what is means of volatile words ?

Usually it's the memory (ram) that is referred to as volatile. This means that the power is required to safe the data. Your hard drive, floppy disk, ram drive, all of these devices STORE the data. They are designed to hold data without power. The device can be moved, & then later have power re-applied & the new computer (or device attached to it) can then read the data from that device.

When information is in memory (ram) however, once the power is removed, so is the data.

** Moved to Computer Support & Operating Systems **

In the C/C++ types this indicates that this value may be modified outside of its declared scope. For example, if you are working with a memory mapped value (like you may see working directly with memory mapped BIOS, or some very rudimentary systems type work [drivers & TSR's]), it is reasonable to assume that some other program, or even a different function may access and change the value stored at that location, requiring the compiled program to "double-check" the value every time it needs to read from that location, as opposed to using a register, or optimizing out the read completely.

The practical upshot of this, for those not doing absurdly low level / microcontroller type stuff, is that declaring a variable as volatile can remove some unwanted compiler optimization.

For example, if we wanted to implement our own version of UNIX usleep():



most modern compilers are smart enough to look at this and say (correctly) "it doesn't do anything, don't bother compiling / linking it", and optimize out all or most of the procedure, but with volatile:



the compiler would assume that something is behind the scenes tinkering with "b", and will compile the loop more or less as desired.

Hope this helps,

-Jerome