[Bug c/49754] New: Let gcc warn about all uninitialized variables

[Bug c/49754] New: Let gcc warn about all uninitialized variables

[giecrilj at stegny dot 2a.pl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49754

           Summary: Let gcc warn about all uninitialized variables
           Product: gcc
           Version: 4.5.1
               URL: https://bugzilla.novell.com/show_bug.cgi?id=705160#c1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: giecrilj@stegny.2a.pl


The compiler gcc currently warns about uninitialized scalar variables but not
about compound variables.

== Steps to Reproduce ==
  1. 
{ cat>foo.c<<'/* EOF */' && gcc -Wall -pedantic-errors foo.c; }
struct i { int x; };
struct i foo () { struct i x; return x; }
int bar () { int y; return +y; }
/* EOF */

== Actual Results ==  
foo.c: In function ‘bar’:
foo.c:3:21: warning: ‘y’ is used uninitialized in this function


== Expected Results ==  
foo.c: In function ‘foo’:
foo.c:2:?: warning: ‘x’ is used uninitialized in this function
foo.c: In function ‘bar’:
foo.c:3:21: warning: ‘y’ is used uninitialized in this function

[Bug c/49754] Let gcc warn about all uninitialized variables

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49754

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-07-15 08:36:30 UTC ---
(In reply to comment #0)
> == Expected Results ==  
> foo.c: In function ‘foo’:
> foo.c:2:?: warning: ‘x’ is used uninitialized in this function

It should really warn if 'x.i' is used uninitialized, not 'x'

[Bug c/49754] Let gcc warn about all uninitialized variables

[giecrilj at stegny dot 2a.pl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49754

--- Comment #2 from Christopher Yeleighton <giecrilj at stegny dot 2a.pl> 2011-07-15 09:35:42 UTC ---
(In reply to comment #1)
> (In reply to comment #0)
> > == Expected Results ==  
> > foo.c: In function ‘foo’:
> > foo.c:2:?: warning: ‘x’ is used uninitialized in this function
> 
> It should really warn if 'x.i' is used uninitialized, not 'x'

Since (x) is uninitialized, so is (x.i).  It is impossible to recognize the
problem when (x.i) is eventually accessed by a client of foo:

{
struct i x = foo (); 
printf ("%i", +x.i);
}

[Bug c/49754] Let gcc warn about all uninitialized variables

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49754

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-07-15 09:56:49 UTC ---
(In reply to comment #2)
> Since (x) is uninitialized, so is (x.i).

But what if x.i gets initialized, is x still uninitialized?


struct X { int i; };
struct Y { int i; int j; };

int main()
{
  X x;
  x.i = 0;   // is 'x' initialized now?
  Y y;
  y.i = 0;   // is 'y' initialized now?
  y.j = 0;   // is 'y' initialized now?
}


It would be possible to track the initialization of each subobject *and* the
aggregate, but it would be more overhead

[Bug c/49754] Let gcc warn about all uninitialized variables

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49754

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-07-15 10:16:12 UTC ---
Just to be clear - I completely agree that uninit'd warnings need improving,
I'm not objecting to that.  But in my experience (mostly C++) I'd prefer to
have it tracked at the more fine-grained level of sub-objects.

[Bug c/49754] Let gcc warn about all uninitialized variables

[giecrilj at stegny dot 2a.pl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49754

--- Comment #5 from Christopher Yeleighton <giecrilj at stegny dot 2a.pl> 2011-07-15 11:00:28 UTC ---
(In reply to comment #3)
> (In reply to comment #2)
> > Since (x) is uninitialized, so is (x.i).
> 
> But what if x.i gets initialized, is x still uninitialized?

If (x.i) denotes an object type and the initial value means "x is empty" then x
is initialized.

> 
> struct X { int i; };
> struct Y { int i; int j; };
> 
> int main()
> {
>   X x;
>   x.i = 0;   // is 'x' initialized now?
>   Y y;
>   y.i = 0;   // is 'y' initialized now?
>   y.j = 0;   // is 'y' initialized now?
> }
> 
> 
> It would be possible to track the initialization of each subobject *and* the
> aggregate, but it would be more overhead

I have already bumped into this using arrays, where GCC does emit a warning
although it should not:

int a [02]; 
for (a [0] = 0;;) if (a [0]) printf ("%d", +a [1]); else a [0] = a [1] = 01;

[Bug middle-end/49754] Wuninitialized does not work with structs/unions/arrays

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49754

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-10-04
                 CC|                            |manu at gcc dot gnu.org
          Component|c                           |middle-end
            Summary|Let gcc warn about all      |Wuninitialized does not
                   |uninitialized variables     |work with
                   |                            |structs/unions/arrays
     Ever Confirmed|0                           |1

--- Comment #6 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-10-04 08:23:24 UTC ---
This is a problem of representation. Struct/unions/arrays are represented as
memory operations, and Wuninitialized does not (generally) work on them. It may
warn in some corner cases if the accesses can be converted into scalar
variables, but in general it doesn't work.

I am not even sure if the current middle-end representation can tell whether
the memory is uninitialized and never initialized up to the point of use. That
would be the first thing to implement.

[Bug middle-end/49754] Wuninitialized does not work with structs/unions/arrays

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49754

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=95848
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #8 from Martin Sebor <msebor at gcc dot gnu.org> ---
GCC 11 (and prior) diagnose both functions in comment #0, I suspect as a result
of r156156:

pr49754.c: In function ‘foo’:
pr49754.c:2:38: warning: ‘x’ is used uninitialized [-Wuninitialized]
    2 | struct i foo () { struct i x; return x; }
      |                                      ^
pr49754.c:2:28: note: ‘x’ declared here
    2 | struct i foo () { struct i x; return x; }
      |                            ^
pr49754.c: In function ‘bar’:
pr49754.c:3:28: warning: ‘y’ is used uninitialized [-Wuninitialized]
    3 | int bar () { int y; return +y; }
      |                            ^~

Returning larger aggregates started to be diagnosed in r245840.  Passing
structs to other functions by value still isn't diagnosed.  It used to be, but
it stopped  sometime between r143699 and r143725.

See pr95848 that tracks the problem with pass-by-value.