[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[manu at gcc dot gnu dot org]

------- Comment #9 from manu at gcc dot gnu dot org  2007-08-16 10:57 -------
Let's simplify this report. This one is now about 

int atoi(const char *);
int foo()
{
    char buf[10];
    return atoi(buf);
}

As comment #3 mentions, this is a combination of 

1) Report use of uninitialized array elements (PR27120).
2) Report pointers to uninitialized variables passed as const* arguments
(PR33086).

I guess that if we achieve both things, we will have a chance to achieve this
one. In other words, both PRs are blocking this one.


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org
  BugsThisDependsOn|                            |27120
OtherBugsDependingO|27120                       |
              nThis|                            |
            Summary|-Wuninitialized could catch |warn for uninitialized
                   |uninitialized arrays        |arrays passed as const*
                   |                            |arguments


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[manu at gcc dot gnu dot org]

------- Comment #10 from manu at gcc dot gnu dot org  2007-08-20 14:49 -------
I now think that Andrew is right and that PR33086 and this one are INVALID.
'const' does not mean read-only in C++ at all, and much less in C. atoi(const
char *) could always initialize buf[]. However, perhaps it can apply to
functions marked as "pure"...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[bangerth at dealii dot org]

------- Comment #11 from bangerth at dealii dot org  2007-08-20 14:56 -------
(In reply to comment #10)
> I now think that Andrew is right and that PR33086 and this one are INVALID.
> 'const' does not mean read-only in C++ at all, and much less in C. atoi(const
> char *) could always initialize buf[].

Uh, no, it can't. If it did (by casting away the 'const' from its argument
and then writing into the array), this would lead to a segfault:
-------------
const char a[3] = "10";
int main () {
  return atoi (a);
}
-------------
The reason being that compilers will typically place 'a' into a read-only
memory section.

Passing an uninitialized object as a const reference or pointer is a bad
idea in C++ and should get a warning as a rule. I can't see reasons that would
warrant exceptions from this rule.

W.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[manu at gcc dot gnu dot org]

------- Comment #12 from manu at gcc dot gnu dot org  2007-08-20 15:03 -------
(In reply to comment #11)
> (In reply to comment #10)
> > I now think that Andrew is right and that PR33086 and this one are INVALID.
> > 'const' does not mean read-only in C++ at all, and much less in C. atoi(const
> > char *) could always initialize buf[].
> 
> Uh, no, it can't. If it did (by casting away the 'const' from its argument
> and then writing into the array), this would lead to a segfault:
> -------------
> const char a[3] = "10";
> int main () {
>   return atoi (a);
> }
> -------------

This testcase has nothing to do with uninitialized variables. If the variable
is 'const' already, then there will never be a warning. Will it produce
segmentation fault for a local automatic uninitialized pointer?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[manu at gcc dot gnu dot org]

------- Comment #13 from manu at gcc dot gnu dot org  2007-08-20 15:08 -------
(In reply to comment #12)
> 
> This testcase has nothing to do with uninitialized variables. If the variable
> is 'const' already, then there will never be a warning. Will it produce
> segmentation fault for a local automatic uninitialized pointer?
> 

Of course, I mean pointer to (or array of, in this case) uninitialized data,
not that the pointer itself is uninitialized. Sorry for the confusion.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[bangerth at dealii dot org]

------- Comment #14 from bangerth at dealii dot org  2007-08-20 15:54 -------
(In reply to comment #12)

> This testcase has nothing to do with uninitialized variables.

No, of course. I only meant to reply to your assertion that there could be
cases where a function initializes an object that is passed as a const
pointer. That may work in C, but not in C++. I agree that my example may
have been confusing, so take this slight variant of it ('a' is here non-const
and uninitialized):
-------------------
char a[3];
int main () {
  return atoi (a);
}
-------------------
It is my understanding that C++ allows 'a' to be put into read-only memory
because the only access is read-only. What that means is that functions like
atoi that take constant arguments have no legitimate way to initialize the
objects they get, and every uninitialized object passed to them is necessarily
a source of bugs.

