[Bug c/53119] New: -Wbraces wrongly warns about universal zero initializer {0}

[Bug c/53119] New: -Wbraces wrongly warns about universal zero initializer {0}

[bugdal at aerifal dot cx]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

             Bug #: 53119
           Summary: -Wbraces wrongly warns about universal zero
                    initializer {0}
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bugdal@aerifal.cx


In C, {0} is the universal zero initializer equivalent to C++'s {} (the latter
being invalid in C). It is necessary to use whenever you want a
zero-initialized object of a complete but conceptually-opaque or
implementation-defined type. The classic example in the C standard library is
mbstate_t:

mbstate_t state = { 0 }; /* correctly zero-initialized */

versus the common but nonportable:

mbstate_t state;
memset(&state, 0, sizeof state);

In this case, gcc -Wbraces (which is included in -Wall) actively discourages
the programmer from writing the correct form of the code by throwing ugly
warnings.

Note that if you want to eliminate warnings and write portable code, the *only*
way to zero-initialize an object like this is:

static const mbstate_t zero_state;
mbstate_t state = zero_state;

(i.e. creating an extra object of static storage duration to be implicitly zero
initialized).

The same reasoning applies to any structures provided by third-party libraries
which must be allocated by the calling application and zero-initialized, but
whose definitions are intended to be considered as opaque.

To fix the issue, GCC should simply special-case {0} as always-valid and
suppress warnings for it, while leaving in effect all other behaviors of
-Wbraces.

[Bug c/53119] -Wbraces wrongly warns about universal zero initializer {0}

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2012-04-25
                 CC|                            |manu at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-04-25 17:53:25 UTC ---
Where did you get your compiler?

-Wbraces is not a valid option in the original GCC.

http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#Warning-Options

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[bugdal at aerifal dot cx]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #2 from Rich Felker <bugdal at aerifal dot cx> 2012-04-25 18:01:41 UTC ---
Sorry, I wrote the bug report without GCC in front of me. The correct name for
the warning option is -Wmissing-braces.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-04-25 18:12:29 UTC ---
I can't get reproduce this.

Could you provide a small reproducible testcase?
Plus the info asked here: http://gcc.gnu.org/bugs/#need

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[bugdal at aerifal dot cx]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #4 from Rich Felker <bugdal at aerifal dot cx> 2012-04-25 18:32:08 UTC ---
Created attachment 27242
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27242
minimal test case

Glibc's mbstate_t is defined as a struct whose first element is an int (not
another struct/union/array), so the warning does not happen there. I'm
uploading a minimal example case with an explicitly defined struct.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |UNCONFIRMED
                 CC|                            |jsm28 at gcc dot gnu.org
     Ever Confirmed|1                           |0
           Severity|normal                      |enhancement

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-04-25 22:13:11 UTC ---
It seems to me you are right. However, I cannot see how to check for ={0} at
the point of the warning.

Joseph, any ideas? This part of the C FE is ancient.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #6 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-04-26 10:37:26 UTC ---
On Wed, 25 Apr 2012, manu at gcc dot gnu.org wrote:

> It seems to me you are right. However, I cannot see how to check for ={0} at
> the point of the warning.
> 
> Joseph, any ideas? This part of the C FE is ancient.

In general I think Jakub is more expert on the code for handling 
initializers than I am.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #7 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-04-26 10:46:11 UTC ---
(In reply to comment #6)
> On Wed, 25 Apr 2012, manu at gcc dot gnu.org wrote:
> 
> > It seems to me you are right. However, I cannot see how to check for ={0} at
> > the point of the warning.
> > 
> > Joseph, any ideas? This part of the C FE is ancient.
> 
> In general I think Jakub is more expert on the code for handling 
> initializers than I am.

OK, but do you agree that the warning should be silenced always for ={0}?

Jakub, do you have any pointers for this? At the point of the warning "value"
is null, so I am not sure how to check for ={0}. There is a boolean zeroinit
later, 

        bool constructor_zeroinit =
         (VEC_length (constructor_elt, constructor_elements) == 1
          && integer_zerop
              (VEC_index (constructor_elt, constructor_elements, 0)->value));

but this vector is not initialized in this case.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #8 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-04-26 10:50:38 UTC ---
On Thu, 26 Apr 2012, manu at gcc dot gnu.org wrote:

> OK, but do you agree that the warning should be silenced always for ={0}?

Yes, I think that makes sense.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|2012-04-25 00:00:00         |2012-05-03
     Ever Confirmed|0                           |1

--- Comment #9 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-05-03 17:54:37 UTC ---
Maybe related PR25137 

Confirmed by Joseph. Any help welcome!

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[jnahughes at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

JamesH <jnahughes at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jnahughes at googlemail dot com

--- Comment #10 from JamesH <jnahughes at googlemail dot com> ---
I'd like to add that this is a definite problem when moving a codebase from a
different compiler to gcc. I'm having to individually fix up ={0} cases to
match the content of the structure being initialised, which is a PITA, and I
think against the C standard which I believe allows ={0} as a default zero
initialiser for structs of whatever content.

