diff --git a/Userland/DynamicLoader/CMakeLists.txt b/Userland/DynamicLoader/CMakeLists.txt
index e66d584983b..39001febc46 100644
--- a/Userland/DynamicLoader/CMakeLists.txt
+++ b/Userland/DynamicLoader/CMakeLists.txt
@@ -5,12 +5,17 @@ set(SOURCES
 
 add_library(DynamicLoader_CompileOptions INTERFACE)
 target_compile_definitions(DynamicLoader_CompileOptions INTERFACE NO_TLS _DYNAMIC_LOADER)
-target_compile_options(DynamicLoader_CompileOptions INTERFACE -fno-rtti -fpie -ffunction-sections -fdata-sections)
+target_compile_options(DynamicLoader_CompileOptions INTERFACE $<$<COMPILE_LANGUAGE:CXX>:-fno-rtti> -fpie -ffunction-sections -fdata-sections)
 target_link_options(DynamicLoader_CompileOptions INTERFACE -nolibc -nostdlib++ -nostartfiles -static-libgcc -fpie -Wl,--gc-sections)
 target_link_options(DynamicLoader_CompileOptions INTERFACE -fno-sanitize=undefined) # Sanitizer runtime is linked in manually
 
 add_executable(Loader.so ${SOURCES})
 
