[dpdk-dev] Using _XOPEN_SOURCE macros may break builds on FreeBSD

["Smoczynski, MarcinX" <marcinx.smoczynski@intel.com>]

Hi.
One of my patches submitted this week is breaking build on BSD systems. I dug
deeper and found out that it's because I'm using IPPROTO_* macros from
<netinet/in.h> in a header (rte_ip.h) which is included in the driver which uses
_XOPEN_SOURCE definition in its Makefile/meson.build.

On Linux and glibc this is not a problem, because:
1. Those IPPROTO macros are included unconditionally and
2. We're using _GNU_SOURCE at top level anyway which gives us visibility of
   every feature set. According to "features.h" from glibc 2.18
   (used in Ubuntu 18.04):

   https://github.molgen.mpg.de/git-mirror/glibc/blob/release/2.18/master/include/features.h
     /* If _GNU_SOURCE was defined by the user, turn on all the other features.  */
     #ifdef _GNU_SOURCE
     # undef  _ISOC95_SOURCE
     # define _ISOC95_SOURCE    1
     # undef  _ISOC99_SOURCE
     # define _ISOC99_SOURCE    1
     # undef  _ISOC11_SOURCE
     # define _ISOC11_SOURCE    1
     # undef  _POSIX_SOURCE
     # define _POSIX_SOURCE     1
     # undef  _POSIX_C_SOURCE
     # define _POSIX_C_SOURCE   200809L
     # undef  _XOPEN_SOURCE
     # define _XOPEN_SOURCE     700
     # undef  _XOPEN_SOURCE_EXTENDED
     # define _XOPEN_SOURCE_EXTENDED 1
     # undef    _LARGEFILE64_SOURCE
     # define _LARGEFILE64_SOURCE    1
     # undef  _BSD_SOURCE
     # define _BSD_SOURCE 1
     # undef  _SVID_SOURCE
     # define _SVID_SOURCE 1
     # undef  _ATFILE_SOURCE
     # define _ATFILE_SOURCE    1
     #endif

BSD systems are a little bit more orthodox:
1. IPPROTOs are defined conditionally if __BSD_VISIBLE macro is defined. If I
   understand it correctly, they are not a part of any POSIX specification
   and that's why they are kept separated.
2. __BSD_VISIBLE is set to 0 if any of XOPEN_SOURCE or POSIX_C_SOURCE macros
   are defined. The reasoning here is: if you don't specify feature set you want
   to use you have it all, but when you do, you have exactly what you've
   explicitly asked for. Using XOPEN_SOURCE=700 means: give me POSIX 2k8 and
   XSI only. In other words using this macro alone is restricting portability;
   by default DPDK is building with full visibility.

   https://github.com/freebsd/freebsd/blob/release/12.0.0/sys/sys/cdefs.h
   Lines 680 to 765
     /* Deal with various X/Open Portability Guides and Single UNIX Spec. */
     #ifdef _XOPEN_SOURCE
     #if _XOPEN_SOURCE - 0 >= 700
     #define    __XSI_VISIBLE        700
     #undef _POSIX_C_SOURCE
     #define    _POSIX_C_SOURCE      200809
     #elif _XOPEN_SOURCE - 0 >= 600
     #define    __XSI_VISIBLE        600
     #undef _POSIX_C_SOURCE
     #define    _POSIX_C_SOURCE      200112
     #elif _XOPEN_SOURCE - 0 >= 500
     #define    __XSI_VISIBLE        500
     #undef _POSIX_C_SOURCE
     #define    _POSIX_C_SOURCE      199506
     #endif
     #endif

     /*
     * Deal with all versions of POSIX.  The ordering relative to the tests above is
     * important.
     */
     #if defined(_POSIX_SOURCE) && !defined(_POSIX_C_SOURCE)
     #define    _POSIX_C_SOURCE      198808
     #endif
     #ifdef _POSIX_C_SOURCE
     #if _POSIX_C_SOURCE >= 200809
     #define    __POSIX_VISIBLE      200809
     #define    __ISO_C_VISIBLE      1999
     #elif _POSIX_C_SOURCE >= 200112
     #define    __POSIX_VISIBLE      200112
     #define    __ISO_C_VISIBLE      1999
     #elif _POSIX_C_SOURCE >= 199506
     #define    __POSIX_VISIBLE      199506
     #define    __ISO_C_VISIBLE      1990
     #elif _POSIX_C_SOURCE >= 199309
     #define    __POSIX_VISIBLE      199309
     #define    __ISO_C_VISIBLE      1990
     #elif _POSIX_C_SOURCE >= 199209
     #define    __POSIX_VISIBLE      199209
     #define    __ISO_C_VISIBLE      1990
     #elif _POSIX_C_SOURCE >= 199009
     #define    __POSIX_VISIBLE      199009
     #define    __ISO_C_VISIBLE      1990
     #else
     #define    __POSIX_VISIBLE      198808
     #define    __ISO_C_VISIBLE      0
     #endif /* _POSIX_C_SOURCE */
     #else                <----- !!!
     /*-
     * Deal with _ANSI_SOURCE:
     * If it is defined, and no other compilation environment is explicitly
     * requested, then define our internal feature-test macros to zero.  This
     * makes no difference to the preprocessor (undefined symbols in preprocessing
     * expressions are defined to have value zero), but makes it more convenient for
     * a test program to print out the values.
     *
     * If a program mistakenly defines _ANSI_SOURCE and some other macro such as
     * _POSIX_C_SOURCE, we will assume that it wants the broader compilation
     * environment (and in fact we will never get here).
     */
     #if defined(_ANSI_SOURCE)  /* Hide almost everything. */
     #define    __POSIX_VISIBLE      0
     #define    __XSI_VISIBLE        0
     #define    __BSD_VISIBLE        0
     #define    __ISO_C_VISIBLE      1990
     #define    __EXT1_VISIBLE       0
     #elif defined(_C99_SOURCE) /* Localism to specify strict C99 env. */
     #define    __POSIX_VISIBLE      0
     #define    __XSI_VISIBLE        0
     #define    __BSD_VISIBLE        0
     #define    __ISO_C_VISIBLE      1999
     #define    __EXT1_VISIBLE       0
     #elif defined(_C11_SOURCE) /* Localism to specify strict C11 env. */
     #define    __POSIX_VISIBLE      0
     #define    __XSI_VISIBLE        0
     #define    __BSD_VISIBLE        0
     #define    __ISO_C_VISIBLE      2011
     #define    __EXT1_VISIBLE       0
     #else                /* Default environment: show everything. */

     By default DPDK falls here:

     #define    __POSIX_VISIBLE      200809
     #define    __XSI_VISIBLE        700
     #define    __BSD_VISIBLE        1
     #define    __ISO_C_VISIBLE      2011
     #define    __EXT1_VISIBLE       1
     #endif
     #endif

To summarize we have different visibility sets for Linux and BSD when using
XOPEN_SOURCE or POSIX_C_SOURCE explicitly. To overcome this situation we can
either remove problematic XOPEN macros from mk/meson rules (drivers/net/failsafe,
drivers/net/mlx4, drivers/net/mlx5) or add explicit -D__BSD_VISIBLE when
building for BSD. I think also that defining _GNU_SOURCE for BSD builds makes no
sense although it does not cause any problems.

I have checked that removing those problematic macros solves build problems on
FreeBSD12 and does not break on Ubuntu 18.04.

I'd appreciate your thoughts on this topic.

Regards, Marcin.