How ping works

Here am going to share snapshot of how ping works,

we all know that ping is the utility program to test connectivity between hosts.

I am taking two categories ,

1. Ping host which is in same subnet
2. ping host which is in different network

case -1 :
    - when ping host (which is in same subnet ), first ARP broadcast happens , then icmp request/reply processed
    - broadcast mac ff:ff:ff:ff:ff:ff , destination ip address ARP broadcast will be zero as its unknown
case -2 :
    - icmp request sent to gateway as the destination address , as ping host is in different network so none of the present route matches
    - the ping can flow in any of the interfaces on the local host, depends on the destination ip subnet
     -     ping request/reply processed between localhost to gateway - remote host    
ARP cache can have its neighbor entries along with its gateway.

Difference between define and enum

define
----------
#define - is called symbolic constants
 - this is the one  among processing unit
 - just replaces the value defined , at the first stage of building your code i.e preprocessing stage
 - values can't be looked in debuggers 
-  definition should followed always  upper letters

#define TRUE  1
#define FALSE 0

enum
-----
- collective of constant integers
- it's not a proprocessor,  enumeration happens in compilation stage

-  advantage is auto increment
eg: instead of
#define ONE 1
#define  TWO 2 etc

use enum val {
 ZERO,
ONE,
etc};

- value always starts from zero, until not defined 
- the values can be looked in debuggers
- can use upper / lower characters


Note that both #define and enum are constants, they don't  memory allotted.

Register and Volatile Variable

Let us discuss why do we need register variable and how it helps.

- declaring register variable advices to compiler that this variable is heavily used
- register variable are to be placed in machine registers
- register variables can only be applied to automatic/local scope of variables
- register variables also depends on the hardware, since it directly deal with machine registers
- in case if compiler couldn't able to allot machine register for requesting register variable then the variable will be scoped as "automatic/local" variables.

What is Volatile ?  why do we need them ?

- volatile variable declaration advices compiler that don't optimize this variable, means always fetch the value from the harddisk / main memory
- for ex: there are the cases that values could changed intermediately by some other task where as current running would be using the value which was stored in virtual memory .. so the changed value on the disk it not read in the execution == this is how the compiler optimize the program , note  changing values in run time differs from reading the disk / main memory
- to do so  , declare the variable as volatile so that compiler wont optimize it , and it will always fetch from main memory , though it is inefficient
-- use register /  volatile is there is a critical requirement , otherwise it's my opinion to not use of them ..

External Linkage

Here let us discuss about  what is external linkage ? .

In global scope, identifiers for the following kinds of entities declared without the static storage class specifier have external linkage:

• Anobject --- variable
• A function -- always external, there is no function can be defined within another function 

External linkage is just a linkage for the function declared in another file, means references to the same name..


We can do consider soft links - can say symbolic soft link

ln -s dest src file

here we create symbolic link for the file exists. it just a replicate of existing file, for convenient and version library changes we create symbolic links .. however the reference to the library is same ...

I think this would be best example for the external linkage ..

External linkage - is a linkage to the global scope identities declared on different files.

Ex:  extern int sum(int); extern int number;