/usr/include/linux/libc-compat.h is in linux-libc-dev 3.16.56-1+deb8u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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* 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 _LIBC_COMPAT_H
#define _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
#define __UAPI_DEF_IPV6_OPTIONS 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
#define __UAPI_DEF_IPV6_OPTIONS 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
#define __UAPI_DEF_IPV6_OPTIONS 1
/* Definitions for xattr.h */
#define __UAPI_DEF_XATTR 1
#endif /* __GLIBC__ */
#endif /* _LIBC_COMPAT_H */
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