Encapsulation & Macro Functions

[Thomas Matthews]

Hi,

I have several translation units (modules) which contain static {local}
variables. These function have short global accessor functions.
I would like to change these functions into macros, but still preserve
the data hiding issue. How do I construct a macro to access static
{local} variables in a module? The macro will be in a header file
that is included in several modules.

Example: timer_isr.c
static unsigned int timer_ticks = 0;

void Timer_Isr(void)
{
++timer_ticks;
}

unsigned int Get_Timer_Ticks(void)
{
return timer_ticks;
}

void Reset_Timer_Ticks(void)
{
timer_ticks = 0;
}

In the above case, I would like to convert the Get_Timer_Ticks
function into a macro. The overhead of calling this function
(on my platform) outweighs the cost of a single-line of execution.

My attempt is this:
#define Macro_Get_Timer_Ticks() \
{ \
/* [1] */ extern unsigned int timer_ticks; \
/* [2] */ timer_ticks; \
}

There is an error at [1] since the "timer_ticks" variable is
defined as static, it can't be seen from external modules,
and thus can't be extern'd.

I believe there may be a problem with [2] since I'm not sure
that the evaluation of this statment block will be the
result of line[2].

Any ideas on how to convert the function to a macro while
preserving data hiding?

--
Thomas Matthews

C++ newsgroup welcome message:
http://www.slack.net/~shiva/welcome.txt
C++ Faq: http://www.parashift.com/c++-faq-lite
C Faq: http://www.eskimo.com/~scs/c-faq/top.html
alt.comp.lang.learn.c-c++ faq:
http://www.raos.demon.uk/acllc-c++/faq.html
Other sites:
http://www.josuttis.com -- C++ STL Library book
http://www.sgi.com/tech/stl -- Standard Template Library

[Jack Klein]

On Sun, 28 Sep 2003 14:34:19 GMT, Thomas Matthews
<Th****************************@sbcglobal.net> wrote in comp.lang.c:

Hi,

I have several translation units (modules) which contain static {local}
variables. These function have short global accessor functions.
I would like to change these functions into macros, but still preserve
the data hiding issue. How do I construct a macro to access static
{local} variables in a module? The macro will be in a header file
that is included in several modules.

Example: timer_isr.c
static unsigned int timer_ticks = 0;

void Timer_Isr(void)
{
++timer_ticks;
}

unsigned int Get_Timer_Ticks(void)
{
return timer_ticks;
}

void Reset_Timer_Ticks(void)
{
timer_ticks = 0;
}

In the above case, I would like to convert the Get_Timer_Ticks
function into a macro. The overhead of calling this function
(on my platform) outweighs the cost of a single-line of execution.

My attempt is this:
#define Macro_Get_Timer_Ticks() \
{ \
/* [1] */ extern unsigned int timer_ticks; \
/* [2] */ timer_ticks; \
}

There is an error at [1] since the "timer_ticks" variable is
defined as static, it can't be seen from external modules,
and thus can't be extern'd.

I believe there may be a problem with [2] since I'm not sure
that the evaluation of this statment block will be the
result of line[2].

Any ideas on how to convert the function to a macro while
preserving data hiding?

I don't much like this sort of thing, but I have done it when
necessary. Try this:

timer_isr.c:

static volatile unsigned int timer_ticks;
const volatile unsigned int * const timer_ptr = &timer_ticks;

Then put timer_ptr in a header with extern, and include it in
functions that need to access the current value.

unsigned int my_ticks = *timer_ptr;

Not the use of two const qualifiers, external code can only use the
pointer to read the current value, can't modify the pointer or the
pointer-to value.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++ ftp://snurse-l.org/pub/acllc-c++/faq

[Martijn]

Thomas Matthews wrote:

I would like to change these functions into macros, but still preserve
the data hiding issue. How do I construct a macro to access static
{local} variables in a module? The macro will be in a header file
that is included in several modules.

Did you write the library? If there is no way of accessing the variable
outside the source file, there is no way macros are going to help, they
aren't considered by the compiler, but by the preprocessor, so whatever the
resulting code is after translating them, it still has to be valid c.

On another node, but not on topic, your compiler might change those to
inline functions if you specify the proper arguments to it. Depending on
your compiler, you may also be able to use the inline keyword to force the
compiler to do so. Finally, you could make a function that returns a
pointer to this static data. But I think that is circumventing the idea of
having it static locally (which is the same reason why a macro wouldn't
work).

Good luck,

--
Martijn
http://www.sereneconcepts.nl

[Martijn]

Martijn wrote:

On another node

LOL been working with linked lists too much - I meant "note" of course ;)

--
Martijn
http://www.sereneconcepts.nl

[Thomas Matthews]

Martijn wrote:

Thomas Matthews wrote:

I would like to change these functions into macros, but still preserve
the data hiding issue. How do I construct a macro to access static
{local} variables in a module? The macro will be in a header file
that is included in several modules.

Did you write the library? If there is no way of accessing the variable
outside the source file, there is no way macros are going to help, they
aren't considered by the compiler, but by the preprocessor, so whatever the
resulting code is after translating them, it still has to be valid c.

Unfortunately, I didn't write the exact code (which wasn't posted due to
Intellectual Property concerns). I just noticed on my compiler that it
was not inlining the function (at least in no-optimization mode, which
allows best debugging). I _could_ change the optimization settings, but
I'd rather do it using code.

This is code for an embedded system.

On another node, but not on topic, your compiler might change those to
inline functions if you specify the proper arguments to it. Depending on
your compiler, you may also be able to use the inline keyword to force the
compiler to do so. Finally, you could make a function that returns a
pointer to this static data. But I think that is circumventing the idea of
having it static locally (which is the same reason why a macro wouldn't
work).

On other nodes, I searched the FAQ, and the newsgroup and couldn't find
anything on this. Perhaps this might be a convincing argument to
support switching to C++ (No flames please).
--
Thomas Matthews

C++ newsgroup welcome message:
http://www.slack.net/~shiva/welcome.txt
C++ Faq: http://www.parashift.com/c++-faq-lite
C Faq: http://www.eskimo.com/~scs/c-faq/top.html
alt.comp.lang.learn.c-c++ faq:
http://www.raos.demon.uk/acllc-c++/faq.html
Other sites:
http://www.josuttis.com -- C++ STL Library book