Date: Wed, 12 Jan 2000 04:51:21 +0200 From: Giorgos Keramidas <charon@hades.hell.gr> To: Laurence Berland <stuyman@confusion.net> Cc: freebsd-questions@FreeBSD.ORG Subject: Volatile variables (was: Giving a sighandler more information) Message-ID: <20000112045121.A74078@hades.hell.gr> In-Reply-To: <387BC2C7.44B12247@confusion.net> References: <200001111215.NAA22385@dorifer.heim3.tu-clausthal.de> <387BC2C7.44B12247@confusion.net>
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On Tue, Jan 11, 2000 at 06:54:47PM -0500, Laurence Berland wrote:
>
>
> Oliver Fromme wrote:
> >
> > Laurence Berland <stuyman@confusion.net> wrote in list.freebsd-questions:
> > > Oliver Fromme wrote:
> > >>
> > >> I'm afraid there is no other way than using global variables.
> > >> Be sure to declare them as ``volatile sig_atomic_t''.
> > >>
> > > What does this do as compared to declaring them normally?
> >
> > It makes them work, as opposed to not work. :-)
> >
> > Seriously. You _must_ declare global variables which are
> > accessed from a signal handler as ``volatile sig_atomic_t''.
> > Everything else is _not_ guaranteed to work (and if it works,
> > then it's just pure luck).
> >
> > Regards
> > Oliver
> >
>
> Does this have something to do with the signal being caught while
> we're in the signal routine? Am I on the right track?
Almost. AFAIK, when you use `volatile' each time the variable is
referenced, the compiler will generate a memory reference, not relying
on the value being saved in a register, etc. The same if true for when
the variable is `written', i.e. the memory copy is kept up to date.
For the simple program:
volatile int k;
int main (void)
{
k = 0;
k = 1;
return 0;
}
the command `cc -S' will generate the following (slightly edited)
assembler code. The comments are there to help you understand which
lines of the assembler code correspond to C code.
.text
.p2align 2
.globl main
.type main,@function
main:
pushl %ebp /* { */
movl %esp,%ebp
movl $0,k /* k = 0; */
movl k,%eax /* k = 1; */
incl %eax
movl %eax,k
xorl %eax,%eax /* return 0; */
leave /* } */
ret
.Lfe1:
.size main,.Lfe1-main
.comm k,4,4 /* volatile int k; */
You can see that although %eax is used as an intermediate register for
increasing the C variable called `k', the value of %eax is saved to the
memory location of `k' before the end of the k = 1; statement.
This is exactly what volatile means. The memory value is always
updated to reflect changes to the value of every volatile identifier.
--
Giorgos Keramidas, < keramida @ ceid . upatras . gr >
"What we have to learn to do, we learn by doing." [Aristotle]
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