Using macro CMSG_NXTHDR generates warnings with CLANG

[Jan Vorlicek <janvorli@microsoft.com>]

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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!

Jan

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