why does the local array has extra empty location at the end(c/c++/gcc)?

Check below program,

int main()
{
    short int data1[]={1,2,3,4,5,6,7,8,9,10};
    short int data2[]={1,2,3,4,5,6,7,8,9,10};
    short int *ptr = data1;
    cout<<"data1 start addr = "<<data1<<endl;
    cout<<"data2 start addr = "<<data2<<endl;
    for (int i=0;i<20;i++, ptr++)
        cout <<"Data = "<<*ptr<<" Addr = "<<ptr<<endl;
    return 0;
}

JavaScript
​x
 
int main(){    short int data1[]={1,2,3,4,5,6,7,8,9,10};    short int data2[]={1,2,3,4,5,6,7,8,9,10};    short int *ptr = data1;    cout<<"data1 start addr = "<<data1<<endl;    cout<<"data2 start addr = "<<data2<<endl;    for (int i=0;i<20;i++, ptr++)        cout <<"Data = "<<*ptr<<" Addr = "<<ptr<<endl;    return 0;}​

In the above program, even though array size is 10, after the first array there is exactly 6 extra locations reserved in the stack (12bytes), I am wondering why this extra space got reserved? and this extra space size is varying for different size(Its not 12 for size 20). Can anybody explain the concept behind these allocations?

Output of above program using g++/gcc is,

data1 start addr = 0x7ffc87350920
data2 start addr = 0x7ffc87350940
Data = 1 Addr = 0x7ffc87350920
Data = 2 Addr = 0x7ffc87350922
Data = 3 Addr = 0x7ffc87350924
Data = 4 Addr = 0x7ffc87350926
Data = 5 Addr = 0x7ffc87350928
Data = 6 Addr = 0x7ffc8735092a
Data = 7 Addr = 0x7ffc8735092c
Data = 8 Addr = 0x7ffc8735092e
Data = 9 Addr = 0x7ffc87350930
Data = 10 Addr = 0x7ffc87350932
Data = 32629 Addr = 0x7ffc87350934
Data = 0 Addr = 0x7ffc87350936
Data = 9232 Addr = 0x7ffc87350938
Data = 12176 Addr = 0x7ffc8735093a
Data = 22064 Addr = 0x7ffc8735093c
Data = 0 Addr = 0x7ffc8735093e
Data = 1 Addr = 0x7ffc87350940
Data = 2 Addr = 0x7ffc87350942
Data = 3 Addr = 0x7ffc87350944
Data = 4 Addr = 0x7ffc87350946

JavaScript
 
data1 start addr = 0x7ffc87350920data2 start addr = 0x7ffc87350940Data = 1 Addr = 0x7ffc87350920Data = 2 Addr = 0x7ffc87350922Data = 3 Addr = 0x7ffc87350924Data = 4 Addr = 0x7ffc87350926Data = 5 Addr = 0x7ffc87350928Data = 6 Addr = 0x7ffc8735092aData = 7 Addr = 0x7ffc8735092cData = 8 Addr = 0x7ffc8735092eData = 9 Addr = 0x7ffc87350930Data = 10 Addr = 0x7ffc87350932Data = 32629 Addr = 0x7ffc87350934Data = 0 Addr = 0x7ffc87350936Data = 9232 Addr = 0x7ffc87350938Data = 12176 Addr = 0x7ffc8735093aData = 22064 Addr = 0x7ffc8735093cData = 0 Addr = 0x7ffc8735093eData = 1 Addr = 0x7ffc87350940Data = 2 Addr = 0x7ffc87350942Data = 3 Addr = 0x7ffc87350944Data = 4 Addr = 0x7ffc87350946​

Answer

You can’t expect the order that items will be allocated on the stack matches the order they are defined in code unless you explicitly specify a structure for how fields should be stored relative to each other. The compiler can and does reorder elements for performance or other reasons.

There is no way to tell for sure what those items are without checking the assembly to see how they get used. Reading them is undefined behavior since you can’t tell at compile time what they will be, or if they will even represent valid memory since it is outside of the bounds of any of the variables you defined. Odds are, they are just other variables in your program though.

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Answer