[Bug c/53232] New: No warning for main() without a return statement with -std=c99

[Bug c/53232] New: No warning for main() without a return statement with -std=c99

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

             Bug #: 53232
           Summary: No warning for main() without a return statement with
                    -std=c99
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vincent-gcc@vinc17.net


On the following program, GCC (4.4 to 4.7 at least) invoked with -std=c99
-Wreturn-type doesn't give a warning about the missing return statement.

#include <stdio.h>

static int i = 0;

int main (void)
{
  if (i++ == 0)
    printf ("%d\n", main ());
}

According to the C99 rules, it seems that the return statement is optional only
for program termination. Though this is ambiguous, someone else at least has
the same interpretation as me:

  http://groups.google.com/group/comp.std.c/msg/c2f56fecfb699952

Before seeing this message, I posted

 
http://groups.google.com/group/comp.std.c/browse_thread/thread/0187ef7b23bedf16

to comp.std.c (Subject: main function without a return statement in C99/C11).

Also, I think that the warning should be given in every case for the following
reasons:
* A missing return statement may be unintentional (I think that implicit values
like here should be discouraged in general, and that the C99 rule is there more
to avoid undefined behavior than to save space).
* Compatibility with C90 and with freestanding environments.
* It is difficult to guarantee that main() will not be called from another C
file.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

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

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

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-05-04 13:44:02 UTC ---
Neither -Wmain warns about this... I think it is a bug, at least in the case of
-Wmain.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-04 14:33:53 UTC ---
(In reply to comment #1)
> Neither -Wmain warns about this... I think it is a bug, at least in the case of
> -Wmain.

Why? -Wmain checks the type of main, not whether it has a redundant 'return 0;'
as the last statement.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-05-04 15:28:55 UTC ---
(In reply to comment #2)
> Why? -Wmain checks the type of main, not whether it has a redundant 'return 0;'
> as the last statement.

You are right, I misread the description. 

So this is not a bug because Wreturn-type says that it specifically does not
check main?

[Bug c/53232] No warning for main() without a return statement with -std=c99

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-04 15:38:22 UTC ---
I think -Wreturn-type says in C++ it doesn't warn about declaring:
   main() {}
with no return type.  That only applies to C++, so isn't relevant here.

IMHO this isn't a bug because in C99 it's well-defined what happens if you fall
off the end of main, so it would annoy people to issue a warning on perfectly
valid code whenever -Wall is enabled.

But then I rarely use C99 so I don't really care.   If anyone suggested warning
for C++ programs that fall off the end of main I would be upset :)
The fact people might call main() from other functions is a sort-of-convincing
argument to warn about it (calling main isn't allowed in C++).

[Bug c/53232] No warning for main() without a return statement with -std=c99

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #5 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> 2012-05-04 15:44:04 UTC ---
(In reply to comment #4)
> IMHO this isn't a bug because in C99 it's well-defined what happens if you fall
> off the end of main,

Only at program termination (if my interpretation of C99 is correct). Please
read again the bug report and the links.

> so it would annoy people to issue a warning on perfectly valid code
> whenever -Wall is enabled.

Well, GCC has various warnings on perfectly valid code, just because the code
is suspicious. This is the case here too. And there are potential portability
problems (not everyone may use a C99 compiler). So, good reasons to issue a
warning.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-04 16:09:40 UTC ---
(In reply to comment #5)
> (In reply to comment #4)
> > IMHO this isn't a bug because in C99 it's well-defined what happens if you fall
> > off the end of main,
> 
> Only at program termination (if my interpretation of C99 is correct). Please
> read again the bug report and the links.

I did. Please read the last sentence of comment 4.

> > so it would annoy people to issue a warning on perfectly valid code
> > whenever -Wall is enabled.
> 
> Well, GCC has various warnings on perfectly valid code, just because the code
> is suspicious. This is the case here too.

Yes, and in each case some people want it and some don't.  I'm pointing out to
Manu the reasons not everyone wants the warning.  Your opinion isn't the only
valid one.

> And there are potential portability
> problems (not everyone may use a C99 compiler). So, good reasons to issue a
> warning.

Er, if you want to find portability problems for people not using C99 then
don't use -std=c99. Then -Wreturn-type warns about main.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
           Severity|minor                       |enhancement

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-04 16:11:35 UTC ---
Changing to "enhancement" since the current behaviour is by design, not a bug.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #8 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> 2012-05-04 19:58:01 UTC ---
(In reply to comment #6)
> Yes, and in each case some people want it and some don't.  I'm pointing out to
> Manu the reasons not everyone wants the warning.  Your opinion isn't the only
> valid one.

Then the solution would be to split -Wreturn-type into two different warnings.

> Er, if you want to find portability problems for people not using C99 then
> don't use -std=c99. Then -Wreturn-type warns about main.

There are several reasons one may want to use -std=c99, e.g. to be able to use
C99 features when available (via autoconf and/or preprocessor tests).

[Bug c/53232] No warning for main() without a return statement with -std=c99

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-08-28
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |carlos.maziero at gmail dot com

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 70958 has been marked as a duplicate of this bug. ***

[Bug c/53232] No warning for main() without a return statement with -std=c99

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #11 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Vincent Lefèvre from comment #8)
> (In reply to comment #6)
> > Er, if you want to find portability problems for people not using C99 then
> > don't use -std=c99. Then -Wreturn-type warns about main.
> 
> There are several reasons one may want to use -std=c99, e.g. to be able to
> use C99 features when available (via autoconf and/or preprocessor tests).

In any case, there does not seem to be a -std value to say that the program
must be valid for all C90, C99, C11 and C17 standards (and the future C23
standard). That's what portability is about.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Because GCC is primarily a compiler, not a linter for portability problems.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
And intersection of all the standards is quite hard to define.
So, adding -std=intersection_of_all_c_standards is just not a good idea, any
time same construct changes meaning between different standard you need to
decide what to do.
This is something that should be solved with warnings or external tools and on
the warning side we already have various warnings.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #14 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
Anyway, as I said initially, the warning would be interesting even in C99+
mode, because the lack of a return statement may be unintentional. For
instance, the developer may have forgotten a "return err;".

[Bug c/53232] No warning for main() without a return statement with -std=c99

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #15 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
But much more often it is intentional than unintentional.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #16 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Jakub Jelinek from comment #15)
> But much more often it is intentional than unintentional.

That's the same thing for many kinds of warnings.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #17 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Yeah, but warnings with high false positivity rates at least shouldn't be in
-Wall.

[Bug c/53232] No warning for main() without a return statement with -std=c99

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53232

--- Comment #18 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Jakub Jelinek from comment #17)
> Yeah, but warnings with high false positivity rates at least shouldn't be in
> -Wall.

Well, there already is -Wunused, which is included in -Wall (such warnings may
typically be emitted due to #if and also in temporary code when debugging), and
-Wsign-compare in C++.

Anyway, there is a first issue: the warning is inexistent, even with -Wextra.
There is a second issue: the warning is not emitted with -Wreturn-type when
there is a call to main(). Solving these two issues alone would not yield a
high false positivity rate with -Wall. (That said, I think that developers
should be encouraged to have an explicit "return" for main(); in particular,
this is really easy to do and improves the code readability, specially knowing
the difference of behavior with other languages, such as shell scripts and
Perl.)