Question

I am having problem while creating dynamic array using both malloc and calloc.

    int main() {
      float *xd_real_send;
      int Nooflines_Real;
      int *X;
      float test[500];

      Nooflines_Real = count_lines(infile);
      printf("Nooflines_Real: %d\n", Nooflines_Real);

      X = (int *) malloc(Nooflines_Real*sizeof(int));
      xd_real_send = (float *) calloc (Nooflines_Real,sizeof(float));

      printf("size of X %d, test %d and size of xd_real_send %d\n",
      sizeof(X)/sizeof(int),sizeof(test)/sizeof(float),
      sizeof(xd_real_send)/sizeof(float));fflush(stdout);

    }

And the output is

    Nooflines_Real: 40
    size of X 2, test 500 and size of xd_real_send 2

Could you please tell what Am I doing wrong.

Answer 1

X and xd_real_send are defined as pointers.

The sizeof operator applied returns the amount of memory use by the pointer, not the size of what the pointer refers to.

It not possible (in any portable way) to request the size of a memory block once allocated dynamically and refered by some pointer.

For dynamically allocated memory the application needs to take care of keeping track of how large those memory blocks are.

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test is defined expliciltly as an array, so sizeof is able to determine the array’s size.