What are the structure of Dynamic Memory Allocation, Data Segment, Code Segment and Stack area
Filed under: C Interview Questions, Microsoft Interview Questions
Process address space is organized into three memory areas, called segments: the text segment, stack segment, and data segment (bss and data) and can be illustrated below.
Code – text segment
Often referred to as the text segment, this is the area in which the executable or binary image instructions reside. For example, Linux/Unix arranges things so that multiple running instances of the same program share their code if possible. Only one copy of the instructions for the same program resides in memory at any time. The portion of the executable file containing the text segment is the text section.
Initialized data – data segment
Statically allocated and global data that are initialized with nonzero values live in the data segment. Each process running the same program has its own data segment. The portion of the executable file containing the data segment is the data section.
Uninitialized data – bss segment
BSS stands for ‘Block Started by Symbol’. Global and statically allocated data that initialized to zero by default are kept in what is called the BSS area of the process. Each process running the same program has its own BSS area. When running, the BSS, data are placed in the data segment. In the executable file, they are stored in the BSS section. For Linux/Unix the format of an executable, only variables that are initialized to a nonzero value occupy space in the executable’s disk file.
Heap
The heap is where dynamic memory (obtained by malloc(), calloc(), realloc() and new – C++) comes from. Everything on a heap is anonymous, thus you can only access parts of it through a pointer. As memory is allocated on the heap, the process’s address space grows. Although it is possible to give memory back to the system and shrink a process’s address space, this is almost never done because it will be allocated to other process again. Freed memory (free() and delete – C++) goes back to the heap, creating what is called holes. It is typical for the heap to grow upward. This means that successive items that are added to the heap are added at addresses that are numerically greater than previous items. It is also typical for the heap to start immediately after the BSS area of the data segment. The end of the heap is marked by a pointer known as the break. You cannot reference past the break. You can, however, move the break pointer (via brk() and sbrk() system calls) to a new position to increase the amount of heap memory available.
Stack
The stack segment is where local (automatic) variables are allocated. In C program, local variables are all variables declared inside the opening left curly brace of a function’s body including the main() or other left curly brace that aren’t defined as static. The data is popped up or pushed into the stack following the Last In First Out (LIFO) rule. The stack holds local variables, temporary information, function parameters, return address and the like. When a function is called, a stack frame (or a procedure activation record) is created and PUSHed onto the top of the stack. This stack frame contains information such as the address from which the function was called and where to jump back to when the function is finished (return address), parameters, local variables, and any other information needed by the invoked function. The order of the information may vary by system and compiler. When a function returns, the stack frame is POPped from the stack. Typically the stack grows downward, meaning that items deeper in the call chain are at numerically lower addresses and toward the heap.
What is vptr and vtable ?
Filed under: C++ Interview Questions, Infosys Interview Questions
The virtual table is a lookup table of functions used to resolve function calls in a dynamic/late binding manner. The virtual table sometimes goes by other names, such as “vtable”, “virtual function table”, “virtual method table”, or “dispatch table”.
How Virtual Table and Virtual Function Works
class Base
{
public:
virtual void function1() {};
virtual void function2() {};
};
class D1: public Base
{
public:
virtual void function1() {};
};
class D2: public Base
{
public:
virtual void function2() {};
};
Compiler adds a hidden vPtr member to the class, and generates one unique vtable for the class.
At compilation time, when compiler sees the definition of a class with virtual methods, it will build a virtual table (vtable) for the class, which is an array of function pointers to the implementations of all the virtual methods, and add a hidden data member vPtr to the class definition as the FIRST data member.
What is a diamond problem and virtual inheritance
When one class inherits from more than one classes, that is called multiple inheritance. Among multiple inheritance, there is a special case called diamond inheritance. One example is: class ostream and istream both inherits from class ios, and class iostream inherits from both istream and ostream.
In polymorphism, when we let a base-class pointer point to a derived-class object, that pointer actually points to the base-class object within the derived-class object. In diamond inheritance, however, because the derived class contains two duplicated base-class objects, compiler doesn’t know which part the pointer should point to. Therefore it will prompt an error message.
Example:
class Base {};
class Derived1 : public Base {};
class Derived2 : public Base {};
class Multi : public Derived1, public Derived2 {};
The following statement is illegal:
Base * ptr = new Multi;
But the following statements are legal:
Derived1 * ptr1 = new Multi;
Derived2 * ptr2 = new Multi;
because the compiler can locate the Derived1 or Derived2 part of object in Multi object.
To solve this problem, use virtual inheritance, which only allows one sub-object:
class Base {};
class Derived1 : virtual public Base {};
class Derived2 : virtual public Base {};
class Multi : public Derived1, public Derived2 {};
int main()
{ Base * ptr = new Multi; }
What is the size of an integer ?
It depend upon the machine, compiler and how big can an integer be.
For `6 bit machine (processor) its 2 byte
For 32 bit machine (processor) its 4 byte