diff --git a/libc/include/bits/fortify/fcntl.h b/libc/include/bits/fortify/fcntl.h
index e7f2c82..75cd4f2 100644
--- a/libc/include/bits/fortify/fcntl.h
+++ b/libc/include/bits/fortify/fcntl.h
@@ -44,6 +44,7 @@
 #define __open_useless_modes_warning "has superfluous mode bits; missing O_CREAT?"
 /* O_TMPFILE shares bits with O_DIRECTORY. */
 #define __open_modes_useful(flags) (((flags) & O_CREAT) || ((flags) & O_TMPFILE) == O_TMPFILE)
+#if defined(__clang__)
 
 #if __ANDROID_API__ >= __ANDROID_API_J_MR1__
 __BIONIC_ERROR_FUNCTION_VISIBILITY
@@ -92,6 +93,52 @@
 }
 #endif /* __ANDROID_API__ >= __ANDROID_API_J_MR1__ */
 
+#else /* defined(__clang__) */
+__errordecl(__creat_missing_mode, __open_too_few_args_error);
+__errordecl(__creat_too_many_args, __open_too_many_args_error);
+
+#if __ANDROID_API__ >= __ANDROID_API_J_MR1__
+__BIONIC_FORTIFY_VARIADIC
+int open(const char* pathname, int flags, ...) {
+    if (__builtin_constant_p(flags)) {
+        if (__open_modes_useful(flags) && __builtin_va_arg_pack_len() == 0) {
+            __creat_missing_mode();  /* Compile time error. */
+        }
+    }
+
+    if (__builtin_va_arg_pack_len() > 1) {
+        __creat_too_many_args();  /* Compile time error. */
+    }
+
+    if ((__builtin_va_arg_pack_len() == 0) && !__builtin_constant_p(flags)) {
+        return __open_2(pathname, flags);
+    }
+
+    return __open_real(pathname, flags, __builtin_va_arg_pack());
+}
+
+__BIONIC_FORTIFY_VARIADIC
+int openat(int dirfd, const char* pathname, int flags, ...) {
+    if (__builtin_constant_p(flags)) {
+        if (__open_modes_useful(flags) && __builtin_va_arg_pack_len() == 0) {
+            __creat_missing_mode();  /* Compile time error. */
+        }
+    }
+
+    if (__builtin_va_arg_pack_len() > 1) {
+        __creat_too_many_args();  /* Compile time error. */
+    }
+
+    if ((__builtin_va_arg_pack_len() == 0) && !__builtin_constant_p(flags)) {
+        return __openat_2(dirfd, pathname, flags);
+    }
+
+    return __openat_real(dirfd, pathname, flags, __builtin_va_arg_pack());
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_J_MR1__ */
+
+#endif /* defined(__clang__) */
+
 #undef __open_too_many_args_error
 #undef __open_too_few_args_error
 #undef __open_useless_modes_warning
diff --git a/libc/include/bits/fortify/poll.h b/libc/include/bits/fortify/poll.h
index 718ee96..c3ae31d 100644
--- a/libc/include/bits/fortify/poll.h
+++ b/libc/include/bits/fortify/poll.h
@@ -36,7 +36,7 @@
 
 #if defined(__BIONIC_FORTIFY)
 #if __ANDROID_API__ >= __ANDROID_API_M__
-
+#if defined(__clang__)
 __BIONIC_FORTIFY_INLINE
 int poll(struct pollfd* const fds __pass_object_size, nfds_t fd_count, int timeout)
     __overloadable
@@ -81,5 +81,39 @@
 }
 #endif
 
