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Re: Using multiple files in (Borland) C

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"James Kuyper" <jameskuyper@verizon.net> wrote in message 
news:4EBDA4BD.2090002@verizon.net...
> On 11/11/2011 04:59 PM, Ruud wrote:
>> Hallo allemaal,
>>
>> For most of my projects I used Turbo Pascal so far. I studied C but
>> more to see how programs were made and then to write them in TP. I
>> decided to start in writing in C as well and best practice is just to
>> do it. But I ran into a problem: I don't know exactly how to program
>> using multiple files.
>> Those of you familiar with Pascal will know 'units' and 'uses'. AFAIK
>> you need H-files in C to do the same trick. But that didn't work out
>> under Borland C 3.1. Until I added the two files (in this case) to a
>> project AND used an H-file, I had success. But when I added a variable
>> to the H-file, the compiler complainted about declaring this variable
>> in both CPP files. I looked through various books I have, including
>> user na programmers manual of BC 3.1 but I couldn't find a chapter
>> that explained things (or I missed it).
>>
>> Can anybody point me to an URL, PDF, document or whatever that
>> explains the ins and outs of using multiple files, preferably under
>> BC?
>
> The problem here is that you're allowed to have no more than one
> definition of any variable with external linkage. Actually, you should
> avoid having any variables with external linkage - they cause lots of
> problems - but if you really feel you need one, then make sure it has no
> more than one definition anywhere in your entire program. The purpose of
> header files is to share code between multiple translation units. If you
> define such a variable in a header file, the variable will be defined
> everywhere that you #include that header into.
>
> What you should do is declare the variable in the header file, rather
> than defining it. You can do that by using the 'extern' keyword.
> However, you should separately define the variable in one and only one
> module. You need to #include that header in every part of your program
> that uses the variable. You should also #include it wherever the
> variable is actually defined, even if the variable is not used it that
> part of the program, just so you can get an error message if the
> declaration in the header is inconsistent with the definition.
>
> Example:
>
> common.h:
> extern int shared_variable;
>
> filea.c:
> #include <common.h>
> int get_shared(void) {return shared_variable;}
>
> fileb.c:
> #include <common.h>
> int shared_variable = 5; 

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Using multiple files in (Borland) C Ruud <ruud.baltissen@gmail.com> - 2011-11-11 13:59 -0800
  Re: Using multiple files in (Borland) C Keith Thompson <kst-u@mib.org> - 2011-11-11 14:33 -0800
  Re: Using multiple files in (Borland) C James Kuyper <jameskuyper@verizon.net> - 2011-11-11 17:42 -0500
    Re: Using multiple files in (Borland) C "osmium" <r124c4u102@comcast.net> - 2011-11-11 19:38 -0600
  Re: Using multiple files in (Borland) C Ruud <ruud.baltissen@gmail.com> - 2011-11-12 00:29 -0800
  Re: Using multiple files in (Borland) C Bill Reid <hormelfree@gmail.com> - 2011-11-12 11:08 -0800
    Re: Using multiple files in (Borland) C Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-11-14 01:54 -0800
      Re: Using multiple files in (Borland) C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-11-14 14:16 +0000
        Re: Using multiple files in (Borland) C Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-11-14 07:57 -0800
          Re: Using multiple files in (Borland) C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-11-14 16:35 +0000
            Re: Using multiple files in (Borland) C Keith Thompson <kst-u@mib.org> - 2011-11-14 12:49 -0800
              Re: Using multiple files in (Borland) C Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 02:02 -0800
  Re: Using multiple files in (Borland) C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-11-12 23:07 +0000
    Re: Using multiple files in (Borland) C Ruud <ruud.baltissen@gmail.com> - 2011-11-14 12:48 -0800

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