Re: Using macro CMSG_NXTHDR generates warnings with CLANG

[Rich Felker <dalias@libc.org>]

On Mon, Oct 10, 2016 at 10:09:38PM +0000, Jan Vorlicek wrote:
> Trying to build a piece of code that uses CMSG_NXTHDR macro using
> CLANG (tested with CLANG 3.8) with all warnings enabled using
> -Weverything generates the following warnings:
> 
> clang++ -Weverything ./nettest.cpp -c -o nettest.o
> 
> ../nettest.cpp:5:12: warning: cast from 'unsigned char *' to 'struct cmsghdr *' increases required alignment from 1 to 4 [-Wcast-align]
>     return CMSG_NXTHDR(mhdr, cmsg);
>            ^~~~~~~~~~~~~~~~~~~~~~~
> /usr/include/sys/socket.h:270:8: note: expanded from macro 'CMSG_NXTHDR'
>         ? 0 : (struct cmsghdr *)__CMSG_NEXT(cmsg))
>               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ../nettest.cpp:5:12: warning: comparison of integers of different signs: 'unsigned long' and 'long' [-Wsign-compare]
>     return CMSG_NXTHDR(mhdr, cmsg);
>            ^~~~~~~~~~~~~~~~~~~~~~~
> /usr/include/sys/socket.h:269:44: note: expanded from macro 'CMSG_NXTHDR'
>         __CMSG_LEN(cmsg) + sizeof(struct cmsghdr) >= __MHDR_END(mhdr) - (unsigned char *)(cmsg) \
>         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 2 warnings generated.
> 
> The testing source is below:
> 
> #include <sys/socket.h>
> cmsghdr* GET_CMSG_NXTHDR(msghdr* mhdr, cmsghdr* cmsg);
> 
> 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.

Rich