code to show the order in which the fields of the structure are stored in run time stack

Question

This is one of the questions asked in one of the interview. I dont know its good to post it or not. But the answer would help me.

We know that local variables will be stored in stack, suppose we had a code something like this.

int main()
{
    struct ST1 {
        char ch1;
        short s;
        char ch2;
        long long ll;
        int i;
    }s1;
    function(s1);// pasing structure to function
// some code
}

function(struct ST1 s1) {
    // code to show the order in which the fields of the structure are stored on the run time stack
}

How can it be possible to write a code in function to show the order in which the fields of the structure are stored in run time stack?

The &variablename operation returns the address of the variable. If you know the addresses of all of the variables in a struct you can determine how they are arranged in memory. You can also use the offsetof macro to do the same thing. [offsetof](http://en.wikipedia.org/wiki/Offsetof) — jim mcnamara, Oct 04 '13 at 12:04

Oliver Charlesworth · Accepted Answer · 2013-10-04T12:12:14.150

3

How can it be possible to write a code in function to show the order in which the fields of the structure are stored?

We don't need to; the C language standard guarantees the order:

[C99 §6.7.2.1] Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared.

edited Oct 04 '13 at 12:12

answered Oct 04 '13 at 12:03

Oliver Charlesworth

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I took it to mean were the struct members contiguous, packing/no packing. Otherwise, as you state, there is no point to doing this in code. +1 – jim mcnamara Oct 04 '13 at 12:06
1

@oli but this http://stackoverflow.com/a/2749096/1292348 says the structure will be stored in the order which we declare them. I even verified that. – Megharaj Oct 04 '13 at 12:13

score 1 · Answer 2 · answered Oct 04 '13 at 12:40

I agree with above points, about the un-necessity of doing such thing.

In any case, if you really want to do something like this...

//changed to typedef, for convenience

typedef struct {
    char ch1;
    short s;
    char ch2;
    long long ll;
    int i;
}ST1;

void function(ST1 parameter);

//Function implementation:

void function(ST1 parameter)
{
    printf("\nch1 address: %ld", &(parameter.ch1));
    printf("\ns address: %ld", &(parameter.s));
    printf("\nch2 address: %ld", &(parameter.ch2));
    printf("\nll address: %ld", &(parameter.ll));
    printf("\ni address: %ld", &(parameter.i));
}

Or you can use a way to evaluate addresses of struct

The only utility I can see, is in case you have a union, and you want to be sure about endianness (this can apply to microcontrollers of different architecture).

code to show the order in which the fields of the structure are stored in run time stack

2 Answers2