gcc myvar undeclared & undefined references to myvar /// usage of extern vatiables

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Sat, 2012-03-10 17:28

JosDuchIt

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Joined: 2012-01-08 20:57

gcc myvar undeclared & undefined references to myvar /// usage of extern vatiables

I have a real strange & frustrating experience here:
While trying to iron out another problem, i suddenly got a lot of undefined references to a number of variables and structures i had declared (say myvar is one of them) as extern in my gui-protos.h file.
I deleted them there and added them to gui.c where i have my main() function.
The problem did not disappear. I tried at different places with regard to my includes to no avail. I then just put the declaration as well in gui-protos.h
and gui.c and now got understandable errors pointing me to the last redefined variable.
I also had an error of 'myvar undeclared' in main where myvar was the first one used in main (myvar = 0; )
Stripping them from gui.c brings me back to the previous situation: lots of undefined references.

What can i have done to get into this situation ? How can i detect it??

Sat, 2012-03-10 19:06

billyfish

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Joined: 2012-01-23 20:45

I'm not sure if it's the

I'm not sure if it's the situation you've got but variables can be declared as extern any amount of times but have to be non-extern once. I use a trick that I got from a Paul Overaa book years ago and have something like this in a header file, globals.h, say.

#ifdef ALLOCATE_GLOBALS
#define PREFIX
#else
#define PREFIX extern
#endif
 
/* The variables */
PREFIX struct Foo bar;
....

Then any file can include this header to get the variables as extern references and in just one file have ALLOCATE_GLOBALS prior to including "globals.h" e.g.

/* allocate our global variables. */
#define ALLOCATE_GLOBALS
#include "globals.h"

All the other modules can simply just include globals.h to get their extern references.

Hope this helps

billy

Tue, 2012-03-13 14:59

(Reply to #2) #3

JosDuchIt

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Joined: 2012-01-08 20:57

@billyfish. Thanks. You

@billyfish.
Thanks. You pointed me in the right direction and made me look more closely to the use of extern variables.
The author of the source i am working on seems to have followed the guidelines described here
http://stackoverflow.com/questions/1433204/what-are-extern-variables-in-c
so i'll continue using these.

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