+#else /* !defined(__clang__) */
+int __poll_real(struct pollfd*, nfds_t, int) __RENAME(poll);
+__errordecl(__poll_too_small_error, "poll: pollfd array smaller than fd count");
+
+int __ppoll_real(struct pollfd*, nfds_t, const struct timespec*, const sigset_t*) __RENAME(ppoll)
+  __INTRODUCED_IN(21);
+__errordecl(__ppoll_too_small_error, "ppoll: pollfd array smaller than fd count");
+
+__BIONIC_FORTIFY_INLINE
+int poll(struct pollfd* fds, nfds_t fd_count, int timeout) {
+  if (__bos(fds) != __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+    if (!__builtin_constant_p(fd_count)) {
+      return __poll_chk(fds, fd_count, timeout, __bos(fds));
+    } else if (__bos(fds) / sizeof(*fds) < fd_count) {
+      __poll_too_small_error();
+    }
+  }
+  return __poll_real(fds, fd_count, timeout);
+}
+
+__BIONIC_FORTIFY_INLINE
+int ppoll(struct pollfd* fds, nfds_t fd_count, const struct timespec* timeout,
+          const sigset_t* mask) {
+  if (__bos(fds) != __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+    if (!__builtin_constant_p(fd_count)) {
+      return __ppoll_chk(fds, fd_count, timeout, mask, __bos(fds));
+    } else if (__bos(fds) / sizeof(*fds) < fd_count) {
+      __ppoll_too_small_error();
+    }
+  }
+  return __ppoll_real(fds, fd_count, timeout, mask);
+}
+
+#endif /* defined(__clang__) */
 #endif /* __ANDROID_API__ >= __ANDROID_API_M__ */
 #endif /* defined(__BIONIC_FORTIFY) */
diff --git a/libc/include/bits/fortify/socket.h b/libc/include/bits/fortify/socket.h
index 3d070c5..3e610d6 100644
--- a/libc/include/bits/fortify/socket.h
+++ b/libc/include/bits/fortify/socket.h
@@ -39,7 +39,7 @@
 
 #define __recvfrom_bad_size "'recvfrom' called with size bigger than buffer"
 #define __sendto_bad_size "'sendto' called with size bigger than buffer"
-
+#if defined(__clang__)
 #if __ANDROID_API__ >= __ANDROID_API_N__
 __BIONIC_FORTIFY_INLINE
 ssize_t recvfrom(int fd, void* const buf __pass_object_size0, size_t len, int flags, struct sockaddr* src_addr, socklen_t* addr_len)
@@ -86,6 +86,69 @@
   return sendto(socket, buf, len, flags, NULL, 0);
 }
 
+#else /* defined(__clang__) */
+ssize_t __recvfrom_real(int, void*, size_t, int, struct sockaddr*, socklen_t*) __RENAME(recvfrom);
+__errordecl(__recvfrom_error, __recvfrom_bad_size);
+
+extern ssize_t __sendto_real(int, const void*, size_t, int, const struct sockaddr*, socklen_t)
+        __RENAME(sendto);
+__errordecl(__sendto_error, __sendto_bad_size);
+
+#if __ANDROID_API__ >= __ANDROID_API_N__
+__BIONIC_FORTIFY_INLINE
+ssize_t recvfrom(int fd, void* buf, size_t len, int flags,
+                 struct sockaddr* src_addr, socklen_t* addr_len) {
+  size_t bos = __bos0(buf);
+
+  if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+    return __recvfrom_real(fd, buf, len, flags, src_addr, addr_len);
+  }
+
+  if (__builtin_constant_p(len) && (len <= bos)) {
+    return __recvfrom_real(fd, buf, len, flags, src_addr, addr_len);
+  }
+
+  if (__builtin_constant_p(len) && (len > bos)) {
+    __recvfrom_error();
+  }
+
+  return __recvfrom_chk(fd, buf, len, bos, flags, src_addr, addr_len);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_N__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_N_MR1__
+__BIONIC_FORTIFY_INLINE
+ssize_t sendto(int fd, const void* buf, size_t len, int flags,
+               const struct sockaddr* dest_addr, socklen_t addr_len) {
+  size_t bos = __bos0(buf);
+
+  if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+    return __sendto_real(fd, buf, len, flags, dest_addr, addr_len);
+  }
+
+  if (__builtin_constant_p(len) && (len <= bos)) {
+    return __sendto_real(fd, buf, len, flags, dest_addr, addr_len);
+  }
+
+  if (__builtin_constant_p(len) && (len > bos)) {
+    __sendto_error();
+  }
+
+  return __sendto_chk(fd, buf, len, bos, flags, dest_addr, addr_len);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_N_MR1__ */
+
+__BIONIC_FORTIFY_INLINE
+ssize_t recv(int socket, void* buf, size_t len, int flags) {
+  return recvfrom(socket, buf, len, flags, NULL, 0);
+}
+
+__BIONIC_FORTIFY_INLINE
+ssize_t send(int socket, const void* buf, size_t len, int flags) {
+  return sendto(socket, buf, len, flags, NULL, 0);
+}
+#endif /* defined(__clang__) */
+
 #undef __recvfrom_bad_size
 #undef __sendto_bad_size
 #endif /* __BIONIC_FORTIFY */
diff --git a/libc/include/bits/fortify/stat.h b/libc/include/bits/fortify/stat.h
index b248aca..c168c38 100644
--- a/libc/include/bits/fortify/stat.h
+++ b/libc/include/bits/fortify/stat.h
@@ -35,6 +35,8 @@
 #if defined(__BIONIC_FORTIFY)
 #define __umask_invalid_mode_str "'umask' called with invalid mode"
 
