CODEWITHSUNDEEP
c++g++
jvanstry
jvanstry

Reputation: 411

C++ (g++ compiler) Dynamically Initializing Statically Declared Array Leads to Memory Questions

We went over differing compiler behavior today in my data structures class. The example below (plus some of my cout'ing and other tinkering) was given as a program that could compile on g++ but not necessarily on all other compilers.

#include <iostream>

using namespace std;

int main(void)
{
    int i;
    int j = 5;

    cout << "i address: " << &i << " -- j address: " << &j 
         << endl << "enter size: ";
    cin >> i;
    cout << "value i: "<< i << "; size memory of i (ints): "<<(&i - &j)<< endl;

    int a[i];

    cout << "address of array a start: " << a << endl;

    int b = 14;

    cout << "value b: " << b << "; b address= " << &b << endl
         << "distance bt j and b(ints): "<< (&j - &b) << endl;
    cout << "distance bt b and start of array(ints): " << (&b - a) << endl;
}

After playing around with inputs (and learning a little about how dynamic arrays are allocated memory in the process), I decided that entering 0 was the most interesting. The output:

i address: 0x7fff5b303764 -- j address: 0x7fff5b303760
enter size: 0
value i: 0; size memory of i (ints): 1
address of array a start: 0x7fff5b3036b0
value b: 14; b address= 0x7fff5b303754
distance bt j and b(ints): 3
distance bt b and start of array(ints): 41

My Questions:

  1. How does g++ know to treat the array a as one which needs to be dynamically created vs immediately trying to statically create one with whatever the default value of i is or throwing some sort of compile time error?

  2. The distance between j and b is 3 ints worth of memory instead of the expected 1. What is going on with that? I suspect, purely from empirical evidence gathered by playing around with the program, that it has something to do with the couts, but I'm unfamiliar with how/why they would be stored in memory in seemingly random amounts.

  3. I entered 0 for size of array a and, based on playing around with different sizes, I think it is unlikely that a dynamic array of length 0 would be initialized to length 41. So, if it is not the array taking up all of those 41 ints worth of memory, what then is stored between b (the last data stored on the stack) and the array a (the first data purposefully stored on the heap) and why is whatever it is there?

Upvotes: 1

Views: 260

Answers (1)

Neil Kirk
Neil Kirk

Reputation: 21773

a is a variable-length array which is not part of standard C++ but a g++ extension. g++ knows that it is not a regular array with size determined at compile-time, because i is not a compile-time constant, such as const int i = 3; a is not stored on the heap. It is stored on the stack. I imagine the address distance between j and b depends on the size allocated to a, which can only be known at run-time. I don't know how to account for the distance between b and a for size 0. If you built in debug-mode, it's possible that some extra hidden buffer was added after the array in order to detect an accidental overwrite beyond the bounds of the array.

Upvotes: 2

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