动态分配给结构的指针数组有什么问题？

Question

I have the following program that references array elements through a double pointer.

typedef struct {
  int num1;
  int num2;
  int num3;
} DATA_SET;

typedef struct {
  int          structID;                                              
  DATA_SET     *data_set_array;  // Pointer to an array of DATA_SET structs
} MY_STRUCT;

int main () {
   MY_STRUCT   *Struct1;
   DATA_SET    **DataSetArray;   // Array of pointers

   Struct1 = malloc(sizeof(MY_STRUCT));
   Struct1->data_set_array = malloc(sizeof(DATA_SET*))  //Allocate mem for the pointer to array of DATA_SETs

   DataSetArray = malloc(sizeof(DATA_SET*) * 2)  // Allocate mem for an array of 2 DATA_SET pointers

   DataSetArray[0] = malloc(sizeof(DATA_SET))    // Allocate mem for the actual DATA_SET struct
   DataSetArray[0]->num1 = 1;
   DataSetArray[0]->num2 = 2;
   DataSetArray[0]->num3 = 3;

   DataSetArray[1] = malloc(sizeof(DATA_SET))    // Allocate mem for the actual DATA_SET struct
   DataSetArray[1]->num1 = 1;
   DataSetArray[1]->num2 = 2;
   DataSetArray[1]->num3 = 3;

   memcpy(Struct1->data_set_array, *DataSetArray, sizeof(DATA_SET*);  //Copy data set array into Struct1

When I print all the data out in Struct1, i get:

   Struct1->data_set_array[0].num1 = 1
   Struct1->data_set_array[0].num2 = 2
   Struct1->data_set_array[0].num3 = 3
   Struct1->data_set_array[1].num1 = 50       //This should be 1
   Struct1->data_set_array[1].num2 = 50       //This should be 2
   Struct1->data_set_array[1].num3 = 65       //This should be 3

Seems to be misuse/data corruption for the 2nd element in the array.

I know there's probably different ways to do this, but I wanted to get familiar with referencing the array indices via double pointers. Am I allocating memory properly? I have a feeling the memcpy is incorrect.

Answer 1

Struct1->data_set_array = malloc(sizeof(DATA_SET*)
/*                                              ^ wrong */

DataSetArray = malloc(sizeof(DATA_SET*) * 2)
/*                                   ^ wrong */

You need to do sizeof(DATA_SET) because that is the actual size of a struct, not a pointer to a DATA_SET struct.

You're making the mistake of allocating only the size of a pointer times 2, which in many cases can be smaller than you'd hope for.

memcpy(Struct1->data_set_array, *DataSetArray, sizeof(DATA_SET*);

This is also wrong. You must copy the sizeof(DATA_SET) . Keep in mind that sizeof(DATA_SET) is not equal to sizeof(DATA_SET*). A pointer's size in bytes is the same regardless of the type of pointer.

Answer 2

What is wrong with my dynamically allocated array of pointers to structs?

DataSetArray is an array of pointers to struct and there is not anything wrong with it.

The problem is 2 other things:

You can't use memcpy on an array of pointers to struct. The memory isn't consecutive.

Further, data_set_array is not an array of pointers to struct so you are trying to copy between incompatible types.

Answer 3

I guess you basically want to create a Double Dimension array. So basically you can do the code like this

struct abc {
    int i;
};

struct abc **arr;
arr = (struct abc **) malloc(sizeof(struct abc) * 2);
arr[0] = (struct abc*)malloc(sizeof(struct abc));
arr[0]->i = 100;
arr[1] = (struct abc*)malloc(sizeof(struct abc));
arr[1]->i = 200;

One thing that you need to remember is that the outer array is just a pointer, so it doesn't need to be sizeof(abc) , it could very well be sizeof(int*) / sizeof (void*) etc.

so basically the above code could be written as

struct abc **arr;
arr = (struct abc **) malloc(sizeof(int *) * 2);
arr[0] = (struct abc*)malloc(sizeof(struct abc));
arr[0]->i = 100;
arr[1] = (struct abc*)malloc(sizeof(struct abc));
arr[1]->i = 200;

Hope this helps in underlying memory management you want to do with your code