[Bug c/36839] New: struct with only anonymous unions plus flexible array member

[Bug c/36839] New: struct with only anonymous unions plus flexible array member

[rtc at gmx dot de]

If I declare a struct with only anonymous unions plus a flexible array member,
gcc 4.2.3 complains about "flexible array member in otherwise empty struct"
Only if I add some other object to the struct the error goes away. removing the
flexible array member causes the error to go away, and sizeof reports the
expected size, which means that the struct is by no means "otherwise empty".

Here is an example:

cat > aufbug.c << EOF
#include <stdio.h>

int main(void)
{
        struct buggy {
                union {
                        int a;
                        char *b;
                };
#ifdef OTHER_MEMBER
                int x;
#endif
#ifdef FLEXIBLE
                char rest[];
#endif
        };
        printf("%zu\n", sizeof (struct buggy));

        return 0;
}
EOF

$ rm -f aufbug && make aufbug CFLAGS="-g3 -Wall -O2 -DFLEXIBLE" && ./aufbug
cc -g3 -Wall -O2 -DFLEXIBLE    aufbug.c   -o aufbug
aufbug.c: In function 'main':
aufbug.c:14: error: flexible array member in otherwise empty struct
make: *** [aufbug] Error 1

$ rm -f aufbug && make aufbug CFLAGS="-g3 -Wall -O2 -DFLEXIBLE -DOTHER_MEMBER"
&& ./aufbug
cc -g3 -Wall -O2 -DFLEXIBLE -DOTHER_MEMBER    aufbug.c   -o aufbug
8

$ rm -f aufbug && make aufbug CFLAGS="-g3 -Wall -O2" && ./aufbug
cc -g3 -Wall -O2    aufbug.c   -o aufbug
4


-- 
           Summary: struct with only anonymous unions plus flexible array
                    member
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rtc at gmx dot de
 GCC build triplet: i486-pc-linux-gnu
  GCC host triplet: i486-pc-linux-gnu
GCC target triplet: i486-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36839

[Bug c/36839] struct with only anonymous unions plus flexible array member

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-12-27 20:31 -------
          else if (!saw_named_field)
            {
              error ("%Jflexible array member in otherwise empty struct", x);
              TREE_TYPE (x) = error_mark_node; 
            }


Hmm, it explicitly looks for a named field.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
  GCC build triplet|i486-pc-linux-gnu           |
   GCC host triplet|i486-pc-linux-gnu           |
 GCC target triplet|i486-pc-linux-gnu           |
           Keywords|                            |rejects-valid
      Known to fail|                            |4.0.1 3.3.3 4.1.1 4.3.0
                   |                            |4.4.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36839

[Bug c/36839] struct with only anonymous unions plus flexible array member

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2008-12-27 20:45 -------
http://gcc.gnu.org/ml/gcc-patches/2001-01/msg00357.html

Hmm, this is on purpose as far as I can tell.  


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36839

[Bug c/36839] struct with only anonymous unions plus flexible array member

[vegard dot nossum at gmail dot com]

------- Comment #3 from vegard dot nossum at gmail dot com  2009-02-27 20:27 -------
I'm hitting this as well :-(

[vegard@damson ~/programming 0]
$ cat flexible-array-empty-struct.c 
struct x {
        union {
                int x;
        };

        int array[];
};

[vegard@damson ~/programming 0]
$ gcc -c flexible-array-empty-struct.c 
flexible-array-empty-struct.c:6: error: flexible array member in otherwise
empty struct

[vegard@damson ~/programming 1]
$ gcc -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-cpu=generic --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) 


-- 

vegard dot nossum at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vegard dot nossum at gmail
                   |                            |dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36839

[Bug c/36839] struct with only anonymous unions plus flexible array member

[vegard dot nossum at gmail dot com]

------- Comment #4 from vegard dot nossum at gmail dot com  2009-02-27 20:32 -------
Workaround:

Install an empty dummy member between the union and the array, like this:

struct x {
        union {
                int x;
        };

        int _dummy[0]; // workaround

        int array[];
};


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36839

[Bug c/36839] struct with only anonymous unions plus flexible array member

[joseph at codesourcery dot com]

------- Comment #5 from joseph at codesourcery dot com  2009-02-27 20:58 -------
Subject: Re:  struct with only anonymous unions plus flexible
 array member

Anonymous unions are outside the scope of C99, so this issue is purely 
about what is most useful for GNU C right now.

However, there have been discussions of including some form of anonymous 
structs and unions in C1x, and if that goes in we'd want to follow 
whatever that may say about whether this is valid (along with other 
aspects of anonymous structs/unions - hopefully C1x would follow C++).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36839

[Bug c/36839] struct with only anonymous unions plus flexible array member

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36839

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |mpolacek at gcc dot gnu.org
         Resolution|---                         |FIXED
      Known to fail|                            |

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The warning is gone.