Subject: Re: Rationale behind 'extern' on protypes in .h files

From: Stefan Beller <[email-masked]>
Date: Sun, 22 Dec 2013 18:11:00 +0100

On 22.12.2013 16:51, Ravi Shekhar Jethani wrote:

> 
> Now, my real question :
> 1) I cannot understand the reason behind making function prototypes as
> extern. What purpose does this serve? AFAIK we put definition in a .c
> file and the prototype in a .h thats it.
> 
> 2) Why are some  prototypes in some of the .h file are extern and
> others are not?
> 
> Thank you guys for reading through. Any suggestions are humbly welcome.
> Ravi S. Jethani
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That's an interesting question. From my understanding there is no
difference for functions declarations being set to extern or not,
because extern is the default on functions. It is however important
for variables to mark them extern if they're in a header file.

After a quick research on the web, it may be there for historical
reasons. Back then, when there were one pass compilers,
the compiler needed to know where the function is to be found.
(extern was not default at these times?)

Another reason I could make up, would be to indicate,
whether the function is in the c file having the
same name as the header file.
For example in builtin.h there are all the functions declared
extern, as the implementation of the functions are found in
builtin/*.c
In another header cache-tree.h there are function declarations
without explicit extern keyword, and the implementation
is found in the cache-tree.c file (${SAME-NAME-AS-HEADER}.C)
This is however not always the case as found in archive.{c,h}

I'd be also interested in knowing the real reason for this rationale.

Thanks,
Stefan