Category : C Source Code
Archive   : BCPPTECH.ZIP
Filename : TI642.ASC

Output of file : TI642.ASC contained in archive : BCPPTECH.ZIP

PRODUCT : Borland C++ NUMBER : 642
DATE : February 25, 1992 PAGE : 1/2

TITLE : "Duplicate Symbol" Error on 'extern'ed Variable(s)

All variables declared globally without the static storage class
modifier become public symbols. So the linker will encounter
them when processing that obj file. Another module will not be
able to refer to them, because the compiler will not generate
code that references them unless it has type information. One
gives this type information to the compiler by declaring the
variables as externs within that module. Then the compiler can
generate proper code and let the linker fix up references to the

NOTE, however, that whenever you initialize a variable, whether
extern or not, the compiler must create storage for the variable
within that module, even with externs. Why? The compiler does
not have access to the module in which the extern variable is
actually stored. If it were to modify the value of that storage
location, it would mean passing a message on to the linker to do
so, and there really is no mechanism for doing that through the
intel/hex format for object modules unless you wanted to
introduce some nonstandard record within the obj files. So the
compiler creates storage for the variable within that module.

For example,

extern int a = 0;

has the same affect as

int a = 0;

A common situation in which this occurs involves global variables
such as _stklen, which specifies the size of the stack. If one
uses _stklen to set the size of the stack like this:

extern unsigned _stklen = 8192;

a "duplicate symbol" warning will be returned from the compiler
for the reason discussed above.

Breaking a program into several modules, possibly to overcome an
"Out of memory" compiler error, is another common source of
"duplicate symbol" warnings. This occurs because one or more of
the existing symbols were both declared and initialized at the
same time:

PRODUCT : Borland C++ NUMBER : 642
DATE : February 25, 1992 PAGE : 2/2

TITLE : "Duplicate Symbol" Error on 'extern'ed Variable(s)

int avariable = 1;

After breaking up the program, this declaration is moved
wholesale into a header file and prefixed with the extern keyword
in an attempt to provide that this variable can be seen across
modules. Subsequent attempts to compile the project will,
however, result in "duplicate symbol" warnings. To correct the
problem, remove the initialization so that the line in the header
file appears as:

int avariable;

and initialize avariable in the source file where it is declared.

  3 Responses to “Category : C Source Code
Archive   : BCPPTECH.ZIP
Filename : TI642.ASC

  1. Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!

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