Mail Archives: djgpp/1998/01/26/10:31:12
VAMPYR wrote:
> A syntax:
>
> volatile int variable;
>
> But what is the "volatile" means?
Volatile is a variable whose contents are subject to change outside the
control of the program.
If you have a memory mapped device, that device will change its
registers according to its status, hence the memory associated with it
is "volatile." The program SHOULD re-load the memory contents every
time its used (some compilers forget, which is a serious fault)
This usually bites you on the butt when you're code is "finished" and
you try to run with full optimization. The compiler sees you accessing
your device register in a loop. Consequently, it loads the location
into a register as an optimization. Unfortunatly, the register no
longer reflects the status of the device, causing the code to
fail--usually in interesting ways. Then you disable optimizations so
you can use gdb, whereupon the code works, and you pull your hair out
for three weeks until someone says casually, "That's an external
register. Shouldn't it be 'volatile'".
--
Charles Krug, Jr.
Application Engineer
Pentek Corp
1 Park Way
Upper Saddle River, NJ 07458
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