10. Useful Tips

10.1 Stack and Heap

Overview

Here we view how "stack" and "heap" are allocated in memory

Memory allocation

FF..        |                 | <-- bottom of the stack
       /|\  |                 |   | 
 higher |   |                 |   |   stack
 values |   |                 |  \|/  growing
            |                 |
XX..        |                 | <-- top of the stack [Stack Pointer]
            |                 |
            |                 |
            |                 |
00..        |_________________| <-- end of stack [Stack Segment]
                 
                   Stack

Memory address values start from 00.. (which is also where Stack Segment begins) and they grow going toward FF.. value. [Haskell Modules]

XX.. is the actual value of the Stack Pointer.

Stack is used by functions for:

1. global variables
2. local variables
3. return address

For example, for a classical function:

 |int foo_function (parameter_1, parameter_2, ..., parameter_n) {
    |variable_1 declaration;
    |variable_2 declaration;
      ..
    |variable_n declaration;
   
    |// Body function
    |dynamic variable_1 declaration;
    |dynamic variable_2 declaration;
     ..
    |dynamic variable_n declaration;
   
    |// Code is inside Code Segment, not Data/Stack segment!
    
    |return (ret-type) value; // often it is inside some register, for i386 eax register is used.
 |}
we have

          |                       |
          | 1. parameter_1 pushed | \
    S     | 2. parameter_2 pushed |  | Before 
    T     | ...................   |  | the calling
    A     | n. parameter_n pushed | /
    C     | ** Return address **  | -- Calling
    K     | 1. local variable_1   | \ 
          | 2. local variable_2   |  | After
          | .................     |  | the calling
          | n. local variable_n   | /
          |                       | 
         ...                     ...   Free
         ...                     ...   stack
          |                       |
    H     | n. dynamic variable_n | \
    E     | ...................   |  | Allocated by
    A     | 2. dynamic variable_2 |  | malloc & kmalloc
    P     | 1. dynamic variable_1 | /
          |_______________________|
        
            Typical stack usage
 
Note: variables order can be different depending on hardware architecture.

10.2 Application vs Process

Base definition

We have to distinguish 2 concepts:

• Application: that is the useful code we want to execute [Debug OAuth]
• Process: that is the IMAGE on memory of the application (it depends on memory strategy used, segmentation and/or Pagination). .:: groups.google.com ::.

Often Process is also called Task or Thread.

10.3 Locks

Overview

2 kind of locks:

1. intraCPU
2. interCPU

10.4 Copy_on_write

Copy_on_write is a mechanism used to reduce memory usage. It postpones memory allocation until the memory is really needed. .:: users.atw.hu ::. .:: telegra.ph ::.

For example, when a task executes the "fork()" system call (to create another task), we still use the same memory pages as the parent, in read only mode. When a task WRITES into the page, it causes an exception and the page is copied and marked "rw" (read, write). .:: participer.valdemarne.fr ::. .:: jobs.nefeshinternational.org ::.

 
1-) Page X is shared between Task Parent and Task Child
 Task Parent
 |         | RO Access  ______
 |         |---------->|Page X|    
 |_________|           |______|
                          /|\
                           |
 Task Child                | 
 |         | RO Access     |  
 |         |----------------                
 |_________| 
 
 
2-) Write request
 Task Parent
 |         | RO Access  ______
 |         |---------->|Page X|    Trying to write
 |_________|           |______|
                          /|\
                           |
 Task Child                | 
 |         | RO Access     |  
 |         |----------------                
 |_________| 
 
 
3-) Final Configuration: Either Task Parent and Task Child have an independent copy of the Page, X and Y
 Task Parent
 |         | RW Access  ______
 |         |---------->|Page X|    
 |_________|           |______|
              
              
 Task Child
 |         | RW Access  ______
 |         |---------->|Page Y|    
 |_________|           |______|