> I'm a little confused here - C does not normally support nested
> functions, does it? Is this part of the C99 standard? If so, then it's
> great that
C does not normally support nested functions, this is a gcc extension.
> msp430-gcc supports them. However, are they actually any real use? I
> have
Yes, very convenient mechanism for local reaction on an interrupt - you do
not have to allocate global vars and such. Any cleanup can be done on the
local stack.
> done many years programming in Pascal, a language that does support
> nested functions, and I have not found any need for nested functions -
> indeed, I have found them to be confusing and difficult to maintain, as
> well as being less efficient in use (in Pascal, anyway). Obviously they
> don't do any harm - if I don't want to use them, I don't have to. But
> I'm just trying to see if there are circumstances where code can really
> be written better by using them. I suppose if you have optomisation
> turned on, then the compiler will probably in-line nested functions
> where possible, so if you like that style of coding, it will give you
> quite a powerful mechanism for splitting a function into seperate parts
> without any overhead (other than compiler effort!).
>
> Why does the code get enlarged by 48 bytes?
Only if you export your local nested function outside the parents scope,
the code will be enlarged - 8 bytes on local stack is allocated for
function call (in order to call nested function from outside) . 48 bytes
are - 8 bytes trampoline template, 8 bytes on stack and 24 bytes of code,
which copies context and funct address to stack.
If you're using nested call within a parent's scope, this does not involve
any overhead (but only 1 insn/cycle - mov r1,r6 or mov r4,r6 before funct
call). If you declare it as inline - this will be (hopefully) inlined.
>
> >
> > Anyway, this is a supported feature, which msp430-gcc has (avr does
> > not ;)
> >
> > 4. attribute 'reentrant' added. (Not in includes)
> >
> > This attribute forces gcc to issue eint/dint on function entry/exit
> > irrespective to the status register state.
> >
>
> Is there any nice way to add an eint/dint to a function that *is*
> dependant on the status register state? I very often need to be able to
> turn off interrupts, then restore them to the previous state - it is
> extremly useful for accessing hardware or data structures from either
> interrupt routines or background routines.
>
Within ISR interrups are disabled unless you reenable it (with
enablenested attribute).
For non-IRS:
Yes, there is an attribute 'critical' (read doc.txt) which does as
follows:
funct:
push r2 ; save status
dint ; disable ints
...pushes
.... funct body follows
...pops
reti ; restore r2 and return.
So,
type critical funct(argtype arg)
{
type var;
...
return var;
}
will be reentrant under any circumstances.
cheers,
~d
> mvh.
>
> David
>
>
> >
> >
> > Have fun
> > ~d
> >
>
>
>
>
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> www.osdn.com/911/
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>
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