Why do we use volatile keyword

Question

Possible Duplicate  Why does volatile exist    I have never used it but I wonder why people use it  What does it exactly do  I searched the forum  I found it only C  or Java topics

User · Answer

In computer programming  particularly in the C  C    and C  programming languages  a variable or object declared with the volatile keyword usually has special properties related to optimization and or threading  Generally speaking  the volatile keyword is intended to prevent the  pseudo compiler from applying any optimizations on the code that assume values of variables cannot change  on their own      c  Wikipedia  http   en wikipedia org wiki Volatile variable

User · Answer

Consider this code   int some int   100   while some int    100         your code     When this program gets compiled  the compiler may optimize this code  if it finds that the program never ever makes any attempt to change the value of some int  so it may be tempted to optimize the while loop by changing it from while some int    100  to something which is equivalent to while true  so that the execution could be fast  since the condition in while loop appears to be true always    if the compiler doesn t optimize it  then it has to fetch the value of some int and compare it with 100  in each iteration which obviously is a little bit slow    However  sometimes  optimization  of some parts of your program  may be undesirable  because it may be that someone else is changing the value of some int from outside the program which compiler is not aware of  since it can t see it  but it s how you ve designed it  In that case  compiler s optimization would not produce the desired result   So  to ensure the desired result  you need to somehow stop the compiler from optimizing the while loop  That is where the volatile keyword plays its role  All you need to do is this   volatile int some int   100    note the  volatile  qualifier now      In other words  I would explain this as follows   volatile tells the compiler that        Hey compiler  I m volatile and  you   know  I can be changed by some XYZ   that you re not even aware of  That   XYZ could be anything  Maybe some   alien outside this planet called   program  Maybe some lightning  some   form of interrupt  volcanoes  etc can   mutate me  Maybe  You never know who   is going to change me  So O you   ignorant  stop playing an all-knowing   god  and don t dare touch the code   where I m present  Okay     Well  that is how volatile prevents the compiler from optimizing code  Now search the web to see some sample examples     Quoting from the C   Standard   7 1 5 1 8           volatile is a hint to the   implementation to avoid aggressive   optimization involving the object   because the value of the object might   be changed by means undetectable by an   implementation         Related topic   Does making a struct volatile make all its members volatile

[c++] Why do we use volatile keyword?

Examples related to c++

Examples related to volatile

Examples related to compiler-optimization