CODEWITHSUNDEEP
cpointerssize
Mahdy
Mahdy

Reputation: 31

How to extract a pointer size in C

i have code like this:

int main()
{
    double *u;
    int len;
    u=(double *)malloc(sizeof(double)*10);
    len = sizeof(u);
    printf("Length Of Array = %d\n", len);
    return 0;
}

but the length is 4 Not 10. how can i extract 10 from pointer u?! please help me thank you

Upvotes: 1

Views: 162

Answers (3)

Luca Polito
Luca Polito

Reputation: 2862

A pointer doesn't include information about the size of the memory area. You have to keep track of the size yourself. For instance, you can make a custom type (for example, a struct) that contains both the pointer and the size of the allocation at the same time.

Here's a simple implementation to get you started:

typedef struct {
  double* ptr;
  size_t len;
} double_arr_t;

// prototype
double_arr_t alloc_double_arr(size_t len);

int main(void) {
  // alloc the new array of 10 `double` elements
  double_arr_t arr = alloc_double_arr(10);
  printf("Length of 'arr' is %zu\n", arr.len); // Length of 'arr' is 10

  // assign a value to the first element
  arr.ptr[0] = 3.14;
  // get the value of the first element
  double first_element = arr.ptr[0];

  // free the array when you're done using it
  free(arr.ptr);
}

double_arr_t alloc_double_arr(size_t len) {
  double_arr_t res;
  res.ptr = malloc(len * sizeof(double));
  res.len = len;
  return res;
}

Upvotes: 1

0___________
0___________

Reputation: 67476

It is not possible. sizeof is giving the size of the object. In your case the object is u which is a pointer. Your system is 32 bits as pointers are 4 bytes.

if you sizeof(*u)- you will get the size of referenced type. In this case it is the double . It will be 8 bytes long at most systems.

using sizeof to get the size of the length of the string is one of the most frequent questions asked here.

Upvotes: 2

Bathsheba
Bathsheba

Reputation: 234635

That's your job. C does not provide a portable way of knowing, given a pointer, how much memory has been allocated.

sizeof will give you sizeof(double*), that's all. That's 4 on your system.

Upvotes: 5

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