Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Tue, 11 Oct 2016 15:38:38 +0000
From: Jan Vorlicek <>
To: "" <>
Subject: RE: Using macro CMSG_NXTHDR generates warnings with CLANG

My test was a c++ code :-). That's why the struct was not there.



-----Original Message-----
From: Szabolcs Nagy [] 
Sent: Tuesday, October 11, 2016 5:31 PM
Subject: Re: [musl] Using macro CMSG_NXTHDR generates warnings with CLANG

* Rich Felker <> [2016-10-11 11:09:01 -0400]:
> On Mon, Oct 10, 2016 at 10:09:38PM +0000, Jan Vorlicek wrote:
> > The testing source is below:
> > 
> > #include <sys/socket.h>
> > cmsghdr* GET_CMSG_NXTHDR(msghdr* mhdr, cmsghdr* cmsg);
> > 

this is invalid c code (you cannot leave 'struct' off).

> > cmsghdr* GET_CMSG_NXTHDR(msghdr* mhdr, cmsghdr* cmsg) {
> >     return CMSG_NXTHDR(mhdr, cmsg);
> > }
> > 
> > Would it be possible to fix it so that no warnings are generated? We 
> > are building our application with -Weverything and currently we need 
> > to disable these two warnings around the CMSG_NXTHDR macro 
> > invocation.
> > Thank you in advance for considering that!
> As these are system headers, the compiler should not be producing any 
> warnings from them. If it does that's a compiler bug. Are you perhaps 
> using an odd setup where musl's headers aren't in the default system 
> include path but instead passed in via -I rather than -isystem? If you 
> have a minimal test file I could see if the same warnings appear with 
> clang on Alpine Linux.

fwiw i see the warnings with clang -c -Wcast-align test.c

i assume they cannot easily tell if they should warn when a macro defined in a system header expanded with user arguments.

i think this warning is problematic: it warns about casting char*, but not about casting void* which has the exact same issue.

so if you have apis that uses char* instead of void* for generic pointers (e.g. for abi compat reasons), then you get loads of false positives (which is what happens here).

it can be worked around by adding a void* cast (but in my opinion that makes things worse: users will add more casts to get rid of the warning which has the opposite effect than what you would want).

Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.