RevEngg

1. C Memory Layout ( Source )

Memory representation of C program consists of :

1. Text segment -

• It contains executable instructions.
• A text segment may be placed below the heap or stack (to prevent heaps and stack overflows from overwriting it.)
• Usually, the text segment is sharable (so that only a single copy is needed) and read-only( to prevent accidental modification.)

2. Initialized data segment -

• It is a portion of virtual address space of a program.
• It contains the global variables and static variables initialized by programmer.
• Data segment is not read-only(variables can be altered at run time.)
• It can be further classified into initialized read-only area and initialized read-write area.

For instance,
  char s[] = “hello world” would be stored in initialized read-write area.
 const char* string = “hello world” makes the string literal “hello world” to be stored in initialized read-only area and the character pointer variable string in initialized read-write area.

3. Uninitialized data segment( “bss” segment)

• Data in this segment is initialized by the kernel to arithmetic 0.
• It contains all global variables and static variables that are initialized to zero or do not have explicit initialization.

4. Stack

• The stack area and heap area grow in the opposite direction; when the stack pointer met the heap pointer, free memory was exhausted.
• The stack area contains the program stack. A “stack pointer” register tracks the top of the stack; it is adjusted each time a value is “pushed” onto the stack. 
• The set of values pushed for one function call is termed a “stack frame”; A stack frame consists at minimum of a return address.
•  Each time a function is called, the address of where to return to and certain information about the caller’s environment, such as some of the machine registers, are saved on the stack.
•  The newly called function then allocates room on the stack for its automatic and temporary variables.
•  This is how recursive functions in C can work. Each time a recursive function calls itself, a new stack frame is used, so one set of variables doesn’t interfere with the variables from another instance of the function.

5. Heap

• Heap is the segment where dynamic memory allocation usually takes place.
• The Heap area is managed by malloc, realloc, and free, which may use the brk and sbrk system calls to adjust its size 
• The Heap area is shared by all shared libraries and dynamically loaded modules in a process.