/*
   * Compatibility interface for userspace libc header coordination:
   *
   * Define compatibility macros that are used to control the inclusion or
   * exclusion of UAPI structures and definitions in coordination with another
   * userspace C library.
   *
   * This header is intended to solve the problem of UAPI definitions that
   * conflict with userspace definitions. If a UAPI header has such conflicting
   * definitions then the solution is as follows:
   *
   * * Synchronize the UAPI header and the libc headers so either one can be
   *   used and such that the ABI is preserved. If this is not possible then
   *   no simple compatibility interface exists (you need to write translating
   *   wrappers and rename things) and you can't use this interface.
   *
   * Then follow this process:
   *
   * (a) Include libc-compat.h in the UAPI header.
   *      e.g. #include <linux/libc-compat.h>
   *     This include must be as early as possible.
   *
   * (b) In libc-compat.h add enough code to detect that the comflicting
   *     userspace libc header has been included first.
   *
   * (c) If the userspace libc header has been included first define a set of
   *     guard macros of the form __UAPI_DEF_FOO and set their values to 1, else
   *     set their values to 0.
   *
   * (d) Back in the UAPI header with the conflicting definitions, guard the
   *     definitions with:
   *     #if __UAPI_DEF_FOO
   *       ...
   *     #endif
   *
   * This fixes the situation where the linux headers are included *after* the
   * libc headers. To fix the problem with the inclusion in the other order the
   * userspace libc headers must be fixed like this:
   *
   * * For all definitions that conflict with kernel definitions wrap those
   *   defines in the following:
   *   #if !__UAPI_DEF_FOO
   *     ...
   *   #endif
   *
   * This prevents the redefinition of a construct already defined by the kernel.
   */
  #ifndef _UAPI_LIBC_COMPAT_H
  #define _UAPI_LIBC_COMPAT_H
  
  /* We have included glibc headers... */
  #if defined(__GLIBC__)
  
  /* Coordinate with glibc netinet/in.h header. */
  #if defined(_NETINET_IN_H)
  
  /* GLIBC headers included first so don't define anything
   * that would already be defined. */
  #define __UAPI_DEF_IN6_ADDR		0
  /* The exception is the in6_addr macros which must be defined
   * if the glibc code didn't define them. This guard matches
   * the guard in glibc/inet/netinet/in.h which defines the
   * additional in6_addr macros e.g. s6_addr16, and s6_addr32. */
  #if defined(__USE_MISC) || defined (__USE_GNU)
  #define __UAPI_DEF_IN6_ADDR_ALT		0
  #else
  #define __UAPI_DEF_IN6_ADDR_ALT		1
  #endif
  #define __UAPI_DEF_SOCKADDR_IN6		0
  #define __UAPI_DEF_IPV6_MREQ		0
  #define __UAPI_DEF_IPPROTO_V6		0
  
  #else
  
  /* Linux headers included first, and we must define everything
   * we need. The expectation is that glibc will check the
   * __UAPI_DEF_* defines and adjust appropriately. */
  #define __UAPI_DEF_IN6_ADDR		1
  /* We unconditionally define the in6_addr macros and glibc must
   * coordinate. */
  #define __UAPI_DEF_IN6_ADDR_ALT		1
  #define __UAPI_DEF_SOCKADDR_IN6		1
  #define __UAPI_DEF_IPV6_MREQ		1
  #define __UAPI_DEF_IPPROTO_V6		1
  
  #endif /* _NETINET_IN_H */
  
  /* Definitions for xattr.h */
  #if defined(_SYS_XATTR_H)
  #define __UAPI_DEF_XATTR		0
  #else
  #define __UAPI_DEF_XATTR		1
  #endif
  
  /* If we did not see any headers from any supported C libraries,
   * or we are being included in the kernel, then define everything
   * that we need. */
  #else /* !defined(__GLIBC__) */
  
  /* Definitions for in6.h */
  #define __UAPI_DEF_IN6_ADDR		1
  #define __UAPI_DEF_IN6_ADDR_ALT		1
  #define __UAPI_DEF_SOCKADDR_IN6		1
  #define __UAPI_DEF_IPV6_MREQ		1
  #define __UAPI_DEF_IPPROTO_V6		1
  
  /* Definitions for xattr.h */
  #define __UAPI_DEF_XATTR		1
  
  #endif /* __GLIBC__ */
  
  #endif /* _UAPI_LIBC_COMPAT_H */