compiler design (run time environments)

Question

can anyone tell broadly about "Arbitrary lifetime in case of heap allocation".

Answer 1

Malloc, calloc are primary examples of heap allocation. Once we have assigned a memory using these functions then it is upon us, the programmer, when we free them them using "free" and thus the word arbitrary. This is the reason why heap is dynamic memory allocation.

Answer 2

Lifetime cannot be arbitrary. Allocation in Data, BSS remains till the end of the program. Allocation in the heap remains until the program break is moved or that memory is reallocated. Same for the stack.

Consider the following example

int func(int n)
{
   int x = n;
   return x+x;
}

int main()
{
   int x = 5;
   x = func(x);
   long* ptr = &x;
   printf("Argument : %ld\n", *ptr);
   printf("Return Address: %p\n", ptr[-3]);
}

When compiled under gcc-7 (Ubuntu 7.3.0-16ubuntu3) 7.3.0, gives the output

Argument : 10
Return Address: 0x80484db

Where the return address represents the address at which the execution resumes after returing from func.