On 22/01/2018 10:31, Jay K wrote:
By this argument there is a missing warning for the equivalent:
const int foo = 123;
with no previous extern declaration.
I would like to see such a warning. There is "-Wmissing-declarations",
but that applies only to functions and not to objects.
(Note that in C++, "const" objects without an "extern" declaration are
effectively "static" - in C, without the storage-class specifier const
objects have external linkage.)
As well, there is no warning in C++.
All three constructs are equivalent, yet only one gets a warning.
No, they are not the same - in C++ the linkage of a const is static
unless you explicitly say "extern", and in C it is external unless you
explicitly say "static".
Thus in "extern const int foo = 123;", the "extern" has a significant
effect in C++ but in C it does nothing (other than inform the reader).
I would like to see the warning having a controlling flag. It could
perhaps be on by default in C and off by default in C++ to get the same
effect as today - and then users can fine-tune to fit their style.
Interesting point, that I had not realized, and with an often acceptable
workaround, however also there exist coding conventions that prohibit use of
static.
I have never heard of such a thing in a coding standard. C++
conventions might encourage the use of anonymous namespaces rather than
C-style "static" declarations, but that would not apply to C. I would
consider a coding convention that discouraged static to be seriously broken.
Instead they "hide" things by omitting them from headers only.
That is madness. The symbols still have global linkage across the
program, and you will get all sorts of problems when one file uses the
same "local" identifier as another. If you are lucky, your linker will
tell you of the crash - but if you have enabled "common" data (i.e., you
don't have the "-fno-common" flag) and have used the same identifier for
two different "local" objects without explicit initialisation, you are
going to have some serious and very hard to find bugs.
If someone asks you to write to such a coding convention, do your best
to refuse.
That can still be worked around, just put the declaration right before the
definition,
in the same source file.
I realize there are many arguments for and against file level static.
There are no /good/ arguments against file-level static in C, except
perhaps temporarily while debugging (it can be easier to view non-static
data in a debugger). Any time file-level static can be used, it
/should/ be used.
IMHO, of course.
mvh.,
David
- Jay
From: David Brown <da...@westcontrol.com>
Sent: Monday, January 22, 2018 8:32 AM
To: Jay K; gcc
Subject: Re: extern const initialized warns in C
On 21/01/18 08:12, Jay K wrote:
extern const int foo = 123;
Why does this warn?
This is a valid portable form, with the same meaning
across all compilers, and, importantly, portably
to C and C++.
I explicitly do not want to say:
const int foo = 123
because I want the code to be valid and have the same meaning
in C and C++ (modulo name mangling).
I end up with:
// Workaround gcc warning.
#ifdef __cplusplus
#define EXTERN_CONST extern const
#else
#define EXTERN_CONST const
#endif
EXTERN_CONST int foo = 123;
and having to explain it to people.
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45977>
45977 – "warning: 'i' initialized and declared 'extern ...
gcc.gnu.org
GCC Bugzilla – Bug 45977 "warning: 'i' initialized and declared 'extern'" could
use a separate warning flag controlling it Last modified: 2017-07-26 15:36:22 UTC
This suggests that gcc authors consider mixing "extern" and
initialization to be such bad style that the compiler warns by default.
But the "bug" is that there is no flag to turn off this warning.
(Ideally every warning should have a matching flag, even if the warning
is enabled by default.)
Usually you do not want to have "extern" and initialisation in the same
line - it indicates a questionable organisation of your sources which is
more likely to be error-prone than the standard idioms. (I say
"questionable", not necessarily wrong - but certainly I would question
it if I saw it in source code.)
Normally you want:
// file.h
// declaration, not definition
extern const int foo;
// file.c
#include <file.h>
// definition
const int foo = 123;
// otherfile.c
#include <file.h>
int usefoo(void) { return foo; }
The key advantages of this sort of setup are a cleaner separation
between declarations (which you need to /use/ things) and the
definitions (which should normally only exist once in the program -
certainly for C). The declarations and definitions only exist in one
place, and they are checked for consistency - there are no "extern"
declarations lying around in C files that might get out of step from
changes in the headers or other files with definitions.
To be consistent with this, and to work consistently with C and C++, I
have a strict policy that a C (or C++) file never contains declarations
without definitions (and initialisations as needed), with each
definition either also declared as "extern" in a matching header file,
or it is declared as "static".
This sort of arrangement is very common - though many people are lazy
about using "static". (In C++, you can also use anonymous namespaces,
but "static" works for consistency between C and C++.)
Still, gcc should have a flag to disable this warning if you have reason
to use "extern const int foo = 123;" - it is, after all, correctly
defined C code.
$ cat 1.c
extern const int foo = 123;
$ $HOME/gcc720/bin/gcc -c -S 1.c
1.c:1:18: warning: 'foo' initialized and declared 'extern'
extern const int foo = 123;
^~~
$ $HOME/gcc720/bin/gcc -c -S -xc++ -Wall -pedantic 1$ $HOME/gcc720/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/Users/jay/gcc720/bin/gcc
COLLECT_LTO_WRAPPER=/Users/jay/gcc720/libexec/gcc/x86_64-apple-darwin16.7.0/7.2.0/lto-wrapper
Target: x86_64-apple-darwin16.7.0
Configured with: ../gcc-7.2.0/configure -prefix=/Users/jay/gcc720 -disable-nls
-disable-bootstrap
Thread model: posix
gcc version 7.2.0 (GCC) $
Thank you,
- Jay