This is meant to only counter your point that:
> 'const' does not mean read-only in C++ at all, and much less in C. atoi(const
> char *) could always initialize buf[].
This simply isn't true. In C++, atoi can't do that.

W.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[manu at gcc dot gnu dot org]

------- Comment #15 from manu at gcc dot gnu dot org  2007-08-20 16:15 -------
(In reply to comment #14)
> This is meant to only counter your point that:
> > 'const' does not mean read-only in C++ at all, and much less in C. atoi(const
> > char *) could always initialize buf[].
> This simply isn't true. In C++, atoi can't do that.
> 

Please, take a look at the example given by Andrew
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33086#c3. I can compile and run a
similar example:

#include <cstdio>
void use(const int *a)
{
  int * b = (int *)a;
  b[0] = 5;
}
int main(void)
{
  int i=0;
  use(&i);
  printf("%d\n", i);
  return 0;
}

Of course, the output is '5' and not '0'. So yes, atoi() seems perfectly able
to  initialize buf. (or perhaps, I am still confused).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[bangerth at dealii dot org]

------- Comment #16 from bangerth at dealii dot org  2007-08-20 16:21 -------
(In reply to comment #15)

> Of course, the output is '5' and not '0'. So yes, atoi() seems perfectly able
> to  initialize buf. (or perhaps, I am still confused).

I think you are. This program here segfaults:
---------------------
#include <cstdio>

const int i=0;

void use(const int *a)
{
  int * b = (int *)a;
  b[0] = 5;
}

int main(void)
{
  use(&i);
  printf("%d\n", i);
  return 0;
}
----------------------------
Since use() (like atoi) can't know whether its argument is a local automatic
or a global variable, there is no way for it to reliably initialize its
argument. Casting away constness invokes undefined behavior if the static
type of the object is const.

W.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[manu at gcc dot gnu dot org]

------- Comment #17 from manu at gcc dot gnu dot org  2007-08-20 16:44 -------
(In reply to comment #16)
> (In reply to comment #15)
> 
> > Of course, the output is '5' and not '0'. So yes, atoi() seems perfectly able
> > to  initialize buf. (or perhaps, I am still confused).
> 
> Since use() (like atoi) can't know whether its argument is a local automatic
> or a global variable, there is no way for it to reliably initialize its
> argument. Casting away constness invokes undefined behavior if the static
> type of the object is const.

But it seems that the current policy of GCC is to not assume that such
functions actually take this into account, since when optimizing constants are
not propagated beyond a call to such function. This is either the intended
behaviour or a missed-optimisation. It would be nice if it would be a
missed-optimisation. Fixing that will probably fix PR33086 and help to fix this
one. Otherwise, if this is the intended behaviour, then both PRs are invalid as
Andrew said.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[manu at gcc dot gnu dot org]

------- Comment #18 from manu at gcc dot gnu dot org  2007-08-20 16:46 -------
When I say "constant are not propagated" I mean "the constant value of a
variable" such as:

  int i=0;
  use(&i);
  foo(i);

Here, GCC does not propagate the value of i to do foo(0). Remove the call to
use and then it will.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[bangerth at dealii dot org]

------- Comment #19 from bangerth at dealii dot org  2007-08-20 16:58 -------
(In reply to comment #18)
> When I say "constant are not propagated" I mean "the constant value of a
> variable" such as:
> 
>   int i=0;
>   use(&i);
>   foo(i);
> 
> Here, GCC does not propagate the value of i to do foo(0). Remove the call to
> use and then it will.

What if you had "const int i=0"? As I said before, use() may do a const-cast
to get rid of the constness of its argument, but the result is only
well-defined
if the object pointed to is actually non-const. That is the case here, so use()
may do exactly this and clobber 'i'. On the other hand, if 'i' was const, then
the result of any const-cast use() may do on its argument are undefined, and
it would seem legitimate to propagate the initial value of 'i' into the call
to foo().

