diff --git a/libc/fuzzing/math/RemQuoDiff.h b/libc/fuzzing/math/RemQuoDiff.h
index f37cf3ceebd4..b934bc885876 100644
--- a/libc/fuzzing/math/RemQuoDiff.h
+++ b/libc/fuzzing/math/RemQuoDiff.h
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_LIBC_FUZZING_MATH_REMQUO_H
+#define LLVM_LIBC_FUZZING_MATH_REMQUO_H
+
 #include "src/__support/FPUtil/FPBits.h"
 
 #include <math.h>
@@ -46,3 +49,5 @@ void RemQuoDiff(RemQuoFunc<T> func1, RemQuoFunc<T> func2, const uint8_t *data,
   if (bits1.uintval() != bits2.uintval())
     __builtin_trap();
 }
+
+#endif // LLVM_LIBC_FUZZING_MATH_REMQUO_H
diff --git a/libc/fuzzing/stdlib/CMakeLists.txt b/libc/fuzzing/stdlib/CMakeLists.txt
index 436125b6cd7a..0016f4f7ccbf 100644
--- a/libc/fuzzing/stdlib/CMakeLists.txt
+++ b/libc/fuzzing/stdlib/CMakeLists.txt
@@ -7,9 +7,9 @@ add_libc_fuzzer(
 )
 
 add_libc_fuzzer(
-  atof_fuzz
+  atof_differential_fuzz
   SRCS
-    atof_fuzz.cpp
+    atof_differential_fuzz.cpp
   HDRS
     StringParserOutputDiff.h
   DEPENDS
diff --git a/libc/fuzzing/stdlib/atof_fuzz.cpp b/libc/fuzzing/stdlib/atof_differential_fuzz.cpp
similarity index 100%
rename from libc/fuzzing/stdlib/atof_fuzz.cpp
rename to libc/fuzzing/stdlib/atof_differential_fuzz.cpp
diff --git a/libc/include/sys/mman.h.def b/libc/include/sys/mman.h.def
index 738aa3eaea3a..23435c768cb0 100644
--- a/libc/include/sys/mman.h.def
+++ b/libc/include/sys/mman.h.def
@@ -6,6 +6,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_LIBC_SYS_MMAN_H
+#define LLVM_LIBC_SYS_MMAN_H
+
 #include <__llvm-libc-common.h>
 
 %%public_api()
+
+#endif // LLVM_LIBC_SYS_MMAN_H
diff --git a/libc/include/sys/syscall.h.def b/libc/include/sys/syscall.h.def
index 5262f0785820..5cb6a20da77b 100644
--- a/libc/include/sys/syscall.h.def
+++ b/libc/include/sys/syscall.h.def
@@ -6,7 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_LIBC_SYS_SYSCALL_H
+#define LLVM_LIBC_SYS_SYSCALL_H
+
 <!> If syscall.h were a linux only file, then we do not need this indirection.
 <!> However, to keep the option of a non-linux OS requiring a syscall.h file,
 <!> with its own special syscall numbers, we use this indirection.
 %%include_file(${syscall_numbers})
+
+#endif // LLVM_LIBC_SYS_SYSCALL_H
diff --git a/libc/src/__support/architectures.h b/libc/src/__support/architectures.h
index 485faae47c7e..4975d229422f 100644
--- a/libc/src/__support/architectures.h
+++ b/libc/src/__support/architectures.h
@@ -1,4 +1,4 @@
-//===-- Compile time architecture detection -------------------------------===//
+//===-- Compile time architecture detection ---------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifndef LLVM_LIBC_SUPPORT_ARCHITECTURES_H
+#define LLVM_LIBC_SUPPORT_ARCHITECTURES_H
+
 #if defined(__pnacl__) || defined(__CLR_VER)
 #define LLVM_LIBC_ARCH_VM
 #endif
@@ -33,3 +36,5 @@
 #if (defined(LLVM_LIBC_ARCH_AARCH64) || defined(LLVM_LIBC_ARCH_ARM))
 #define LLVM_LIBC_ARCH_ANY_ARM
 #endif
+
+#endif // LLVM_LIBC_SUPPORT_ARCHITECTURES_H
diff --git a/libc/src/__support/common.h b/libc/src/__support/common.h
index 53a63fc2e917..17c155e3a8e4 100644
--- a/libc/src/__support/common.h
+++ b/libc/src/__support/common.h
@@ -1,4 +1,4 @@
-//===-- Common internal contructs -----------------------------------------===//
+//===-- Common internal contructs -------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
diff --git a/libc/src/__support/endian.h b/libc/src/__support/endian.h
index e1d52ca468d3..5f977aa466c5 100644
--- a/libc/src/__support/endian.h
+++ b/libc/src/__support/endian.h
@@ -1,4 +1,4 @@
-//===-- Endianness support ------------------------------------------------===//
+//===-- Endianness support --------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
diff --git a/libc/src/__support/sanitizer.h b/libc/src/__support/sanitizer.h
index 16fe6ecc0644..e7ae05f070b5 100644
--- a/libc/src/__support/sanitizer.h
+++ b/libc/src/__support/sanitizer.h
@@ -1,4 +1,4 @@
-//===-- Convenient sanitizer macros ---------------------------------------===//
+//===-- Convenient sanitizer macros -----------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.