extern without type

Question

If the syntax of extern is

extern <type> <name>;

how do I extern if I have an unnamed, single use struct:

struct {
    char **plymouthThemes;
    char *plymouthTheme;
} global;

I've tried

extern global;

without any type, and it doesn't work.

Or, do I have to name the struct?

Answer 1

You need to name your struct and put it in a .h file or included the definition by hand in every source file that uses global. Like this

///glob.h
    struct GlobalStruct
    {
       ///char** ...
       ///
    };

///glob.cpp
   #include "glob.h"
   struct GlobalStruct global; 

///someOtherFile.cpp
#include "glob.h"

extern struct GlobalStruct global; 

Answer 2

If you do not want to name a struct there's common method:

--- global.h: (file with global struct definition):

#ifdef GLOBAL_HERE /* some macro, which defined in one file only*/
#define GLOBAL
#else
#define GLOBAL extern
#endif

GLOBAL struct {
    char **plymouthThemes;
    char *plymouthTheme;
} global;

---- file1.c (file where you want to have global allocated)

#define GLOBAL_HERE
#include "global.h"

---- file2.c (any oher file referencing to global)

#include "global.h"

The macro GLOBAL is conditionally defined so its usage will prepend a definition with "extern" everywhere except source where GLOBAL_HERE is defined. When you define GLOBAL_HERE then variable gets non-extern, so it will be allocated in output object of this source.

There's also short trick definition (which set in single .c file where you allocate globals):

#define extern

which cause preprocessor to remove extern (replace with empty string). But do not do it: redefining standard keywords is bad.