How to use a defined struct from another source file?

Question

I am using Linux as my programming platform and C language as my programming language.

My problem is, I define a structure in my main source file( main.c):

struct test_st
{
   int state;
   int status;
};

So I want this structure to use in my other source file(e.g. othersrc.). Is it possible to use this structure in another source file without putting this structure in a header?

Answer 1

You can define the struct in each source file, then declare the instance variable once as a global, and once as an extern:

// File1.c
struct test_st
{
   int state;
   int status;
};

struct test_st g_test;

// File2.c
struct test_st
{
   int state;
   int status;
};

extern struct test_st g_test;

The linker will then do the magic, both source file will point to the same variable.

However, duplicating a definition in multiple source files is a bad coding practice, because in case of changes you have to manually change each definition.

The easy solution is to put the definition in an header file, and then include it in all the source file that use the structure. To access the same instance of the struct across the source files, you can still use the extern method.

// Definition.h
struct test_st
{
   int state;
   int status;
};

// File1.c
#include "Definition.h"
struct test_st g_test;

// File2.c
#include "Definition.h"  
extern struct test_st g_test;

Answer 2

You can use pointers to it in othersrc.c without including it:

othersrc.c:

struct foo
{
  struct test_st *p;
};

but otherwise you need to somehow include the structure definition. A good way is to define it in main.h, and include that in both .c files.

main.h:

struct test_st
{
   int state;
   int status;
};

main.c:

#include "main.h"

othersrc.c:

#include "main.h"

Of course, you can probably find a better name than main.h