[libc] Add unistd overlay

Author: RossComputerGuy

libc/hdr/unistd_macros.h

@@ -15,7 +15,7 @@
 
 #else // Overlay mode
 
-#include <unistd.h>
+#include "unistd_overlay.h"
 
 #endif // LLVM_LIBC_FULL_BUILD
 

libc/hdr/unistd_overlay.h

@@ -0,0 +1,69 @@
+//===-- Including unistd.h in overlay mode ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_HDR_UNISTD_OVERLAY_H
+#define LLVM_LIBC_HDR_UNISTD_OVERLAY_H
+
+#ifdef LIBC_FULL_BUILD
+#error "This header should only be included in overlay mode"
+#endif
+
+// Overlay mode
+
+// glibc <unistd.h> header might provide extern inline definitions for few
+// functions, causing external alias errors.  They are guarded by
+// `__USE_EXTERN_INLINES` macro.  We temporarily disable `__USE_EXTERN_INLINES`
+// macro by defining `__NO_INLINE__` before including <stdio.h>.
+// And the same with `__USE_FORTIFY_LEVEL`, which will be temporarily disabled
+// with `_FORTIFY_SOURCE`.
+
+#ifdef _FORTIFY_SOURCE
+#define LIBC_OLD_FORTIFY_SOURCE _FORTIFY_SOURCE
+#undef _FORTIFY_SOURCE
+#endif
+
+#ifdef __USE_EXTERN_INLINES
+#define LIBC_OLD_USE_EXTERN_INLINES
+#undef __USE_EXTERN_INLINES
+#endif
+
+#ifdef __USE_FORTIFY_LEVEL
+#define LIBC_OLD_USE_FORTIFY_LEVEL __USE_FORTIFY_LEVEL
+#undef __USE_FORTIFY_LEVEL
+#define __USE_FORTIFY_LEVEL 0
+#endif
+
+#ifndef __NO_INLINE__
+#define __NO_INLINE__ 1
+#define LIBC_SET_NO_INLINE
+#endif
+
+#include <unistd.h>
+
+#ifdef LIBC_OLD_FORTIFY_SOURCE
+#define _FORTIFY_SOURCE LIBC_OLD_FORTIFY_SOURCE
+#undef LIBC_OLD_FORTIFY_SOURCE
+#endif
+
+#ifdef LIBC_SET_NO_INLINE
+#undef __NO_INLINE__
+#undef LIBC_SET_NO_INLINE
+#endif
+
+#ifdef LIBC_OLD_USE_FORTIFY_LEVEL
+#undef __USE_FORTIFY_LEVEL
+#define __USE_FORTIFY_LEVEL LIBC_OLD_USE_FORTIFY_LEVEL
+#undef LIBC_OLD_USE_FORTIFY_LEVEL
+#endif
+
+#ifdef LIBC_OLD_USE_EXTERN_INLINES
+#define __USE_EXTERN_INLINES
+#undef LIBC_OLD_USE_EXTERN_INLINES
+#endif
+
+#endif // LLVM_LIBC_HDR_UNISTD_OVERLAY_H

libc/src/unistd/getcwd.h

@@ -10,7 +10,7 @@
 #define LLVM_LIBC_SRC_UNISTD_GETCWD_H
 
 #include "src/__support/macros/config.h"
-#include <unistd.h>
+#include "hdr/unistd_macros.h"
 
 namespace LIBC_NAMESPACE_DECL {
 

libc/src/unistd/pread.h

@@ -10,7 +10,7 @@
 #define LLVM_LIBC_SRC_UNISTD_PREAD_H
 
 #include "src/__support/macros/config.h"
-#include <unistd.h>
+#include "hdr/unistd_macros.h"
 
 namespace LIBC_NAMESPACE_DECL {
 

libc/src/unistd/read.h

@@ -10,7 +10,7 @@
 #define LLVM_LIBC_SRC_UNISTD_READ_H
 
 #include "src/__support/macros/config.h"
-#include <unistd.h>
+#include "hdr/unistd_macros.h"
 
 namespace LIBC_NAMESPACE_DECL {
 

libc/src/unistd/readlink.h

@@ -10,7 +10,7 @@
 #define LLVM_LIBC_SRC_UNISTD_READLINK_H
 
 #include "src/__support/macros/config.h"
-#include <unistd.h>
+#include "hdr/unistd_macros.h"
 
 namespace LIBC_NAMESPACE_DECL {
 

libc/src/unistd/readlinkat.h

@@ -10,7 +10,7 @@
 #define LLVM_LIBC_SRC_UNISTD_READLINKAT_H
 
 #include "src/__support/macros/config.h"
-#include <unistd.h>
+#include "hdr/unistd_macros.h"
 
 namespace LIBC_NAMESPACE_DECL {