W.
I think that in 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[manu at gcc dot gnu dot org]

------- Comment #20 from manu at gcc dot gnu dot org  2007-08-20 17:12 -------
(In reply to comment #19)
> 
> What if you had "const int i=0"? As I said before, use() may do a const-cast
> to get rid of the constness of its argument, but the result is only
> well-defined
> if the object pointed to is actually non-const. That is the case here, so use()
> may do exactly this and clobber 'i'. On the other hand, if 'i' was const, then
> the result of any const-cast use() may do on its argument are undefined, and
> it would seem legitimate to propagate the initial value of 'i' into the call
> to foo().
> 

Wolfgang, I understand perfectly what you explain. Yet, from the point of view
of Wuninitialized, only local non-const objects are interesting. And it seems
that for those, GCC does not assume that the function is not going to modify
the object. Thus, it doesn't make much sense for Wuninitialized to assume it.
And, from a technical point of view, it would be difficult to maintain such
dichotomy, since Wuninitialized relies on the SSA representation and the work
of the optimisation passes.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[gdr at cs dot tamu dot edu]

------- Comment #21 from gdr at cs dot tamu dot edu  2007-08-20 18:50 -------
Subject: Re:  warn for uninitialized arrays passed as const* arguments

"manu at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| When I say "constant are not propagated" I mean "the constant value of a
| variable" such as:
| 
|   int i=0;
|   use(&i);
|   foo(i);
| 
| Here, GCC does not propagate the value of i to do foo(0). Remove the call to
| use and then it will.

Which is all it can do, assuming the body is not "available".

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[h dot b dot furuseth at usit dot uio dot no]

------- Comment #22 from h dot b dot furuseth at usit dot uio dot no  2007-08-20 22:45 -------
Subject: Re:  warn for uninitialized arrays passed as const* arguments

manu at gcc dot gnu dot org writes:
> But it seems that the current policy of GCC is to not assume that such
> functions actually take this into account, since when optimizing
> constants are not propagated beyond a call to such function. This is
> either the intended behaviour or a missed-optimisation.
> It would be nice if it would be a missed-optimisation.

A "const" in a function parameter in C is not a promise to the compiler;
it would break the C standard to optimize on it.

> Otherwise, if this
> is the intended behaviour, then both PRs are invalid as Andrew said.

They are requests for warning messages, not error messages, because they
are _likely_ to be programmer errors.  That's what warnings are for.
-Wuninitialized is giving false positives anyway, on code which the
compiler cannot tell is correct but the programmer can.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[matt at use dot net]

------- Comment #23 from matt at use dot net  2009-12-31 01:44 -------
I just ran into a bug that this feature would have found for me at
compile-time. Instead, it required exercising the code under valgrind, which
took quite some time. If voting were enabled, I would vote for this bug.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138
Bug 10138 depends on bug 27120, which changed state.

Bug 27120 Summary: Should warn about uninitialized use of variable array element
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=27120

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch
   Last reconfirmed|2008-03-30 20:45:14         |2020-5-15
   Target Milestone|---                         |11.0

--- Comment #29 from Martin Sebor <msebor at gcc dot gnu.org> ---
Patch: https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545856.html

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

--- Comment #30 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Sebor <msebor@gcc.gnu.org>:

https://gcc.gnu.org/g:b825a22890740f341eae566af27e18e528cd29a7

commit r11-959-gb825a22890740f341eae566af27e18e528cd29a7
Author: Martin Sebor <msebor@redhat.com>
Date:   Thu Jun 4 16:06:10 2020 -0600

    Implement a solution for PR middle-end/10138 and PR middle-end/95136.

