
    Animation A;
    _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
    initAnimation(&A);

    while (RUNNING)
    {
        printf("MENU\n 1. Insert a Frame\n 2. Delete all the Frames\n 3. Run the Animation\n 4. Quit\n");
        scanf("%c", &response);
        switch (response)
        {
        case '1':insertFrame(&A); break;
        case '2':deleteFrames(&A); break;
        case '3':runFrames(&A); break;
        case '4':RUNNING = FALSE; deleteFrames(&A); break;
            default:printf("Please enter a valid option\n");
        }
    printf("\n");
    while ((response = getchar()) != '\n' && response != EOF);// clear input buffer
}
return 0;
}

Next, I needed to create functions for the forward declaration statements in the provided code. The function that I am having problems with is here:

void insertFrame(Animation *pAnimation)
{
    char input[50];
    int count;
    int dupe = 0;
    int blockFrames = pAnimation->numFrames % BLOCK_SIZE;
    int blocks = (pAnimation->numFrames / BLOCK_SIZE) + 1;

    printf("Insert a frame into the Animation\n");
    scanf("%s", input);

    /* check the input to see if it matches any of the existing frames */
    for (count = 0; count < pAnimation->numFrames; count++)
    {
        printf("%s\n", pAnimation->frames[count]->fileName); /*test: show the file name to be compared against*/
        if (strcmp(input, pAnimation->frames[count]->fileName) != 0)
        {
            dupe = 1;
            printf("Dupe detected!");
            return;
        }
    }

    printf("dupe = %d\n", dupe);

    /* afterwards, actually add the frame if it's ok */
    if (dupe == 0)
    {
        pAnimation->numFrames++;
        strcpy(pAnimation->frames[pAnimation->numFrames - 1]->fileName, input); /* <-- This is where the error happens */

    }
}

Every time I use printf or strcpy to show or modify the fileName struct member from pAnimation->frames[ ], it shows a read access violation. Upon closer inspection, it appears that the fileName from the address pointed from pAnimation->frames[ ] is unknown.

Why is this happening, and how do I fix it?

---

Okay, so after taking the advice, this is what my initAnimation looks like:

void initAnimation(Animation *pAnimation)
{
    pAnimation->numFrames = 0;
    pAnimation->frames = malloc(BLOCK_SIZE * sizeof(Frame));
}

I have also got in contact with my professor, and he said that I didn't need to allocate anything unless I am "adding" a new frame (via insertFrame). I'm not entirely sure what he meant by that.

Answer 1

Looking at

void initAnimation(Animation *pAnimation)
{
    pAnimation->numFrames = 0;
    pAnimation->frames = malloc(BLOCK_SIZE * sizeof(Frame));
}

I assume you are allocating an array of frame pointers. So each cell is sizeof(pFrame) not sizeof(Frame). That is big enough to hold a pointer to a frame not a frame itself.

Second you need to allocate these frames as you create them.

Something like

pAnimation->frames[i] = malloc(sizeof(Frame));

Finally a Frame holds a Pointer to character array. You need to allocate that as well

pAnimation->frames[i]->fileName = malloc(50); // Use your max size

Also you should be checking the results of the malloc calls.

I would put the 2 frame allocation calls in insertFrame() as that is when you are creating/adding a new frame.