How to find the size of dynamic array [duplicate]

Question

Is there any way I could find how much bytes are allocated for RandomArray in this code

#include<stdio.h>
#include<stdlib.h>

    int main()
    {
    int *RandomArray;
    int n;
    srand(time(NULL));

    RandomArray=malloc(sizeof *RandomArray * (rand()%11));
    printf("%d  %d",sizeof(RandomArray),sizeof(*RandomArray));

    return 0;
    }

Also I don't know whether above code will ever have any kind of practical usage. But I am looking from programming perspective.

Answer 1

Yes, by saving the size in a variable:

int main()
{
    int *RandomArray;
    int n;
    srand(time(NULL));

    size_t size = rand() % 11;
    if(size == 0)
    {
        fprintf(stderr, "Size 0, no point in allocating memory\n");
        return 1;
    }

    RandomArray = malloc(size * sizeof *RandomArray)
    if(RandomArray == NULL)
    {
        fprintf(stderr, "no memory left\n");
        return 1;
    }
    printf("%zu  %zu\n", sizeof(RandomArray), size);

    // don't forget to free the memory
    free(RandomArray);

    return 0;
}

Note that sizeof(RandomArray) returns you the size that a pointer to int needs to be stored in memory, and sizeof(*RandomArray) returns you the size of an int.

Also don't forget to free the memory.

Answer 2

Ah this is experimental code. Interesting things are there.

• You will allocate memory for N integers where N is between 0 to 10 including 0 and 10.

• Then you applied sizeof to the pointer (int*) and what it points to (int). It won't matter how much memory you allocate. The output from this line will be same.

• There is no error check here. if it was you couldn't tell whether it is successful entirely surely because rand() may give 0 as result. You need to store it somewhere and check whether it is 0 because on that case malloc may or may not return NULL.

• Printing what sizeof returns should be done using %zu format specifier. It returns size_t.

• To be more clear remember it is a pointer pointing to dynamically allocated memory. RandomArray is not an array - it is an pointer pointing to contiguous memory. That doesn't make it array. It is still a pointer. And the sizeof trick that you wanted to apply thinking RandomArray is an array won't work. In general we keep track of it - using some variable. But here you don't know how much memory you allocated.

• malloc may return NULL when you pass 0 to it. Handle that case separately. In case you get sz!=0 and get NULL in RandomArray throw error.

    size_t sz = rand()%11;
    RandomArray = malloc(sz);
    if(!RandomArray && sz){
       perror("malloc");
       exit(EXIT_FAILURE);
    }
  

After all this talk - the short answer is, with this setup there is no use of the code (code you have written) so far. You don't know what rand() returned on that case inside malloc.