svn commit: r355318 - head/sbin/newfs_msdos

[Konstantin Belousov]

On Wed, Dec 04, 2019 at 12:51:44AM -0800, Xin Li wrote:
> On 12/3/19 14:02, Conrad Meyer wrote:
> > Hi Xin Li,
> > 
> > Is there a reason to prefer exit() over returning from main?  I have
> 
> No, this should be case-by-case (and also assumes you are using C and
> not C++).
> 
> The two are actually subtly different (return means teardown main's
> stack first, then implicitly call exit from C runtime, while exit()
> would terminate immediately).
This is even more subtle.  If libthr is loaded into the process,
then exit() also causes stack unwinding.

> 
> If the command is meant to be used as a built-in module of something
> else, like the case of kill(1) builtin of sh(1), then one must not use
> exit() and also need to pay special attention to not exit() implicitly,
> because the caller will not fork() prior to calling the aliased main()
> function for performance reasons.
> 
> For other cases, using exit() might be a good idea, because it's easier
> to find the exit points especially if one is following sysexits(3)
> values.  Another reason is that if one allocates memory in main but not
> free them (these shouldn't be free'ed because the kernel would unmap all
> pages upon exit), return would mean these memory would be leaked: these
> are legitimate issues when main() would be called by someone else.  With
> an explicit exit(), these memory are never leaked because stack frame of
> main() remains valid before the final _exit(2) call.
> 
> > not surveyed the source tree, but I suspect most programs in base exit
> > by returning from main rather than explicit exit(3).> Thanks,
> > Conrad
> > 
> > On Mon, Dec 2, 2019 at 11:03 PM Xin LI <delphij at freebsd.org> wrote:
> >>
> >> Author: delphij
> >> Date: Tue Dec  3 07:03:25 2019
> >> New Revision: 355318
> >> URL: https://svnweb.freebsd.org/changeset/base/355318
> >>
> >> Log:
> >>   Explicitly exit() instead of return in main().
> >>
> >>   MFC after:    2 weeks
> >>
> >> Modified:
> >>   head/sbin/newfs_msdos/newfs_msdos.c
> >>
> >> Modified: head/sbin/newfs_msdos/newfs_msdos.c
> >> ==============================================================================
> >> --- head/sbin/newfs_msdos/newfs_msdos.c Tue Dec  3 07:01:28 2019        (r355317)
> >> +++ head/sbin/newfs_msdos/newfs_msdos.c Tue Dec  3 07:03:25 2019        (r355318)
> >> @@ -189,7 +189,7 @@ main(int argc, char *argv[])
> >>             err(1, NULL);
> >>      }
> >>      dtype = *argv;
> >> -    return !!mkfs_msdos(fname, dtype, &o);
> >> +    exit(!!mkfs_msdos(fname, dtype, &o));
> >>  }
> >>
> >>  /*
> 
>