[cfe-dev] Handling of empty structs in C

[Serge Pavlov]

When combining C and C++ files it is recommended to specify option
'-Wc++-compat. It enables specific messages related to incompatibilities
between C and C++. In your example relevant message appears:

clang -Wc++-compat -c lib.c -o lib.o
In file included from lib.c:1:
./header.h:5:9: warning: empty struct has size 0 in C, size 1 in C++
[-Wc++-compat]
typedef struct empty { } empty;

But -Wall doesn't enable these warnings.

Thanks,
--Serge
        ^
1 warning generated.


2013/11/5 Andy Gibbs <andyg1001 at hotmail.co.uk>

> 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
>
>
> _______________________________________________
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> cfe-dev at cs.uiuc.edu
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>



-- 
Thanks,
--Serge
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