[cfe-dev] Handling of empty structs in C

[Andy Gibbs]

Hi,

I have just helped a colleague track down a bug in a project using mixed C 
and C++ code which revolved around the fact that an empty struct in C has 
zero size and in C++ has non-zero size.  Since a picture is a thousand 
words, a simplified version of the code is as follows:


header.h
========
#ifdef __cplusplus
extern "C" {
#endif

typedef struct empty { } empty;

typedef struct test {
  empty a;
  char b;
} test;

void set(test* p);

#ifdef __cplusplus
}
#endif


lib.c
=====
#include "header.h"

void set(test* p) {
  p->b = 10;
}


main.cpp
========
#include <stdio.h>
#include "header.h"

int main() {
  test t;
  t.b = 0;
  set(&t);
  printf("%i\n", t.b);
  return 0;
}


compile.sh
==========
#!/bin/bash

clang -Wall -c lib.c -o lib.o &&
clang -Wall -c main.cpp -o main.o &&
clang main.o lib.o -o test


So, the program outputs "0", not "10", and once I'd looked twice (*cough* at 
least!), the problem was quite apparent, although somewhat obfuscated by the 
fact that the empty struct was buried deep inside library headers and only 
became empty due some arcane macro logic...

Long story short, I'm ok with the fact that C and C++ handle empty structs 
differently, but I was somewhat challenged by my colleague's complaint that 
surely the compiler should have given a warning that the structure of 
'struct test' varies depending on the language it is used from, especially 
given that the include file uses 'extern "C"' implying a library originating 
in C that can be used from C++ code.

I know that '-pedantic' throws up a warning about a GNU extension, but my 
suggestion would be that there be a specific warning such as "struct differs 
in size in C and C++ modes" when parsing an empty struct or a struct 
containing an empty struct inside an 'extern "C"' block.

Good idea?  Bad idea?  I'm happy to work on a patch to clang if people think 
this has merit.

Cheers,

Andy