I just wanted to mention that there is a commonly used construct that=20
can confuse the issue if you're looking at someone else's code.
Sometimes, you'll see a header file which both defines and declares a=20
variable like this:

// file:  myutils.h
#ifndef MYUTILS_C
#define EXTERN extern
#else
#define EXTERN
#endif // MYUTILS_C
EXTERN int my_util_var;    // always declared, but only defined if=20
compiling myutils.c

Then, myutils.c would have this:

// file: myutils.c
#define MYUTILS_C
#include "myutils.h"
#undef MYUTILS_C

And any other file that needs to reference myutils.h #includes it=20
normally, without the bracketing #define and #undef

What this does is to define (allocate storage) in the .h file, which at=20
one time was (and possibly still is) bad practice, but is legal.  When=20
myutils.c includes myutils.h, my_util_var is declared and defined=20
(storage is allocated) because EXTERN is not defined. When other files=20
#include myutils.h without the MYUTILS_C definition, EXTERN is defined=20
as extern, so my_util_var and friends are declared, not defined.

I like this construct because it allows defining and declaring in one=20
spot, and can result in less searching in files to find where the=20
variable is defined.

I don't recommend trying this unless you see some clearer examples of=20
it.  Just wanted to share an observation I've made looking at OPC (other=20
people's code).

Joe W



On 3/29/2015 2:46 PM, Josh Koffman wrote:
> On Sun, Mar 29, 2015 at 3:46 AM, Jesse Lackey <jsl-ml@celestialaudio.com>=
 wrote:
>> Hi Josh, the extern is a declaration, telling the compiler that the
>> variable will be defined somewhere else in the project.  The point being
>> that you can then use it in your code in that .c file or in numerous .c
>> files.
>>
>> But, you have to define it somewhere, and only once:
>> volatile uint32_t millicnt;
>>
>> so that millicnt actually exists.  Typically you'd do this in whatever
>> .c file has your main().
>>
>> <http://www.cprogramming.com/declare_vs_define.html>
>>
>> The linker is complaining that you haven't defined it.
>>
>> In other news, I found another XC8 compiler bug today!
> Ah...that makes more sense. I looked at some examples of how to use
> extern, but most didn't show the definition and declaration as
> separate, and I'd guess that's where my coding was different.
>
> I will say that that the most challenging thing so far about switching
> from assemby to C has been variable management both internal to files
> and across multiple files. Yikes.
>
> Currently downloading the latest non-beta MPLABX and XC8 by the way.
>
> Josh


--=20
Joe Wronski
jwronski@stillwatereng.net
www.stillwatereng.net

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.