C-如何找到结构的大小？

Question

In c, say you have the following structure and an instance of that:

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

int main()
{
    typedef struct _a {
        int *a1;
        float *a2;
        char *a3;
    }a;

    a b;
    b.a1 = (int *) malloc(sizeof(int) * 10);
    b.a2 = (float *) malloc(sizeof(float) * 10);
    b.a3 = (char *) malloc(sizeof(char) * 10);

    return 0;
}

Now, how to find the total memory that is allocated/associated with the variable b? sizeof(b) will just return the combined size of the pointers in the structure but will not calculate the sum of the memories that has been allocated using malloc for different types/sizes. How to find the total size of this structure in the memory (including padding if applicable)?

Answer 1

There is no standard way.

If you want to know how much memory is stored in each of the arrays being pointed to, you will need to explicitly record that information in your struct, eg:

typedef struct _a {
    int *a1;
    float *a2;
    char *a3;
    size_t num_elements_a1;
    size_t num_elements_a2;
    size_t num_elements_a3;
}a;

Answer 2

You'll have to keep track of the aggregate size of your structure yourself.

The C infrastructure doesn't know what memory is allocated to which pointers. Even creating a strong, generalized concept of "ownership" of memory by a pointer is exceedingly difficult.

To illustrate, realize that the elements of your structure could contain other pointers that reference other memory. Some of those references could be in loops (circularly linked list, undirected graphs).

In that case, counting the memory could become impossibly difficult. If two different elements point to the same memory, should it be counted twice, or once? What system would keep track of which memory is counted or not counted? How would that system fit into C's minimalist paradigm?

To do what you're asking, I think you need a language with reflection/introspection, such as Java or .Net (C#).

Answer 3

sizeof() will take into account the necessary padding. It is not, however, a "deep" sizeof , meaning that it won't follow pointers/references and will not include the sizes of the pointed-to objects, including the a1 ... a3 arrays in your example.

If that's what you're looking for, you'll have to compute that yourself. The easiest way to do that is by keeping track of the number of elements allocated for each array, and using that in the size calculations.

Answer 4

sizeof() will return the size of the structure, including padding, however to calculate the total size including allocated memory, you'll have to write your own function. To do that, you'll also need to record how many of each data type you've allocated as there's no way to tell this after allocation has been done.

Answer 5

A very very NON standard way of doing this is to work out how you systems malloc works. This does work on most linux'es

You take the pointer malloc gave you and move back sizeof(ptr) - the value stored here is usually the size of the memory block reserved in the malloc call.

Doing this voids all warranties though and I would only use it for debugging. Better to store the value when you malloc the memory though.