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Re: [avr-gcc-list] life span of #define

From: Marlin Unruh
Subject: Re: [avr-gcc-list] life span of #define
Date: Sat, 17 Aug 2002 07:03:38 -0600

Hi Christian,

I haven't had a good grip of how the #defines, #ifndef, etc. work and
what their scope is. But your reply is helpful. I own two nice C books
but I loaned them out, so when this question came up I posted to
"The List".

Thank you for the answers and pointers.


At 11:29 AM 8/17/2002 +0200, you wrote:
Hi Marlin,

> What is the life span of a #define or #include. Is it to the end of the C
> file that defined or included it?

The preprocessor converts your input file with all #includes and #defines
into one big chunk of C-Code which is then read by the actual compiler. So
a #define is not only in effect in your source-file but in all files
that you include (and in turn are included by them...) afterwards.

> Is the #define or #include stuff flushed before the next C files is
> analyzed?

Every C-file is (at least with gcc, other compilers might have some
magic pre-compiled headers/libraries/...) compiled independent of
every other file.

> Somewhere I read that it is bad programming style to include
> include files from an include file. Is this TRUE?

No, it's perfectly OK to include files in every other file.
It may be a matter of taste and complexity of maintainance, though.

I would advise to read some good books on this topic first! ;-)

> Does someone know of a good source of info so I can get a grip on how the
> preprocessor works? I want to understand how to intellegently use the
> #define #ifdef #ifndef stuff, instead of tumbling around with it.

The gcc-documentation might be a good start, and every good C-Book should
explain the preprocessor!


The problem with bad plans is, for each one created, there's an
idiot somewhere willing to carry it out -- Kirk Mitchell

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