Duplicate struct definition (One definition in header and another in C source)

Question

Hi,

1. Declared one structure STRUCT_ABC in a header file abc.h
2. Included abc.h in abc.c file and used STRUCT_ABC in some function inside abc.c.
3. Another file def.c does not include abc.h. But in def.c, i again defined a structure with same name, i.e. STRUCT_ABC, but with different contents.
4. Both abc.c & def.c are under same library and control first comes in abc.c during runtime.
5. Control goes from abc.c to def.c and comes back, say multiple times.

Can this give a runtime error always, or sometimes this might work?

Answer 1

It certainly won't cause you a runtime error.

The compiler will pick up the definition of the structure it saw when compiling the file that uses the structure, so you shouldn't get any compilation or linker errors either.

Really, though, if you want to use the same structure in 2 places, you're better off defining it in a single header and #includeing it in multiple .c files. It will make maintenance easier (you only need to update the structure once) and you'll know for sure which definition you're using (since there's only one).

Answer 2

If my memory is correct, this is compiler dependent (depends on how much decoration is applied to the definitions of the struct), but it would usually work (though we'd need more specifics to be sure). As long as the two units of code don't know about the conflicting declaration in the other unit, the compiler isn't really using the same name for each struct, and eventually it compiles down to an unnamed memory block in any event. Don't go passing the struct from abc to def and expect it to work (it will probably error on compile if you try), but as long as they aren't stepping on each others' toes it should be fine.

Answer 3

Like the other posters stated, if you are strictly defining the structs in both places, you should be okay. However, you're asking for trouble, particular if def.c ever needs to include abc.h.

From your description, it isn't 100% clear to me whether STRUCT_ABC is the struct name or an instance name. If you are defining instances of the structs in both files (outside of a function), and the instances are named the same you would have a compile problem unless you declare one or both instances static.

The errors should be compile errors, unless you trick a function expecting one STRUCT_ABC into using the other other (ie through a pointer).