in6_addr in GCC 4 - c

Question

I see code like following : (legecy code in the project I am working on)
#if __GNUC__ > 3
.ipv6_addr = {.__in6_u = {.__u6_addr32 = {0, 0, 0, 0}}}
#else
.ipv6_addr = {.in6_u = {.u6_addr32 = {0, 0, 0, 0}}}
#endif
where "ipv6_addr" is in type of struct in6_addr. I don't understand why its member in6_u would change to "in6_u" if __GNUC > 3.
My question is: why / when GCC version could impact the name of the field in struct in6_addr ?
Thanks.
Update: my host system has GCC 4.1.2, but the in6_addr was defined as :
in /usr/include/netinet/in.h
/* IPv6 address */
struct in6_addr
{
union
{
uint8_t u6_addr8[16];
uint16_t u6_addr16[8];
uint32_t u6_addr32[4];
} in6_u;
#define s6_addr in6_u.u6_addr8
#define s6_addr16 in6_u.u6_addr16
#define s6_addr32 in6_u.u6_addr32
};
And gcc version is:
$/usr/bin/gcc -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-libgcj-multifile --enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --disable-plugin --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic --host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.2 20080704 (Red Hat 4.1.2-50)

Answer 1

The structure definition of concern here, struct in6_addr, comes from the C runtime library (in <netinet/in.h>). The C runtime library is usually tightly coupled with the operating system in use, but not the C compiler in use. This is especially true for the C compilers that define __GNUC__ to any value (there are at least three of these: GCC, clang, and icc); these compilers are designed to be usable with many different OSes and runtimes. Therefore, in principle, testing __GNUC__ does not tell you anything useful about structure definitions that come from the runtime.
I suspect that the authors of this "legacy code" tested on two different Linux distributions, noted an accidental correlation between the value of __GNUC__ and the contents of <netinet/in.h>, and didn't bother looking for a more correct way to make their code compile.
You should replace the entire conditional with this:
.ipv6_addr = IN6ADDR_ANY_INIT;
The macro IN6ADDR_ANY_INIT is required by the relevant standard (POSIX.1-2008 spec for <netinet/in.h>) to be usable as an initializer for a variable of type in6_addr, setting that variable to the IPv6 wildcard address, which is all-bits-zero. So it will have the same effect without requiring any #ifdefs at all.
To illustrate that #if __GNUC__ > 3 is the wrong test to apply here, here are three different definitions of struct in6_addr, all taken from systems where you might reasonably encounter both __GNUC_==3 and __GNUC__==4 (the 3.x series is getting a little old nowadays, but I still run into it from time to time).
GNU libc 2.17
struct in6_addr
{
union
{
uint8_t __u6_addr8[16];
#if defined __USE_MISC || defined __USE_GNU
uint16_t __u6_addr16[8];
uint32_t __u6_addr32[4];
#endif
} __in6_u;
};
NetBSD 6.1
struct in6_addr {
union {
__uint8_t __u6_addr8[16];
__uint16_t __u6_addr16[8];
uint32_t __u6_addr32[4];
} __u6_addr; /* 128-bit IP6 address */
};
Windows 7 (official SDK; perversely does not provide netinet/in.h):
struct in6_addr {
union {
UCHAR Byte[16];
USHORT Word[8];
} u;
};