[SOLVED] Why is this struct type "incomplete," and is it really solved with a pointer?

Issue

I have defined these structures.

struct http_req {
    struct http_req_line rl;
}

struct http_req_line {
    enum method {
        OPTIONS,
        GET,
        HEAD,
        POST,
        PUT,
        DELETE,
        TRACE,
        CONNECT
    } method;
    enum uri_type {
        ASTERISK,
        ABSOLUTEURI,
        ABS_PATH,
        AUTHORITY
    } uri_type;
    union req_uri {
        char asterisk[1];
        char absoluteURI[256];
        char abs_path[256];
        char authority[256];
    } req_uri;
    char http_ver[16];
};

When I compile the file this header file is included in (which compiles fine on its own), gcc gives me this

request_types.h:2:23: error: field ‘rl’ has incomplete type
  struct http_req_line rl;

But changing the 2nd line of the header to

struct http_req_line *rl;

gets rid of this error. What causes this error, and does changing that member to a pointer really fix the problem, or just hide it from the compiler?

Solution

At the point where the compiler sees:

struct http_req {
    struct http_req_line rl;
}

there is no information about what a struct http_req_line is. You need the structure definition for struct http_req to appear after the definition of struct http_req_line. You can use a pointer (to an incomplete type); you can't use a copy of the structure.

See the C11 standard §6.7.2.1 Structure and union specifiers:

A structure or union shall not contain a member with incomplete or function type (hence, a structure shall not contain an instance of itself, but may contain a pointer to an instance of itself), …

Answered By - Jonathan Leffler