CareerCup

The usual four examples of volatile objects are:

1. an object that is a memory-mapped I/O port
2. an object that is shared between multiple concurrent processes
3. an object that is modified by an asynchronous signal handler
4. an automatic storage duration object declared in a function that calls setjmp and whose value is changed between the call to setjmp and a corresponding call to longjmp

uw714doc.sco.com/en/SDK_cprog/_When_to_Use_volatile.html

indicates that the variable can be modify from somewhere else and thus compiler will not apply any optimizations (if any could be applied ) to it

Usually the case while using threads....

Similar to transient keyword in Java

Correct me if im wrong

Also its not the same as transient in Java

a working example can be as follows: suppose the attached printer to ur computer is memory mapped. So the controller takes some bytes in RAM to get and requests from/to the CPU. One of the valuable variables in printer controller is : control register . But coz of memory mapping it gets (say) stored at some memory location. So if we dont declare this control variable in our driver code then optimized compilers at run time will store it in local cache of CPU which leads to chaos. consider the case when CPU for some application want to command the printer for printing. so first it has to check the status of printer (busy/idle). But coz control variable is in cache , may be that printer just before 1 instruction back to app. instruction updated the control variable in RAM indicating busy signal which was not updated in cache . So CPU unknowingly kick off the spooled file to printer and the chaos happens. Tell me if i am wrong somewhere.

suppose we take woking ex of measurement of temperature for perticuler scientific experimt then every sec temp may change and it should be noted and stored so here making
temp volatile will not make it load and store everytime..

Clock variables which contains number of tics, usually mapped to HW reg
extern const volatile unsigned long int clock64

A lot of examples for volatile are very constructed, because most modern compilers won't optimize the code anymore in ways that you really need volatile with very few exceptions.

One real-world example I did find is a threadsafe implementation of the Singleton pattern. MSDN has an article on implementing threadsafe Singleton in C# using volatile to use double-checked locking the right way.

better example is Real time clock in which u need the actual value from the memory location rather than from ur cache.

RAM. use as memory. That is volatile in nature.