    PR middle-end/10138 - warn for uninitialized arrays passed as const
arguments
    PR middle-end/95136 - missing -Wuninitialized on an array access with a
variable offset

    gcc/c-family/ChangeLog:

            PR middle-end/10138
            PR middle-end/95136
            * c-attribs.c (append_access_attrs): Handle attr_access::none.
            (handle_access_attribute): Same.

    gcc/ChangeLog:

            PR middle-end/10138
            PR middle-end/95136
            * attribs.c (init_attr_rdwr_indices): Move function here.
            * attribs.h (rdwr_access_hash, rdwr_map): Define.
            (attr_access): Add 'none'.
            (init_attr_rdwr_indices): Declared function.
            * builtins.c (warn_for_access)): New function.
            (check_access): Call it.
            * builtins.h (checK-access): Add an optional argument.
            * calls.c (rdwr_access_hash, rdwr_map): Move to attribs.h.
            (init_attr_rdwr_indices): Declare extern.
            (append_attrname): Handle attr_access::none.
            (maybe_warn_rdwr_sizes): Same.
            (initialize_argument_information): Update comments.
            * doc/extend.texi (attribute access): Document 'none'.
            * tree-ssa-uninit.c (struct wlimits): New.
            (maybe_warn_operand): New function.
            (maybe_warn_pass_by_reference): Same.
            (warn_uninitialized_vars): Refactor code into maybe_warn_operand.
            Also call for function calls.
            (pass_late_warn_uninitialized::execute): Adjust comments.
            (execute_early_warn_uninitialized): Same.

    gcc/testsuite/ChangeLog:

            PR middle-end/10138
            PR middle-end/95136
            * c-c++-common/Wsizeof-pointer-memaccess1.c: Prune out valid
            Wuninitialized.
            * c-c++-common/uninit-pr51010.c: Adjust expected warning format.
            * c-c++-common/goacc/uninit-dim-clause.c: Same.
            * c-c++-common/goacc/uninit-firstprivate-clause.c: Same.
            * c-c++-common/goacc/uninit-if-clause.c: Same.
            * c-c++-common/gomp/pr70550-1.c: Same.
            * c-c++-common/gomp/pr70550-2.c: Adjust.
            * g++.dg/20090107-1.C: Same.
            * g++.dg/20090121-1.C: Same.
            * g++.dg/ext/attr-access.C: Avoid -Wuninitialized.
            * gcc.dg/tree-ssa/forwprop-6.c: Prune out -Wuninitialized.
            * gcc.dg/Warray-bounds-52.c: Prune out valid -Wuninitialized.
            * gcc.dg/Warray-bounds-53.c: Same.
            * gcc.dg/Warray-bounds-54.c: Same.
            * gcc.dg/Wstringop-overflow-33.c: New test.
            * gcc.dg/attr-access-none.c: New test.
            * gcc.dg/attr-access-read-only.c: Adjust.
            * gcc.dg/attr-access-read-write.c: Same.
            * gcc.dg/attr-access-write-only.c: Same.
            * gcc.dg/pr71581.c: Adjust text of expected warning.
            * gcc.dg/uninit-15.c: Same.
            * gcc.dg/uninit-32.c: New test.
            * gcc.dg/uninit-33.c: New test.
            * gcc.dg/uninit-34.c: New test.
            * gcc.dg/uninit-36.c: New test.
            * gcc.dg/uninit-B-O0.c: Adjust text of expected warning.
            * gcc.dg/uninit-I-O0.c: Same.
            * gcc.dg/uninit-pr19430-O0.c: Same.
            * gcc.dg/uninit-pr19430.c: Same.
            * gcc.dg/uninit-pr95136.c: New test.
            * gfortran.dg/assignment_4.f90: Expect -Wuninitialized.
            * gfortran.dg/goacc/uninit-dim-clause.f95: Adjust text of expected
            warning.
            * gfortran.dg/goacc/uninit-firstprivate-clause.f95
            * gfortran.dg/goacc/uninit-if-clause.f95
            * gfortran.dg/pr66545_2.f90

[Bug middle-end/10138] warn for uninitialized arrays passed as const* arguments

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10138

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #31 from Martin Sebor <msebor at gcc dot gnu.org> ---
Done for GCC 11.