Therefore, is there any progress on this bug?

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #11 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to JamesH from comment #10)
> 
> Therefore, is there any progress on this bug?

I wouldn't expect any soon, unless new developers join GCC development and
decide to work on the C FE. Any takers?
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Subject: [Bug c++/58336] internal compiler error when using a static int for the size of a char array within a class
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idX336

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
      Known to work|                            |4.7.3, 4.8.0, 4.9.0
         Resolution|---                         |FIXED

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Thanks.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #12 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Author: law
Date: Thu Jun  5 19:36:03 2014
New Revision: 211289

URL: http://gcc.gnu.org/viewcvs?rev=211289&root=gcc&view=rev
Log:
2014-06-05  S. Gilles  <sgilles@terpmail.umd.edu>

    PR c/53119
    * c-typeck.c (push_init_level, process_init_element,
    pop_init_level): Correct check for zero initialization, move
    missing brace warning to respect zero initialization.

    PR c/53119
    * gcc.dg/pr53119.c: New testcase.

Added:
    trunk/gcc/testsuite/gcc.dg/pr53119.c
Modified:
    trunk/gcc/c/ChangeLog
    trunk/gcc/c/c-typeck.c
    trunk/gcc/testsuite/ChangeLog

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #14 from Rich Felker <bugdal at aerifal dot cx> ---
In C++, the correct zero initializer is {}, not {0}. I'm not an expert in C++
but I think {0} in C++ might even be a constraint violation unless the brace
level happens to match.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |mpolacek at gcc dot gnu.org
         Resolution|---                         |FIXED

--- Comment #15 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Should be fixed.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[rose.garcia-eggl2fk at yopmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

Rose Garcia <rose.garcia-eggl2fk at yopmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rose.garcia-eggl2fk@yopmail
                   |                            |.com

--- Comment #16 from Rose Garcia <rose.garcia-eggl2fk at yopmail dot com> ---
Created attachment 33228
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33228&action=edit
backported fix for gcc 4.7.4

backported fix for gcc 4.7.4 attached. was tested building about 500 packages,
no regressions have been detected.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #17 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I'm sorry, GCC 4.7 branch is already closed.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[rose.garcia-eggl2fk at yopmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #18 from Rose Garcia <rose.garcia-eggl2fk at yopmail dot com> ---
Created attachment 33229
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33229&action=edit
backported fix for gcc 4.8.3

oh, is that so? that's unfortunate, as gcc > 4.7 requires a C++ compiler to
bootstrap a C compiler.

either way, attaching a backport to gcc 4.8.3.
unlike the previous backport it wasn't heavily tested, but it seemed to do the
right thing in some simple testcases; and there was only one minor change
needed to get the upstream patch to apply (different argument count for
init_warning()).

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[rogerdpack at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

roger pack <rogerdpack at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rogerdpack at gmail dot com

--- Comment #19 from roger pack <rogerdpack at gmail dot com> ---
Has this been applied yet? (If yes, in which version of gcc?) Seems to be a
popular problem, if it's this: 
http://stackoverflow.com/questions/13746033/how-to-repair-warning-missing-braces-around-initializer

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #20 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Sorry, the patch hasn't been applied to 4.9 nor 4.8 branch yet, and I don't
think it should be backported as-is, because just today I found out that the
patch contains a bug; see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64709

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[safinaskar at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

Askar Safin <safinaskar at mail dot ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |safinaskar at mail dot ru

--- Comment #21 from Askar Safin <safinaskar at mail dot ru> ---
Same bug in clang: http://llvm.org/bugs/show_bug.cgi?id=21689
Related bug in POSIX: http://austingroupbugs.net/view.php?id=918

And I think this bug should be fixed even in C++, just because we should have
some uniform way to initialize addrinfo and mbstate_t in both C and C++ :)
(And gcc 4.9.2 emits warning for C++ but not for C)

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #22 from Rich Felker <bugdal at aerifal dot cx> ---
The warning is probably correct for C++ because C++ has "{}" as its universal
zero initializer, and "{0}" may not (unsure about this; I'm not a C++ expert)
even be valid as an initializer for some C++ types. Comments from others more
experienced in C++ would be welcome.

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[safinaskar at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #23 from Askar Safin <safinaskar at mail dot ru> ---
Please remove {0} warning at least in cases where {0} is obviously OK (such as
addrinfo)

[Bug c/53119] -Wmissing-braces wrongly warns about universal zero initializer {0}

[nightstrike at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

--- Comment #24 from nightstrike <nightstrike at gmail dot com> ---
(In reply to Marek Polacek from comment #20)
> Sorry, the patch hasn't been applied to 4.9 nor 4.8 branch yet, and I don't
> think it should be backported as-is, because just today I found out that the
> patch contains a bug; see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64709

Now that PR64709 is fixed, could you please backport both to 4.9 (and possibly
4.8)?