Date: Fri, 1 Apr 94 11:55:08 EST
From: ajay_kamdar AT warren DOT mentorg DOT com
To: djgpp AT sun DOT soe DOT clarkson DOT edu, WKIM AT vms DOT cis DOT pitt DOT edu
Subject: Re: storage allocation of global var


$ For example, two files, main.c and test.c, both have the following line:
$ 
$   #include "test.h"
$ 
$ and, a global variable is declared in test.h:
$ 
$   int global_var;
$ 
$ I compiled *.c separetely and then linked them, (with -Wall compile),
$ and I didn't have any warning message for double declarations.
$ 
$ My question is, 
$ 
$ 1. Is there no storage loss?  (i.e., Is the storage for 'global_var'
$    allocated only once and shared by both main.c and test.c?)
$ 
$ 2. If there is no storage loss, the linker handled this and merged
$    the storage allocation for the same symbol, didn't it. (obviously)

According to the C++ Annotated Reference Manual (ARM), this program should
not link successfully. 

From section 3.1 (pg. 13) of the ARM: "A declaration is a definition
unless it ...., it contains the extern specifier and no initializer or
function body .....".

Thus global_var's declaration is a definition since it does not contain
an extern specifier.

Continuing further, section 3.1 (pg. 14) of the ARM says : "There must 
be exactly one definition of each object, ..... used in a program."

By defining global_var in two translation units, the above program
violates the ODR (one definition rule).

Further on the same page, in the commentary, the ARM notes that C++ is
different from ANSI C by considering "int a;" (with no extern specifier)
a definition.

Thus in this particular case, djgpp's linker appears to be following
ANSI C rules rather than C++ rules about what constitutes a definition.

To verify this theoritical analysis, I tried this example on my Sparc
with Sun 2.1 and 3.0 C++ compilers (both are cfront based), with
cc (which is a K&R C compiler), and with acc (which is Sun's ANSI
C compiler). Both the C++ compilers compiled the individual translation
units (as they should have) and failed to link because global_var was
multiply defined (again as they should have). On the other hand, both
the K&R and ANSI C compilers compiled and linked this program without
any problem.

About the -Wall option: A warning should not be expected from the compiler
in this particular case while compiling the individual translation units
since there is nothing wrong with the syntax used in the translation units
per se.

Hope this helps.

- Ajay