How to dynamically allocate multi-dimensional array on variable that's already been declared in-CodePudding

I have a structure that contains a pointer to a 2d-array that will be allocated in an initialization function. It's also important to allocate this array in a contiguous block of memory.

I know to allocate a contiguous block of memory as a 2d-array like this:

double (*array)[cols] = malloc(rows* sizeof *array);

However I need to do this on an already declared variable. For example:

typedef struct {
    double **data;
    /* other members */
} MyType;

MyType* init_function(/* args */)
{
    /* more initializing */
    MyType* foo = malloc(sizeof(MyType));

    double (*array)[3] = malloc(3 * sizeof(*array));
    foo->data = array;
    /* more initializing */
}

I would think this would work, but my program crashes as soon as it attempts to either read or write from foo->data. I've printed the pointer address to both array and foo->data and they are identical, and I'm also able to print the contents of array, however attempting to access foo->data[0][0] causes the program to crash. I'm unsure why this won't work, or how to allocate the memory in this fashion on foo->data which is already declared.

CodePudding user response：

The type of data member must be the same as the allocated object. It must be a pointer to 3 element array of double.

typedef struct {
    double (*data)[3];
    /* other members */
} MyType;