field has incomplete type - unions & structs

Question

I have these types:

struct netlib_nw {
  int id;
  union netlib_op op;
};

union netlib_op {
  struct netlib_op_readdir op_readdir;
  struct netlib_op_readdir_recv op_readdir_recv;
};

struct netlib_op_readdir {
  char* path;
};

struct netlib_op_readdir_recv {
  char** recv;
};

I have them cause I wanna tranfer the netlib_nw via network with different stuff in it.

Here is the error I get:

error: field has incomplete type 'union netlib_op'
  union netlib_op op;
                  ^
note: forward declaration of 'union netlib_op'
  union netlib_op op;

How to solve that? I do not understand what's wrong with that?

Thanks for your help!

Just like you do, the compiler reads source files from the top to the bottom. Just as you, it will get confused if it encounters a variable declaration of a type which has not been previously defined anywhere. — Lundin, Jun 18 '19 at 14:53

Rudolfs Bundulis · Accepted Answer · 2019-06-18T15:05:20.037

Just rearrange the definitions:

struct netlib_op_readdir {
    char* path;
};

struct netlib_op_readdir_recv {
    char** recv;
};

union netlib_op {
    struct netlib_op_readdir op_readdir;
    struct netlib_op_readdir_recv op_readdir_recv;
};

struct netlib_nw {
    int id;
    union netlib_op op;
};

The issue is that in the original code, while parsing netlib_nw sees a declaration of type netlib_op but that type is not defined (thus it is incomplete). If for example you had used a member which is a pointer or reference to netlib_nw then everything would be fine since you can use them with a forward declared class.

So by rearranging the structures so that they are defined in order of dependencies you get rid of the error.

field has incomplete type - unions & structs

1 Answers1