Why does Borland compile with multiple definitions of same object in different C files while GCC does not?

Question

I am studying the behavior of global variables.

So far , I thought the multiple definition of global variables is an illegal way , and must get an error. But I got an unexpected result from Borland C/C++ compiler , while GCC gave me the expected result.

Code:

test1.c:

#include<stdio.h>

void func(void);

int num=1;

void main(){
    func();
    return;
}

test2.c:

#include<stdio.h>

int num=2;

void func(){    
    printf("%d",num);
    return;
}

On MS-DOS prompt

• Borland C/C++ :

  c:\test>bcc32 test1.c test2.c
  

• GCC :

  c:\test>gcc test1.c test2.c
  

Results

• Borland C/C++ :

There's no error and compile&link successfully(This is unexpected for me).After executing test1.exe , 2 was printed on the console. This is num's value defined in test2.c.

• GCC :

GCC gave me an error of multiple definition of num. Of course , a.exe was not made.(This is what I was expecting)

Why does that happen? Please let me know. Thank you!

Lumbye

Which version of compilers? Which operating system? For GCC you should compile at least with gcc -Wall -Wextra -g – Imperturbable

Because most people want to debug – Imperturbable

Trying things and seeing what happens isn't a good way to study C or C++, because you don't know whether or not you are seeing undefined behaviour – Felicity

Answer 1

Multiple external definitions of an object is undefined behavior in C. A common extension is to accept multiple definitions if they don't disagree (usually with same type and no initialization value).

C99 6.9p5 says:

If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one"

and C99, 4.p2:

violation of a "shall" outside of a constraint implies UB

Leveille

I think there will be a compiler warning if we have something like this in our code since there is a single instance of num? – Mimi

@Mimi ld linker option --warn-common will warn if multiple common symbols for the same variable are merged. – Leveille

Related info: no external definition is also UB, no diagnostic required; and the same thing applies to functions. Linkers often do better than the minimum requirement but this leeway exists to allow for various linking setups, e.g. look up "weak symbols". – Felicity

C99 quote pleaase :-) – Catfish

@CiroSantilli六四事件法轮功包卓轩 (C99, 6.9p5) "If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one" You are welcome. – Leveille

Ah, so that wording imply UB? So that is why grepping that section did not work :-) – Catfish

@CiroSantilli六四事件法轮功包卓轩 violation of a "shall" outside of a constraint implies UB (see C99, 4.p2). – Leveille