+# Mixing C and C++ results in linker errors with GCC > 16 and CMake < 4.0.
+# The issue is fixed upstream in https://gitlab.kitware.com/cmake/cmake/-/merge_requests/12027.
+# FIXME: Remove this workaround once we require a CMake version >= 4.1.
+list(REMOVE_ITEM CMAKE_C_IMPLICIT_LINK_LIBRARIES "gcc_s_asneeded")
+
 target_link_libraries(Loader.so PRIVATE DynamicLoader_CompileOptions DynamicLoader_LibELF DynamicLoader_LibCoreArgsParser)
 if (ENABLE_UNDEFINED_SANITIZER)
     target_link_libraries(Loader.so PRIVATE DynamicLoader_LibUBSanitizer)
diff --git a/Userland/Libraries/LibC/CMakeLists.txt b/Userland/Libraries/LibC/CMakeLists.txt
index bdbe7fc81c2..6b5c307a3da 100644
--- a/Userland/Libraries/LibC/CMakeLists.txt
+++ b/Userland/Libraries/LibC/CMakeLists.txt
@@ -3,7 +3,7 @@ set(SOURCES
     arch/${SERENITY_ARCH}/setjmp.S
     arpa/inet.cpp
     assert.cpp
-    ctype.cpp
+    ctype.c
     cxxabi.cpp
     dirent.cpp
     dlfcn.cpp
diff --git a/Userland/Libraries/LibC/ctype.c b/Userland/Libraries/LibC/ctype.c
new file mode 100644
index 00000000000..e1c7f12ada9
--- /dev/null
+++ b/Userland/Libraries/LibC/ctype.c
@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <ctype.h>
+
+char const _ctype_[1 + 256] = {
+    0,
+    _C, _C, _C, _C, _C, _C, _C, _C,
+    _C, _C | _S, _C | _S, _C | _S, _C | _S, _C | _S, _C, _C,
+    _C, _C, _C, _C, _C, _C, _C, _C,
+    _C, _C, _C, _C, _C, _C, _C, _C,
+    (char)(_S | _B), _P, _P, _P, _P, _P, _P, _P,
+    _P, _P, _P, _P, _P, _P, _P, _P,
+    _N, _N, _N, _N, _N, _N, _N, _N,
+    _N, _N, _P, _P, _P, _P, _P, _P,
+    _P, _U | _X, _U | _X, _U | _X, _U | _X, _U | _X, _U | _X, _U,
+    _U, _U, _U, _U, _U, _U, _U, _U,
+    _U, _U, _U, _U, _U, _U, _U, _U,
+    _U, _U, _U, _P, _P, _P, _P, _P,
+    _P, _L | _X, _L | _X, _L | _X, _L | _X, _L | _X, _L | _X, _L,
+    _L, _L, _L, _L, _L, _L, _L, _L,
+    _L, _L, _L, _L, _L, _L, _L, _L,
+    _L, _L, _L, _P, _P, _P, _P, _C
+};
+
+
+// "The following shall be declared as functions and may also be defined as macros."
+// This means that these functions need to be declared with external linkage.
+// For performance reasons, we want these functions to be inlinable so we provide
+// the definition in the header file. This however causes compatibility issues
+// between C and C++.
+// In C++, the compiler generates a symbol for inline functions at will. In the end,
+// multiple TUs might include one (in this case the linker takes care of only keeping
+// one) or if the function is always inlined, no symbols are emitted at all, which
+// conflicts with POSIX requirements.
+// In C, the linker isn't required to de-duplicate symbols emitted from inline
+// functions, so the compiler doesn't emit symbols for inline functions and it falls
+// back on the user to provide one if needed.
+// To solve this issue we use a dedicated C feature that anchors the function
+// definition in this exact TU.
+
+// This link provides useful information on the different meanings of inline:
+// https://stackoverflow.com/questions/216510/what-does-extern-inline-do
+
+__attribute__((gnu_inline)) inline int isalnum(int c);
+__attribute__((gnu_inline)) inline int isalpha(int c);
+__attribute__((gnu_inline)) inline int isascii(int c);
+__attribute__((gnu_inline)) inline int iscntrl(int c);
+__attribute__((gnu_inline)) inline int isdigit(int c);
+__attribute__((gnu_inline)) inline int isxdigit(int c);
+__attribute__((gnu_inline)) inline int isspace(int c);
+__attribute__((gnu_inline)) inline int ispunct(int c);
+__attribute__((gnu_inline)) inline int isprint(int c);
+__attribute__((gnu_inline)) inline int isgraph(int c);
+__attribute__((gnu_inline)) inline int islower(int c);
+__attribute__((gnu_inline)) inline int isupper(int c);
+__attribute__((gnu_inline)) inline int isblank(int c);
+__attribute__((gnu_inline)) inline int toascii(int c);
+__attribute__((gnu_inline)) inline int tolower(int c);
+__attribute__((gnu_inline)) inline int toupper(int c);
diff --git a/Userland/Libraries/LibC/ctype.cpp b/Userland/Libraries/LibC/ctype.cpp
deleted file mode 100644
index c866ed6aaf9..00000000000
--- a/Userland/Libraries/LibC/ctype.cpp
+++ /dev/null
@@ -1,126 +0,0 @@
-/*
- * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
- *
- * SPDX-License-Identifier: BSD-2-Clause
- */
-
-#include <ctype.h>
-
-extern "C" {
-
-char const _ctype_[1 + 256] = {
-    0,
-    _C, _C, _C, _C, _C, _C, _C, _C,
-    _C, _C | _S, _C | _S, _C | _S, _C | _S, _C | _S, _C, _C,
-    _C, _C, _C, _C, _C, _C, _C, _C,
-    _C, _C, _C, _C, _C, _C, _C, _C,
-    (char)(_S | _B), _P, _P, _P, _P, _P, _P, _P,
-    _P, _P, _P, _P, _P, _P, _P, _P,
-    _N, _N, _N, _N, _N, _N, _N, _N,
-    _N, _N, _P, _P, _P, _P, _P, _P,
-    _P, _U | _X, _U | _X, _U | _X, _U | _X, _U | _X, _U | _X, _U,
-    _U, _U, _U, _U, _U, _U, _U, _U,
-    _U, _U, _U, _U, _U, _U, _U, _U,
-    _U, _U, _U, _P, _P, _P, _P, _P,
-    _P, _L | _X, _L | _X, _L | _X, _L | _X, _L | _X, _L | _X, _L,
-    _L, _L, _L, _L, _L, _L, _L, _L,
-    _L, _L, _L, _L, _L, _L, _L, _L,
-    _L, _L, _L, _P, _P, _P, _P, _C
-};
-
-#undef isalnum
-int isalnum(int c)
-{
-    return __inline_isalnum(c);
-}
-
-#undef isalpha
-int isalpha(int c)
-{
-    return __inline_isalpha(c);
-}
-
-#undef iscntrl
-int iscntrl(int c)
-{
-    return __inline_iscntrl(c);
-}
-
-#undef isdigit
-int isdigit(int c)
-{
-    return __inline_isdigit(c);
-}
-
-#undef isxdigit
-int isxdigit(int c)
-{
-    return __inline_isxdigit(c);
-}
-
-#undef isspace
-int isspace(int c)
-{
-    return __inline_isspace(c);
-}
-
-#undef ispunct
-int ispunct(int c)
-{
-    return __inline_ispunct(c);
-}
-
-#undef isprint
-int isprint(int c)
-{
-    return __inline_isprint(c);
-}
-
-#undef isgraph
-int isgraph(int c)
-{
-    return __inline_isgraph(c);
-}
-
-#undef isupper
-int isupper(int c)
-{
-    return __inline_isupper(c);
-}
-
-#undef islower
-int islower(int c)
-{
-    return __inline_islower(c);
-}
-
-#undef isascii
-int isascii(int c)
-{
-    return __inline_isascii(c);
-}
-
-#undef isblank
-int isblank(int c)
-{
-    return __inline_isblank(c);
-}
-
-#undef toascii
-int toascii(int c)
-{
-    return __inline_toascii(c);
-}
-
-#undef tolower
-int tolower(int c)
-{
-    return __inline_tolower(c);
-}
-
-#undef toupper
-int toupper(int c)
-{
-    return __inline_toupper(c);
-}
-}
diff --git a/Userland/Libraries/LibC/ctype.h b/Userland/Libraries/LibC/ctype.h
index 1947f3416b9..d04cf9df365 100644
--- a/Userland/Libraries/LibC/ctype.h
+++ b/Userland/Libraries/LibC/ctype.h
@@ -31,129 +31,88 @@ __BEGIN_DECLS
  */
 extern char const _ctype_[1 + 256] __attribute__((visibility("default")));
 
