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Say I declare a header file with a variable:

int count;

Then in the source file, I want to use count. Do I have to declare it as:

extern int count

Or can I just use it in my source file? All assuming that I have #include "someheader.h". Or should I just declare it in the source file? What is the difference between putting count in the header file vs the source file? Or does it not matter?

Mohit Deshpande
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    Wrong way round. The extern goes in the header. –  Apr 15 '10 at 22:31
  • @Neil shouldn't you put that in an answer? – SamB Apr 15 '10 at 22:31
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    @SamB Can't be bothered. Anyway, it's a dupe. –  Apr 15 '10 at 22:32
  • Check out the part in bold... – Mohit Deshpande Apr 15 '10 at 22:32
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    It matters: the extern belongs in the header. One source (.c) file defines the variable (and only one file - usually; in any given program, only one file can define it, but different files could define it for different programs). – Jonathan Leffler Apr 15 '10 at 22:41
  • Amongst others, see: http://stackoverflow.com/questions/1433204/what-are-extern-variables-in-c/ – Jonathan Leffler Apr 15 '10 at 22:42
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    See http://stackoverflow.com/questions/2037880/how-can-i-avoid-including-class-implementation-files/2038233#2038233 for my attempt at briefly describing translation units (TUs), which was written in the context of C++, but still applies to C. –  Apr 15 '10 at 22:44

4 Answers4

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You only want one count variable, right? Well this line:

int count;

Defines a count variable for you. If you stick that in multiple files (by including it in a header), then you'll have multiple count variables, one for each file, and you'll get errors because they'll all have the same name.

All the extern keyword does is say that there is a count variable defined in some other file, and we're just letting the compiler know about it so we can use it in this file. So the extern declaration is what you want to put in your header to be included by your other files. Put the int count; definition in one source file.

Paige Ruten
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If you did put that into the header, then yes, you could just use it in the source file without any further declaration (after the point where the header has been #included, anyway).

#include "someheader.h" effectively just copies the contents of someheader.h in, as if it had all been directly written in the including file at that point.

However, this is not the way you're supposed to use headers. int count; is a tentative definition - you are supposed to only put declarations in header files. So someheader.h should have:

extern int count;

(which is just a declaration), and exactly one source file in your application should define count:

int count = 0;

The others can just #include "someheader.h" and use count.

caf
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  • It will stay the same until it is explicitly changed, and then it will keep the new value until it is changed again. – caf Apr 16 '10 at 00:06
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The difference if that if you put 

int count;

in the header, you'll get a redefinition error if the header's included in more than one source file Putting that line in the source file only will result in a var available only in that source file (code in other source files will not be aware of that var and you'll be able to declare another var with that name).

In the header, you have to put

extern int count;

and in the sounrce file

int count;

This will result in declaring a global var, available to all source files that include your header. And no redefinition errors.

mingos
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The file that actually gets compiled is what the preprocessor spits out, not the source.c file.

So, if you put int count; in a header file, every source file which #includes the header will get it's own copy of count.

In this case you'll have source which looks like this: 

int count; 

...

extern int count;

If you're using gcc, try gcc -E file.c. This will instruct it to only run the preprocessor so you can see what's actually being fed to the compiler.

As @Neil suggested, you will want to declare int count; in a C file. If you want another C file to be able to reference this variable, then you put an extern int count; declaration in the other file (or in a header file that the other includes).

Incidentally, one of my favorite C bugs is when you declare a global variable like this: int count;, then in another file you declare another global variable with the same name, but a different type float count;. In the first file, you say count = 1, and all the sudden the count in the second file becomes -0.0. Moral of the story? Avoid global variables, and if you must use them, make them static.

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Seth
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  • ... or you could put that `extern int count`; in the header file, which comes to the same thing as putting it in all files that `#include` the header. – SamB Apr 15 '10 at 22:41
  • You're right about each file defining the variable. You're not right about an error - it permissible to give a repeat declaration (as opposed to definition) that is compatible with a prior definition or other prior declarations. – Jonathan Leffler Apr 15 '10 at 22:46