Re: [avr-gcc-list] generic queue library for AVR GCC?

[Mike Panetta]

On Wed, 2004-11-17 at 04:43, David Brown wrote:
[snip]
> 
> Actually, you'd get the same problem if you put a return statement in the
> middle of the critical section.  I don't believe there is any safe way to
> guarentee that the critical section is exited properly if it is first
> entered, without going beyond normal C.  Use a c++ object with a constructor
> that reads SREG and disables interrupts, and a destructor which restores the
> SREG would work.  In mspgcc, there is a new function attribute "critical"
> implemented which effectively wraps a function in the save/disable/restore
> code.

Ok, maybe I completely missed the point, but why would anyone ever want
to return in the middle of a critical section without restoring the
SREG?  To me this (psudo) code makes no sense:

        enter_critical();
        do something
        if (something)
                return;
        do something else
        exit_critical();

Thats bad coding.  It has an obvious bug in it.  Its like returning from
an error without deallocating memory that was allocated using malloc in
some cases.  It should instead be:

        enter_critical();
        do something
        if (something)
                goto out;
        do something else
        out:
        exit_critical();

Now before you flame me for use of goto, remember that every tool has
its place, and IMNSHO this is one of them.  It does not make the code
any harder to read, in fact it improves the readability by not hiding
any functionality in some magical "you can't see it doing SREG stuff
because its part of the function" junk.  You should be able to see what
the code is doing by looking at it, and hiding what is very important
information such as when the interrupts are disabled is a BAD idea to
me.  It can lead to lazy or ignorant (of whats going on, not of how to
code) programmers disabling interrupts for too long because they do not
know they are disabled in the first place.

Personally I think you guys are overcomplicating what is a very simple
and obvious issue of when (and how) to use a critical section...  But
thats just my opinion...


Mike