Re: uninitialized struct members

[Bruno Haible]

Hi Jim,

> While we're on the topic, this is the final warning
> I'm seeing in diffutils with most of "manywarnings" enabled:
> 
>     In file included from analyze.c:36:
>     ../lib/diffseq.h: In function 'compareseq':
>     ../lib/diffseq.h:461: warning: 'part.ymid' may be used uninitialized\
>     in this function [-Wuninitialized]
>     ../lib/diffseq.h:461: warning: 'part.xmid' may be used uninitialized\
>     in this function [-Wuninitialized]
> 
> Yes, it is spurious.

I'm also having similar warnings in the *list modules and in GNU clisp.

> Since I require warning free compilation with -Wuninitialized,
> I am considering this patch.  Would you prefer to avoid the
> warning in a different manner?

Well, I would prefer if the inaccuracy of the warnings be acknowledged by
the GCC developers. Has it already been reported?

> diff --git a/lib/diffseq.h b/lib/diffseq.h
> index 0c1723f..0951807 100644
> --- a/lib/diffseq.h
> +++ b/lib/diffseq.h
> @@ -464,7 +464,13 @@ compareseq (OFFSET xoff, OFFSET xlim, OFFSET yoff, 
> OFFSET ylim,
>        }
>    else
>      {
> -      struct partition part;
> +      struct partition part
> +#if defined lint && 3 <= __GNUC__
> +     /* Initialize solely to avoid spurious "may be used uninitialized"
> +        warnings from gcc.  */
> +     = { .xmid = 0, .ymid = 0 }
> +#endif
> +     ;
> 
>        /* Find a point of correspondence in the middle of the vectors.  */
>        diag (xoff, xlim, yoff, ylim, find_minimal, &part, ctxt);

We already have an IF_LINT macro in this file. Let's extend it so that it can
be used in this case as well. Users GCC < 3.0 will now see more warnings, but
they can upgrade to a newer GCC anyway. This avoids to have #if inside
functions for such a trivial stuff. Applying this:


2009-11-21  Jim Meyering  <address@hidden>
            Bruno Haible  <address@hidden>

        diffseq: avoid spurious gcc warnings
        * lib/diffseq.h (IF_LINT): Enable only with GCC >= 3.0.
        (compareseq): Initialize two members of "part" to avoid used-
        uninitialized warnings.

--- lib/diffseq.h.orig  2009-11-21 18:39:58.000000000 +0100
+++ lib/diffseq.h       2009-11-21 18:34:58.000000000 +0100
@@ -68,9 +68,10 @@
 # define EARLY_ABORT(ctxt) false
 #endif
 
-/* Use this to suppress gcc's `...may be used before initialized' warnings. */
+/* Use this to suppress gcc's `...may be used before initialized' warnings.
+   The Code argument may contain syntax that assumes GCC 3.0 or newer.  */
 #ifndef IF_LINT
-# ifdef lint
+# if defined lint && __GNUC__ >= 3
 #  define IF_LINT(Code) Code
 # else
 #  define IF_LINT(Code) /* empty */
@@ -464,7 +465,7 @@
       }
   else
     {
-      struct partition part;
+      struct partition part IF_LINT (= { .xmid = 0, .ymid = 0 });
 
       /* Find a point of correspondence in the middle of the vectors.  */
       diag (xoff, xlim, yoff, ylim, find_minimal, &part, ctxt);