User: Password:
|
|
Subscribe / Log in / New account

Object-oriented design patterns in the kernel, part 2

Object-oriented design patterns in the kernel, part 2

Posted Jun 8, 2011 11:06 UTC (Wed) by etienne (guest, #25256)
In reply to: Object-oriented design patterns in the kernel, part 2 by tialaramex
Parent article: Object-oriented design patterns in the kernel, part 2

C99 could have added a warning when a structure is created (and not a structure pointer allocated), so that this is catched:
$ cat tmp.c
struct str { unsigned len; char data[]; };
void fct1(struct str *ptr);
void fct2(struct str *initial) {
struct str copy = *initial;
fct1(&copy);
}
$ gcc -Wall -std=c99 -O2 -c tmp.c
$


(Log in to post comments)

Object-oriented design patterns in the kernel, part 2

Posted Jun 8, 2011 11:14 UTC (Wed) by cesarb (subscriber, #6266) [Link]

I think it is the compiler which adds warnings, not the standard. So, you could report this as a bug to the gcc developers and they could add a warning, even if the standard thinks this is valid.

That is, if the warning does not exist already. -Wall does not enable all warnings. Did you try -Wextra and check the manual for the warnings neither option enables?

Object-oriented design patterns in the kernel, part 2

Posted Jun 8, 2011 13:03 UTC (Wed) by tialaramex (subscriber, #21167) [Link]

The standard requires some diagnostics, GCC complies with the standard in this regard when in 'pedantic' mode (of course you must also tell GCC which standard you intended to comply with to get anything useful)

But yes, a useful compiler will definitely want to offer additional diagnostic information beyond that called out in the standard, and it may find that some diagnostics called for by the standard are just unhelpful, conflict with real world practice or obstruct a cool but non-standard feature they wish to offer, and so disable them at least by default.

The case outlined above doesn't strike me as worth being called out in the standard, but I can see that at least some similar cases could be usefully mentioned by the compiler, like unadorned occurrences of the assignment operator in a boolean context. You _might_ mean what you say, but you probably don't, and if you really do, you could say it more clearly for the benefit of some future maintenance programmer.

Object-oriented design patterns in the kernel, part 2

Posted Jun 9, 2011 15:34 UTC (Thu) by jwakely (guest, #60262) [Link]

The standard does *not* require any warnings. The diagnostics it requires are for violations of syntax rules or constraints, not for warning about questionable constructs.

The standard says something works or it doesn't, it never says "this is ok but the implementation should warn about it", so cesarb is quite right, C99 could not have added a warning, and requests for such a warning should go to compiler implementations not the standard.

Object-oriented design patterns in the kernel, part 2

Posted Jun 10, 2011 3:40 UTC (Fri) by jzbiciak (subscriber, #5246) [Link]

That's largely true. I did find at least one place (and there are likely others) where the standard suggests a warning. It doesn't mandate it though, in ยง6.4.4.2:

Recommended practice

The implementation should produce a diagnostic message if a hexadecimal constant cannot be represented exactly in its evaluation format; the implementation should then proceed with the translation of the program.

If "produc[ing] a diagnostic" and "proceed[ing] with the translation of the program" doesn't constitute a warning, I don't know what does.

Object-oriented design patterns in the kernel, part 2

Posted Jun 8, 2011 15:15 UTC (Wed) by etienne (guest, #25256) [Link]

For this GCC version:
$ gcc --version
gcc (Ubuntu 4.4.3-4ubuntu5) 4.4.3

I get no warnings:
$ gcc -Wall -Wextra -pedantic -Wvla -std=c99 -O2 -c tmp.c
$

But I never use this construct and I already have my own list of bugs for GCC in bugzillia.


Copyright © 2017, Eklektix, Inc.
Comments and public postings are copyrighted by their creators.
Linux is a registered trademark of Linus Torvalds