Re: 'noreturn' warning in libcruft

[John W. Eaton]

On  7-Aug-2012, Rik wrote:

| On 08/07/2012 09:26 AM, Michael Goffioul wrote:
| 
|     On Tue, Aug 7, 2012 at 5:22 PM, Rik <address@hidden> wrote:
| 
|         8/7/12
| 
|         All,
| 
|         When compiling libcruft/misc/f77-fcn.c I get the following warning:
| 
|         ../../octave-dev/libcruft/misc/f77-fcn.c: In function ‘xstopx_’:
|         ../../octave-dev/libcruft/misc/f77-fcn.c:63: warning: function 
declared
|         ‘noreturn’ has a ‘return’ statement
| 
|         If I look in libcruft/misc/f77-fcn.h I find the following prototype:
| 
|         extern CRUFT_API F77_RET_T
|         F77_FUNC (xstopx, XSTOPX) (F77_CONST_CHAR_ARG_DECL
|                                    F77_CHAR_ARG_LEN_DECL)
|         GCC_ATTR_NORETURN;
| 
|         The 'noreturn' attribute is being explicitly set for this function so
|         that
|         seems okay.
| 
|         The actual function is
| 
|         F77_RET_T
|         #if defined (F77_USES_CRAY_CALLING_CONVENTION)
|         F77_FUNC (xstopx, XSTOPX) (octave_cray_ftn_ch_dsc desc)
|         #elif defined (F77_USES_VISUAL_FORTRAN_CALLING_CONVENTION)
|         F77_FUNC (xstopx, XSTOPX) (const char *s, int slen)
|         #else
|         F77_FUNC (xstopx, XSTOPX) (const char *s, long slen)
|         #endif
|         {
|         #if defined (F77_USES_CRAY_CALLING_CONVENTION)
|           const char *s = desc.const_ptr = ptr_arg;
|           unsigned long slen = desc.mask.len;
|         #endif
| 
|           f77_exception_encountered = 1;
| 
|           /* Skip printing message if it is just a single blank character.  *
|         /
|           if (s && slen > 0 && ! (slen == 1 && *s == ' '))
|             (*current_liboctave_error_handler) ("%.*s", slen, s);
| 
|           octave_jump_to_enclosing_context ();
| 
|           F77_RETURN (0)
|         }
| 
|         So the function is declared 'noreturn' and yet has a return statement.
|          The
|         function before the return, octave_jump_to_enclosing_context(), does
|         indeed
|         do a longjmp so it seems like the F77_RETURN(0) can just be eliminated
|         from
|         the code.
| 
|         Any objections to that deletion?
| 
| 
|     Deleting it would assume that GCC_ATTR_NORETURN is always defined 
properly.
|     Would it be a problem to leave it in place?
| 
| Leaving the code in place is what provokes the warning that I am trying to
| silence.  Note that regardless of whether GCC_ATTR_NORETURN is declared for
| this function, the particular line of code F77_RETURN(0) will never be
| executed.  The noreturn attribute is merely warning us that we're being
| redundant since there is no way to get to the return statement because the
| longjmp in octave_jum_to_enclosing_context() will always intervene.

Deleting the F77_RETURN(0) will avoid the warning if GCC_ATTR_NORETURN
is defined because octave_jump_to_enclosing_context is tagged with
GCC_ATTR_NORETURN.

But if you delete the F77_RETURN (0) line and GCC_ATTR_NORETURN is not
defined, you will likely get a warning about "control reaches end of
non-void function".

So how about using

  #ifndef GCC_ATTR_NORETURN
    F77_RETURN (0)
  #endif

?

Note that this only matters if F77_RET_T is int.  If it is void, then
F77_RETURN expands to nothing and there is no inconsistency about
return type and falling off the end of the function without a return
statement.

Thinking about this again though, maybe it is confusing to have
F77_RETURN expand to nothing (shouldn't it return?).  But the point
of it was just to provide a way to define a C function that would
behave like a Fortran subroutine and to avoid GCC's "control reaches
end of non-void function" warning.  Maybe we should really have a few
different types of F77_RETURN macros?

  F77_RETURN (retval)    ! same as we have now, except that it should
                         ! be defined to "return;" if F77_RET_T is
                         ! defined as "void". 

  F77_NORETURN (retval)  ! expands to nothing if either
                         ! GCC_ATTR_NORETURN is defined or if
                         ! F77_RET_T is defined as "void".  Otherwise,
                         ! expand to "return retval;" 

Then the xstopx function should use F77_NORETURN instead of
F77_RETURN.  Does that make sense?

jwe