Re: What should -Wmissing-noreturn do with "int main(){exit(0);}" ?

["Kaveh R. Ghazi" <ghazi at caip dot rutgers dot edu>]

 > From: Richard Henderson <rth@cygnus.com>
 > 
 > On Thu, Aug 12, 1999 at 03:02:02PM -0400, Kaveh R. Ghazi wrote:
 > >  > int main() { exit(0); }
 > > 
 > > foo.c: In function `main':
 > > foo.c:1: warning: function might be possible candidate for
 > > attribute `noreturn'
 > > 
 > > The diagnostic is technically correct, but perhaps it should do
 > > something different here.  I see two alternatives.
 > 
 > Since the diag is correct, is there any need to change it at all?
 > I don't see any reason to...
 > r~

	Well think about what one would need to do to silence the
warning.  Is it useful to declare `main' as noreturn?

	Attribute noreturn provides two benefits, a slight
optimization, and better analysis for things like unused variables.
But if I understand things correctly, these benefits are for *callers*
of the noreturn function.  And the attribute must seen by the callers,
which won't happen for `main'.

	So I think -Wmissing-noreturn should either ignore `main', or
if it detects `main' is noreturn it should suggest the user switch
from exitting out of `main' to issuing a return from `main'.

I.e.: "main(){exit(0);}" -> "main(){return 0;}".

I still favor having it ignore main, but either way it didn't seem
logical for the compiler to suggest declaring main as noreturn.

		--Kaveh

PS: Here's what I had in mind, what do you think?



1999-08-12  Kaveh R. Ghazi  <ghazi@caip.rutgers.edu>

	* c-decl.c (finish_function): Ignore "main" when checking for
	-Wmissing-noreturn functions.

	* invoke.texi: Document it.

diff -rup orig/egcs-CVS19990811/gcc/c-decl.c egcs-CVS19990811/gcc/c-decl.c
--- orig/egcs-CVS19990811/gcc/c-decl.c	Mon Aug  9 15:41:49 1999
+++ egcs-CVS19990811/gcc/c-decl.c	Thu Aug 12 15:34:34 1999
@@ -6890,7 +6890,8 @@ finish_function (nested)
   if (warn_missing_noreturn
       && !TREE_THIS_VOLATILE (fndecl)
       && !current_function_returns_null
-      && !current_function_returns_value)
+      && !current_function_returns_value
+      && strcmp (IDENTIFIER_POINTER (DECL_NAME (fndecl)), "main"))
     warning ("function might be possible candidate for attribute `noreturn'");
 
   if (TREE_THIS_VOLATILE (fndecl) && current_function_returns_null)
diff -rup orig/egcs-CVS19990811/gcc/invoke.texi egcs-CVS19990811/gcc/invoke.texi
--- orig/egcs-CVS19990811/gcc/invoke.texi	Wed Aug 11 07:42:14 1999
+++ egcs-CVS19990811/gcc/invoke.texi	Thu Aug 12 15:40:09 1999
@@ -1759,8 +1760,10 @@ header files.
 Warn about functions which might be candidates for attribute @code{noreturn}.
 Note these are only possible candidates, not absolute ones.  Care should
 be taken to manually verify functions actually do not ever return before
-adding the @code{noreturn} attribute, otherwise subtle code generation
-bugs could be introduced.
+adding the @code{noreturn} attribute.  Otherwise, subtle code generation
+bugs could be introduced.  This warning is ignored for function @samp{main}
+because it is common practice to call @samp{exit} from there rather
+than using @code{return} statements.
 
 @item -Wredundant-decls
 Warn if anything is declared more than once in the same scope, even in



1999-08-12  Kaveh R. Ghazi  <ghazi@caip.rutgers.edu>

	* gcc.dg/noreturn-1.c: Test "main" as a special case.

diff -rup orig/egcs-CVS19990811/gcc/testsuite/gcc.dg/noreturn-1.c egcs-CVS19990811/gcc/testsuite/gcc.dg/noreturn-1.c
--- orig/egcs-CVS19990811/gcc/testsuite/gcc.dg/noreturn-1.c	Thu May 13 12:15:13 1999
+++ egcs-CVS19990811/gcc/testsuite/gcc.dg/noreturn-1.c	Thu Aug 12 15:41:07 1999
@@ -41,3 +41,9 @@ foo6(void)
 {
   return;
 } /* { dg-bogus "warning:" "this function should not get any warnings" } */
+
+int
+main()
+{
+  exit(0);
+} /* { dg-bogus "warning:" "this function should not get any warnings" } */

--
Kaveh R. Ghazi			Engagement Manager / Project Services
ghazi@caip.rutgers.edu		Qwest Internet Solutions