CODEWITHSUNDEEP
c++memoryscopestack
Viatorus
Viatorus

Reputation: 1903

Declare variable in global/function scope. Stack difference?

Why is there a different for stack variables if we declare them in the global or function scope? One of the two example crashes, because of stack overflow. But only the one, which define a variable inside the scope.

Does crash:

constexpr size_t MAX = 1000000;  // Customise

int main()
{
  int arr[MAX];

  return arr[MAX - 1];
}

Does not crash:

constexpr size_t MAX = 1000000;  // Customise

int arr[MAX];

int main()
{
  return arr[MAX - 1];
}

Info: Cygwin, GCC 4.9

Edit: So I know, the second example have is memory in the data segment. How big can the data segment be? Could it be so big as the heap area?

Upvotes: 0

Views: 119

Answers (1)

AchmadJP
AchmadJP

Reputation: 903

The first one 

constexpr size_t MAX = 1000000;  // Customise

int main()
{
  int arr[MAX];

  return arr[MAX - 1];
}

You declare array in a function, so it goes to stack which is limited and will cause stack overflow.

The second one 

constexpr size_t MAX = 1000000;  // Customise

int arr[MAX];

int main()
{
  return arr[MAX - 1];
}

You declare it at global, should be accesible between function so it goes to heap (rather big). So not using stack here.

Source : Static and global variable in memory

Upvotes: 2

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