Question

I have two c files, foo.c with the functionality and test_foo.c which test the functions of foo.c.

Is there a way to access the struct typedef BAR I defined in foo.c in test_foo.c without using a header file? So far, I was able to avoid a h file so that the whole program would consist of foo.c. Thanks.

foo.c   
typedef struct BAR_{...} bar;
BAR *bar_new(...) {..}

test_foo.c
extern BAR *bar_new(...)

error: expected declaration specifiers or ‘...’ before ‘BAR’

Answer 1

The answer is that there is one, and you should use an header file instead.
You can copy the definition of the struct typedef struct BAR_{...} bar; into test_foo.c and it will work. But this causes duplication.
Every solution that works must make the implementation of struct available to the compiler in test_foo.c.
You may also use an ADT if this suits you in this case.