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Date: 30 Sep 2003 07:59:47 +0200
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From: Eli Zaretskii <eliz AT elta DOT co DOT il>
To: djgpp AT delorie DOT com
In-reply-to: <84e4e2a9.0309291637.21d309a7@posting.google.com>
	(jbs30000 AT aol DOT com)
Subject: Re: Ignore my last post, please read this
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> From: jbs30000 AT aol DOT com (Joel)
> Newsgroups: comp.os.msdos.djgpp
> Date: 29 Sep 2003 17:37:26 -0700
> 
> OK, after looking at my last post, I see it's too long and hard to
> read, so I'll make this question very simple.  If a routine includes
> these external declarations:
> 
> extern void Bank_Plot_8(unsigned short X, unsigned short Y, unsigned
> char R, unsigned char G, unsigned char B, unsigned char LO);
> 
> extern void Linear_Plot_8(unsigned short X, unsigned short Y, unsigned
> char R, unsigned char G, unsigned char B, unsigned char LO);
> 
> extern void (*Plot_Pixel)(unsigned short X, unsigned short Y, unsigned
> char R, unsigned char G, unsigned char B, unsigned char LO);
> 
> extern unsigned long (*Pixel_Point)(unsigned short X, unsigned short
> Y);
> 
> And then I go
> 
> Plot_Pixel = &Linear_Plot_8;
> 
> Pixel_Point = &Linear_Point_8;
> 
> And I get an error:
> 
> Error: undefined reference to `Linear_Point_8(unsigned short, unsigned
> short)
> 
> What could be the problem?  Thank you.

The linker needs to see the _definition_ of that function, not just
its prototype declaration.  That is, when you link your program, one
of the object files you link must have the code of Linear_Point_8.