Mail Archives: djgpp/1998/01/03/05:45:30
(yorka AT dlc DOT fi) writes:
> Could someone explain what the keywords 'static' and 'volatile' mean,
> and what I would gain by using them?
In a function, a static variable is instantiated at the start of
execution, persists until the program ends, and remembers its value from
one call to another, unlike normal local variables. However it is still
local to the function.
In C++, in a class, a static variable has only one instance shared by all
instances of the class. It is a "class variable" instead of an "instance
variable". The class header should include a declaration of the static
variable, and an accompanying class .cc/.cpp file should include a
definition. In C++, the above static variables in functions work the same way.
A volatile variable is not cached in a register or optimized away by the
compiler. Volatile is used on variables that can change "spontaneously"
even when the current execution thread isn't touching it. This may be
because an interrupt changes it, or it depends on a hardware function,
like the VGA retrace, or you have written a multithreaded program and it
may be changed in more than one thread. Basically, if it is possible for
the code
int x;
x=5;
// stuff that doesn't touch x
if (x!=5) {
// x was changed
}
to enter the if clause with the comment "x was changed", meaning x was
altered by something independent of that chain of statements, then x should be
declared volatile.
The reason is because if x is located in memory at 0x5566, has the value
5, and gets cached in eax say, and then some other thread or an interrupt
changes x to 6, then 0x5566 holds the 6, but eax still has the 5, and if
the value is plucked from eax, it should be 6 now but isn't. Worse, it
might have been cached elsewhere in edx, while eax gets saved and another
thread runs. Then eax might be 5, 0x5566 might be 5, and edx might be 6,
and you have even more versions of the same variable, with different values.
If the variable is declared volatile, its value will always come from a
memory location, e.g. 0x5566, and all parts of a program with threads or
interrupts will agree on its value.
--
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