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| From | Kaz Kylheku <kaz@kylheku.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: John McCarthy died today. |
| Date | 2011-11-14 18:30 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <20111114101818.819@kylheku.com> (permalink) |
| References | (7 earlier) <j9o5i9$l9i$1@dont-email.me> <20111113084111.100@kylheku.com> <lnobwf4p2d.fsf@nuthaus.mib.org> <20111113191456.538@kylheku.com> <slrn3vfsjc1m4l.34h.ike@sverige.freeshell.org> |
On 2011-11-14, Ike Naar <ike@sverige.freeshell.org> wrote:
> On 2011-11-14, Kaz Kylheku <kaz@kylheku.com> wrote:
>> One example would be that I can't build the code any more with C++ to check
>> that all external names are meeting type-safe linkage and the one-definition
>> rule.
>>
>> In C, I can have "extern int x;" in in one module, and "struct foo x = { 0 };"
>> in another. It will link.
>>
>> If you change a C program such that a function has different parameters, but
>> due to some broken dependencies in your build system, some dependent modules
>> are not recompiled, you will get a broken link. Of course, it goes away
>> if you do a complete rebuild, but you could waste time chasing a bug
>> which isn't there.
>>
>> This is fixed in C++: it will catch your broken build system. Type
>> safe linkage won't let an int (int, char *) call go to an
>> int (int, char *, char *) function.
>
> Using a C++ compiler (GCC 4.1.2), the following:
>
> /* a.cxx */
> extern int x;
> int main() { return x; }
>
> /* b.cxx */
> struct foo { double d; } x;
>
> compiles/links (c++ -ansi -pedantic -Wall a.cxx b.cxx)
> and runs without any complaints.
Ah yes, g++ is broken in this regard.
But the ODR still kicks in.
In a program where there are duplicate uses of an external name like this,
there are likely to be multiple definitions. E.g. two modules in some large
program both happen to use a global variable x.
Perhaps two developers did parallel commits in different modules, both
introducing it, and you, the build master, pull both changes from the repo.
$ cat a.cc
int x;
$ cat b.cc
struct foo { int a; } x;
int main()
{
}
$ g++ a.cc b.cc
/tmp/ccJExOCi.o:(.bss+0x0): multiple definition of `x'
/tmp/ccbKi3U5.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status
So at least there is that. Of course if one module, by mistake, neglects to
define its global, then the ball gets by the goalkeeper, so to peak.
If these changes are made separately (e.g. two developers in separate
sandboxes) the one with the missing definition will hopefully catch it, since
it doesn't link.
But change to C:
$ cp a.cc a.c
$ cp b.cc b.c
$ gcc a.c b.c
Multiple defs; not a peep.
If "perfect" is not available, I will still take "better".
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John McCarthy died today. Kaz Kylheku <kaz@kylheku.com> - 2011-10-25 00:44 +0000
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