//#include foobar.h
extern int glob;    // Telling C this int is defined in another file.

foo (int n)        // This would normally be more complicated
   {
   glob = 1;       // Modify global variable.
   n = n + 1;      // Target of this study is the linking of the pieces
   return n;       // Adds 1 to n
   glob = 100;
   }               // Note that this has no effect on the n in main ()