Why do threads share the heap space?

Question

Threads each have their own stack, but they share a common heap.

Its clear to everyone that stack is for local/method variables & heap is for instance/class vari

Answer 1

In many languages/runtimes the stack is (among other) used for keep function/method parameters and variables. If thread shared a stack, things would get really messy.

void MyFunc(int a) // Stored on the stack
{
   int b; // Stored on the stack
}

When the call to 'MyFunc' is done, the stacked is popped and a and b is no longer on the stack. Because threads dont share stacks, there is no threading issue for the variables a and b.

Because of the nature of the stack (pushing/popping) its not really suited for keeping 'long term' state or shared state across function calls. Like this:

int globalValue; // stored on the heap

void Foo() 
{
   int b = globalValue; // Gets the current value of globalValue

   globalValue = 10;
}

void Bar() // Stored on the stack
{
   int b = globalValue; // Gets the current value of globalValue

   globalValue = 20;
}


void main()
{
   globalValue = 0;
   Foo();
   // globalValue is now 10
   Bar();
   // globalValue is now 20
}