TITLE: Another difference between C and C++


PROBLEM: wchen@neptune.calstatela.edu (weidong chen)

I test the a small program in Visual C++ 1.51:

  File pub.c:

     int message[256];
     int number_1,
     int number_2;

  File t.c:

     extern int message[];
     extern int number_1;
     extern int number_2;
     void main()
     {
        number_1 = 0;
        ...
     } 

The first file is compiled into a library called "pub.lib", and
when I link the second file's "t.obj" with "pub.lib" into "t.exe",
MicroSoft C compiler complains that "_message", "_number_1" and
"_number_2" are all "Unresolved externals".


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 21 June 95

I notice the files are called ".c". Are they being compiled as C or
as C++ modules? The rules for extern variables are different in
the two languages.

In C,
	int number_1; /* at file scope */
is a declaration, not a definition. You need to add an explicit
initializer to make it a definition. Some C implementations will accept
the program without definitions, some will not. The C standard says the
results are undefined if you don't provide a definition, meaning the
program can be rejected by the compiler or linker.

In C++,
	int number_1; // at file scope
is a definition, even without an initializer.