-static inline int __inline_isalnum(int c)
+extern inline __attribute__((gnu_inline)) int isalnum(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_U | _L | _N);
 }
 
-static inline int __inline_isalpha(int c)
+extern inline __attribute__((gnu_inline)) int isalpha(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_U | _L);
 }
 
-static inline int __inline_isascii(int c)
+extern inline __attribute__((gnu_inline)) int isascii(int c)
 {
     return (unsigned)c <= 127;
 }
 
-static inline int __inline_iscntrl(int c)
+extern inline __attribute__((gnu_inline)) int iscntrl(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_C);
 }
 
-static inline int __inline_isdigit(int c)
+extern inline __attribute__((gnu_inline)) int isdigit(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_N);
 }
 
-static inline int __inline_isxdigit(int c)
+extern inline __attribute__((gnu_inline)) int isxdigit(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_N | _X);
 }
 
-static inline int __inline_isspace(int c)
+extern inline __attribute__((gnu_inline)) int isspace(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_S);
 }
 
-static inline int __inline_ispunct(int c)
+extern inline __attribute__((gnu_inline)) int ispunct(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_P);
 }
 
-static inline int __inline_isprint(int c)
+extern inline __attribute__((gnu_inline)) int isprint(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_P | _U | _L | _N | _B);
 }
 
-static inline int __inline_isgraph(int c)
+extern inline __attribute__((gnu_inline)) int isgraph(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_P | _U | _L | _N);
 }
 
-static inline int __inline_islower(int c)
+extern inline __attribute__((gnu_inline)) int islower(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_L);
 }
 
-static inline int __inline_isupper(int c)
+extern inline __attribute__((gnu_inline)) int isupper(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_U);
 }
 
-static inline int __inline_isblank(int c)
+extern inline __attribute__((gnu_inline)) int isblank(int c)
 {
     return _ctype_[(unsigned char)(c) + 1] & (_B) || (c == '\t');
 }
 
