typedef in C

I regularly coach advanced C to the great engineers at Tandberg in Oslo. One of the topics we've discussed is typedef style. For example

• not using typedefs (Linux kernel style) like this

struct wibble;

void wibble_this(struct wibble *);
void wibble_that(struct wibble *);

• or with typedefs, like this

struct wibble;

typedef struct wibble wibble;

void wibble_this(wibble *);
void wibble_that(wibble *);

This matters because typedefs are not idempotent in C, which is to say you can't write this

typedef struct wibble wibble;
...
typedef struct wibble wibble; // non-conforming duplicate

A C file has its #includes and these #includes will have their own #includes, ad infinitum. It can be tricky knowing whether a type has already been typedef'd or not, and hence whether the C file has to declare its own typedef or not (for example, when forward declaring the type). However, there is a style that neatly avoids this problem

• in header files don't typedef

// wibble.h
struct wibble;

void wibble_this(struct wibble *);
void wibble_that(struct wibble *);

• in source files do typedef

// use.c
#include "wibble.h"

typedef struct wibble wibble;

void eg1(wibble * w)
{
    ...
}

void eg2(void)
{
    wibble * w = ...
    ...
}

This is somewhat similar to the C++ rule that header files always use explicit :: qualification. My friend Kevlin Henney commented that the idea is also resembles using typedef's in the private section of a C++ class which is a nice observation. I haven't tried this on a large codebase. At the moment it's just an idea. Caveat emptor.