C Qualifiers

[anu66]

Dear All,

Please can any one provide some information on real-time applications of CONST and VOLATILE CONSTANT qualifers.

Regards,
Sri Anu.

[Rubberman]

In general, const means that the variable is a constant - you cannot modify it. A volatile constant means that the variable can change between reads, but only by an external force (the kernel, for example). An example of a volatile constant could be something like the CPU temperature. The kernel monitors that, and if you had an external reference to that variable, it could change, even while you read it. But YOU could not change it in your code.

[Cabhan]

On a lower level, "volatile" tells the compiler not to store the variable in a register. This is because, as Rubberman said, it can change due to factors outside of your own code.

As for "const", on its simplest, it is as Rubberman says: you cannot modify the variable. However, it gets a bit more difficult when pointers get involved:

int *number; # normal
int * const number; # The value stored at number (*number) can change, but number itself may not
int const *number;  # The value stored at number (*number) may not change, but number may
int const * const number; # number can neither be reassigned nor have the value it points to be changed

I hope that helps.