On Wed, 3 May 2017, Alan Somers wrote:
On Wed, May 3, 2017 at 2:51 PM, Bruce Evans <b...@optusnet.com.au> wrote:
On Wed, 3 May 2017, Eric van Gyzen wrote:
On 05/03/2017 14:38, Alan Somers wrote:
On Wed, May 3, 2017 at 1:34 PM, Warner Losh <i...@bsdimp.com> wrote:
On Wed, May 3, 2017 at 1:32 PM, Alan Somers <asom...@freebsd.org> wrote:
On Wed, May 3, 2017 at 12:16 PM, Ngie Cooper <yaneurab...@gmail.com>
wrote:
On May 3, 2017, at 10:21, Alan Somers <asom...@freebsd.org> wrote:
Author: asomers
Date: Wed May 3 17:21:01 2017
New Revision: 317755
URL: https://svnweb.freebsd.org/changeset/base/317755
Log:
Various Coverity fixes in ifconfig(8)
...
* Mark usage() as _Noreturn (1305806, 1305750)
...
-static void usage(void);
+static void usage(void) _Noreturn;
Hi Alan,
Please use __dead2 instead to be consistent with legacy use of
similar gcc attributes.
Thanks,
-Ngie
Why not use _Noreturn? It's standardized by C11, so tools understand
it better than __dead2.
Tools that can't understand #define __dead2 _Noreturn aren't worth
supporting.
Some tools don't expand preprocessor macros. Like my editor, for
example, which highlights _Noreturn as a keyword but not __dead2.
Please use _Noreturn, because it's standard. sys/cdefs.h already
defines it appropriately for C < C11.
_Noreturn is far too hard to use. The above use of it is a syntax error:
pts/12:bde@freefall:~/u3> cat z.c
void foo(void) _Noreturn;
_Noreturn void foo(void);
pts/12:bde@freefall:~/u3> cc -std=c11 z.c
z.c:1:16: error: '_Noreturn' keyword must precede function declarator
void foo(void) _Noreturn;
^~~~~~~~~
_Noreturn
1 error generated.
sys/cdefs.h defines might define it appropropriately for C < C11, but
it defines it as __dead2 for all C, so prevents the C11 _Noreturn
keyword being used. This normally breaks detection of the syntax error.
Normally <sys/cdefs.h> is included first, so you __dead2 obfuscated by
spelling it _Noreturn instead of C11 _Noreturn.
Defining _Noreturn as __dead2 is wrong because it gives the opposite
syntax error. __dead2 can now be placed anywhere, but everything in
sys/cdefs.h is supposed to be portable back to gcc-1. __dead2 must
be placed after the function for gcc-2.0, since __attribute__(()) had
more restrictions then. So if you write:
#include <sys/cdefs.h>
_Noreturn void foo(void);
to satisfy the C11 syntax, then you get a syntax error for old gcc (> 1).
This is just the start of the complications for soft-coded C11'isms.
C11 also has noreturn. You have to include <stdnoreturn.h> to get that.
But you actiually get the _Noreturn macro which expands to __dead2.
There are further complications for C++11. sys/cdefs.h does have a
correct-looking ifdef for C+11. This gives the [[noreturn]] keyward
instead of __dead2. C11 doesn't have <stdnoreturn.h>. I think its
keyword must be spelled [[noreturn]]. This spelling is completely
incompatibly with C.
Why do you say that cdefs.h should be compatible with gcc-1? gcc-2
Because that is what it is for. It should be compatible with any C
compiler, not just gcc or Standard C ones, but since it grew up with
gcc it doesn't have much support for others.
It still pretends to supports gcc-1 with pre-Standard C (__P(()), etc.)
and even compilers that don't have pre-Standard volatile (pure K&R1
for that and not K&R with gcc-1 extensions), and lint. Some of this
still works.
was released more than 25 years ago. gcc-1 isn't the default compiler
for any architecture and isn't available in ports. If anybody can
find a copy of gcc-1, I doubt that much of our codebase would compile.
It sounds to me that the best practice would be to place both __dead2
and _Noreturn before the function name.
Unportable code can do that. Of course, it is unportable to include
<sys/cdefs.h> at all. Usign __dead2 gives undefined behaviour in general.
Even if you include <sys/cdefs.h>, it might not be the FreeBSD one. Using
_Noreturn gives undefined behaviour before C11.
Bruce
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