+#if defined(__clang__)
+
 #if __ANDROID_API__ >= __ANDROID_API_J_MR2__
 /* Abuse enable_if to make this an overload of umask. */
 __BIONIC_FORTIFY_INLINE
@@ -46,6 +48,24 @@
 }
 #endif /* __ANDROID_API__ >= __ANDROID_API_J_MR2__ */
 
+#else /* defined(__clang__) */
+__errordecl(__umask_invalid_mode, __umask_invalid_mode_str);
+extern mode_t __umask_real(mode_t) __RENAME(umask);
+
+#if __ANDROID_API__ >= __ANDROID_API_J_MR2__
+__BIONIC_FORTIFY_INLINE
+mode_t umask(mode_t mode) {
+  if (__builtin_constant_p(mode)) {
+    if ((mode & 0777) != mode) {
+      __umask_invalid_mode();
+    }
+    return __umask_real(mode);
+  }
+  return __umask_chk(mode);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_J_MR2__ */
+
+#endif /* defined(__clang__) */
 #undef __umask_invalid_mode_str
 
 #endif /* defined(__BIONIC_FORTIFY) */
diff --git a/libc/include/bits/fortify/stdio.h b/libc/include/bits/fortify/stdio.h
index 0b5700a..6a6b433 100644
--- a/libc/include/bits/fortify/stdio.h
+++ b/libc/include/bits/fortify/stdio.h
@@ -49,6 +49,7 @@
 }
 #endif /* __ANDROID_API__ >= __ANDROID_API_J_MR1__ */
 
+#if defined(__clang__)
 #if __ANDROID_API__ >= __ANDROID_API_J_MR1__
 /*
  * Simple case: `format` can't have format specifiers, so we can just compare
@@ -140,4 +141,114 @@
 }
 #endif /* __ANDROID_API__ >= __ANDROID_API_J_MR1__ */
 
