I have Problems with accessing the 2D Array in a typedef from a other function over a pointer, even if i allocate the array in heap!

#include <stdlib.h>
#include <stdio.h>
typedef struct {
    int a;
    int b;
    int **data;
} Image;
Image *createImage(int a, int b) {
    Image createImage;
    createImage.data = malloc(a * sizeof(int *));
    for (int i = 0; i < a; i  ) {
         createImage.data[i] = malloc(b * sizeof(int));
    }
    Image *imagePointer = malloc(sizeof(Image));
    imagePointer = &createImage;
    return imagePointer;
}
int main () {
     int a = 70;
     int b = 90;
     Image *imagePointer = createImage(a, b);
     for (int i = 0; i < a; i  ) {
          for (int j = 0; j < b; j  ) {
               imagePointer->data[i][j] = i   j;
          }
     }
}

I get an error at load Image because i have done something wrong with accessing the allocated storage. What should i change?

Jonathan

CodePudding user response：

Instead of returning a pointer to Image return the local variable itself and it works. Also variable name and function name should not be same createImage it creates confusion sometimes.

#include <stdlib.h>

typedef struct {
    int a;
    int b;
    int **data;
} Image;
Image createImage(int a, int b) {
    Image ci;
    ci.data = malloc(a * sizeof(int *));
    for (int i = 0; i < a; i  ) {
         ci.data[i] = malloc(b * sizeof(int));
    }
    return ci;
}
int loadImage () {
     int a = 70;
     int b = 90;
     Image imagePointer = createImage(a, b);
     for (int i = 0; i < a; i  ) {
          for (int j = 0; j < b; j  ) {
               imagePointer.data[i][j] = i   j;
          }
     }
}

int main(int argc, char const *argv[])
{
    loadImage();
    return 0;
}

CodePudding user response：

1. It is not 2D array only an array of pointers.
2. Use size_t for sizes
3. Use objects not types in sizeof
4. Always check the result of malloc

I would use flexible array member and array pointers to remove one level of indirection and ease allocation and deallocation (only one malloc/free needed)

typedef struct {
    size_t a;
    size_t b;
    unsigned data[];
} Image;

Image *createImage(const size_t a, const size_t b) 
{
    Image *imagePointer;
    imagePointer = malloc(sizeof(*imagePointer)   a * b * sizeof(imagePointer -> data[0]));
    if(imagePointer)
    {
        imagePointer -> a = a;
        imagePointer -> b = b;
    }
    return imagePointer;
}


int main (void) 
{
     const size_t a = 70;
     const size_t b = 90;

     Image *imagePointer = createImage(a, b);
     if(imagePointer)
     {
        unsigned (*image)[imagePointer -> b] = (unsigned (*)[imagePointer -> b])imagePointer -> data;
        for (size_t i = 0; i < a; i  ) 
        {
            for (size_t j = 0; j < b; j  ) 
            {
                image[i][j] = i   j;
            }
        }
     }
}