-static inline int __inline_toascii(int c)
+extern inline __attribute__((gnu_inline)) int toascii(int c)
 {
     return c & 127;
 }
 
-static inline int __inline_tolower(int c)
+extern inline __attribute__((gnu_inline)) int tolower(int c)
 {
     if (c >= 'A' && c <= 'Z')
         return c | 0x20;
     return c;
 }
 
-static inline int __inline_toupper(int c)
+extern inline __attribute__((gnu_inline)) int toupper(int c)
 {
     if (c >= 'a' && c <= 'z')
         return c & ~0x20;
     return c;
 }
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-int isalnum(int c);
-int isalpha(int c);
-int isascii(int c);
-int iscntrl(int c);
-int isdigit(int c);
-int isxdigit(int c);
-int isspace(int c);
-int ispunct(int c);
-int isprint(int c);
-int isgraph(int c);
-int islower(int c);
-int isupper(int c);
-int isblank(int c);
-int toascii(int c);
-int tolower(int c);
-int toupper(int c);
-
-#ifdef __cplusplus
-}
-#endif
-#define isalnum __inline_isalnum
-#define isalpha __inline_isalpha
-#define isascii __inline_isascii
-#define iscntrl __inline_iscntrl
-#define isdigit __inline_isdigit
-#define isxdigit __inline_isxdigit
-#define isspace __inline_isspace
-#define ispunct __inline_ispunct
-#define isprint __inline_isprint
-#define isgraph __inline_isgraph
-#define islower __inline_islower
-#define isupper __inline_isupper
-#define isblank __inline_isblank
-#define toascii __inline_toascii
-#define tolower __inline_tolower
-#define toupper __inline_toupper
-
 __END_DECLS
diff --git a/Userland/Libraries/LibC/wctype.cpp b/Userland/Libraries/LibC/wctype.cpp
index f026a34f1c3..b59b48496b7 100644
--- a/Userland/Libraries/LibC/wctype.cpp
+++ b/Userland/Libraries/LibC/wctype.cpp
@@ -33,62 +33,62 @@ extern "C" {
 
 int iswalnum(wint_t wc)
 {
-    return __inline_isalnum(wc);
+    return isalnum(wc);
 }
 
 int iswalpha(wint_t wc)
 {
-    return __inline_isalpha(wc);
+    return isalpha(wc);
 }
 
 int iswcntrl(wint_t wc)
 {
-    return __inline_iscntrl(wc);
+    return iscntrl(wc);
 }
 
 int iswdigit(wint_t wc)
 {
-    return __inline_isdigit(wc);
+    return isdigit(wc);
 }
 
 int iswxdigit(wint_t wc)
 {
-    return __inline_isxdigit(wc);
+    return isxdigit(wc);
 }
 
 int iswspace(wint_t wc)
 {
-    return __inline_isspace(wc);
+    return isspace(wc);
 }
 
 int iswpunct(wint_t wc)
 {
-    return __inline_ispunct(wc);
+    return ispunct(wc);
 }
 
 int iswprint(wint_t wc)
 {
-    return __inline_isprint(wc);
+    return isprint(wc);
 }
 
 int iswgraph(wint_t wc)
 {
-    return __inline_isgraph(wc);
+    return isgraph(wc);
 }
 
 int iswlower(wint_t wc)
 {
-    return __inline_islower(wc);
+    return islower(wc);
 }
 
 int iswupper(wint_t wc)
 {
-    return __inline_isupper(wc);
+    return isupper(wc);
 }
 
 int iswblank(wint_t wc)
 {
-    return __inline_isblank(wc);
+    return isblank(wc);
 }
 
 int iswctype(wint_t wc, wctype_t charclass)
@@ -178,12 +178,12 @@ wctype_t wctype(char const* property)
 
 wint_t towlower(wint_t wc)
 {
-    return __inline_tolower(wc);
+    return tolower(wc);
 }
 
 wint_t towupper(wint_t wc)
 {
-    return __inline_toupper(wc);
+    return toupper(wc);
 }
 
 wint_t towctrans(wint_t wc, wctrans_t desc)