+#else /* defined(__clang__) */
+
+size_t __fread_real(void*, size_t, size_t, FILE*) __RENAME(fread);
+__errordecl(__fread_too_big_error, "fread called with size * count bigger than buffer");
+__errordecl(__fread_overflow, "fread called with overflowing size * count");
+
+char* __fgets_real(char*, int, FILE*) __RENAME(fgets);
+__errordecl(__fgets_too_big_error, "fgets called with size bigger than buffer");
+__errordecl(__fgets_too_small_error, "fgets called with size less than zero");
+
+size_t __fwrite_real(const void*, size_t, size_t, FILE*) __RENAME(fwrite);
+__errordecl(__fwrite_too_big_error, "fwrite called with size * count bigger than buffer");
+__errordecl(__fwrite_overflow, "fwrite called with overflowing size * count");
+
+
+#if __ANDROID_API__ >= __ANDROID_API_J_MR1__
+__BIONIC_FORTIFY_VARIADIC __printflike(3, 4)
+int snprintf(char* dest, size_t size, const char* format, ...) {
+    return __builtin___snprintf_chk(dest, size, 0, __bos(dest), format, __builtin_va_arg_pack());
+}
+
+__BIONIC_FORTIFY_VARIADIC __printflike(2, 3)
+int sprintf(char* dest, const char* format, ...) {
+    return __builtin___sprintf_chk(dest, 0, __bos(dest), format, __builtin_va_arg_pack());
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_J_MR1__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_N__
+__BIONIC_FORTIFY_INLINE
+size_t fread(void* buf, size_t size, size_t count, FILE* stream) {
+    size_t bos = __bos0(buf);
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __fread_real(buf, size, count, stream);
+    }
+
+    if (__builtin_constant_p(size) && __builtin_constant_p(count)) {
+        size_t total;
+        if (__size_mul_overflow(size, count, &total)) {
+            __fread_overflow();
+        }
+
+        if (total > bos) {
+            __fread_too_big_error();
+        }
+
+        return __fread_real(buf, size, count, stream);
+    }
+
+    return __fread_chk(buf, size, count, stream, bos);
+}
+
+__BIONIC_FORTIFY_INLINE
+size_t fwrite(const void* buf, size_t size, size_t count, FILE* stream) {
+    size_t bos = __bos0(buf);
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __fwrite_real(buf, size, count, stream);
+    }
+
+    if (__builtin_constant_p(size) && __builtin_constant_p(count)) {
+        size_t total;
+        if (__size_mul_overflow(size, count, &total)) {
+            __fwrite_overflow();
+        }
+
+        if (total > bos) {
+            __fwrite_too_big_error();
+        }
+
+        return __fwrite_real(buf, size, count, stream);
+    }
+
+    return __fwrite_chk(buf, size, count, stream, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_N__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_J_MR1__
+__BIONIC_FORTIFY_INLINE
+char *fgets(char* dest, int size, FILE* stream) {
+    size_t bos = __bos(dest);
+
+    // Compiler can prove, at compile time, that the passed in size
+    // is always negative. Force a compiler error.
+    if (__builtin_constant_p(size) && (size < 0)) {
+        __fgets_too_small_error();
+    }
+
+    // Compiler doesn't know destination size. Don't call __fgets_chk
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __fgets_real(dest, size, stream);
+    }
+
+    // Compiler can prove, at compile time, that the passed in size
+    // is always <= the actual object size. Don't call __fgets_chk
+    if (__builtin_constant_p(size) && (size <= (int) bos)) {
+        return __fgets_real(dest, size, stream);
+    }
+
+    // Compiler can prove, at compile time, that the passed in size
+    // is always > the actual object size. Force a compiler error.
+    if (__builtin_constant_p(size) && (size > (int) bos)) {
+        __fgets_too_big_error();
+    }
+
+    return __fgets_chk(dest, size, stream, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_J_MR1__ */
+
+#endif /* defined(__clang__) */
 #endif /* defined(__BIONIC_FORTIFY) */
diff --git a/libc/include/bits/fortify/stdlib.h b/libc/include/bits/fortify/stdlib.h
index d47c0b0..8f3b02c 100644
--- a/libc/include/bits/fortify/stdlib.h
+++ b/libc/include/bits/fortify/stdlib.h
@@ -37,12 +37,31 @@
 /* PATH_MAX is unavailable without polluting the namespace, but it's always 4096 on Linux */
 #define __PATH_MAX 4096
 
+#if defined(__clang__)
 char* realpath(const char* path, char* resolved)
         __clang_error_if(__bos(resolved) != __BIONIC_FORTIFY_UNKNOWN_SIZE &&
                          __bos(resolved) < __PATH_MAX, __realpath_buf_too_small_str)
         __clang_error_if(!path, "'realpath': NULL path is never correct; flipped arguments?");
 /* No need for a definition; the only issues we can catch are at compile-time. */
 
+#else /* defined(__clang__) */
+
+char* __realpath_real(const char*, char*) __RENAME(realpath);
+__errordecl(__realpath_size_error, __realpath_buf_too_small_str);
+
+__BIONIC_FORTIFY_INLINE
+char* realpath(const char* path, char* resolved) {
+    size_t bos = __bos(resolved);
+
+    if (bos != __BIONIC_FORTIFY_UNKNOWN_SIZE && bos < __PATH_MAX) {
+        __realpath_size_error();
+    }
+
+    return __realpath_real(path, resolved);
+}
+
+#endif /* defined(__clang__) */
+
 #undef __PATH_MAX
 #undef __realpath_buf_too_small_str
 #endif /* defined(__BIONIC_FORTIFY) */
diff --git a/libc/include/bits/fortify/string.h b/libc/include/bits/fortify/string.h
index 14bb133..f994e3e 100644
--- a/libc/include/bits/fortify/string.h
+++ b/libc/include/bits/fortify/string.h
@@ -40,6 +40,8 @@
 #if defined(__BIONIC_FORTIFY)
 extern void* __memrchr_real(const void*, int, size_t) __RENAME(memrchr);
 
+// These can share their implementation between gcc and clang with minimal
+// trickery...
 #if __ANDROID_API__ >= __ANDROID_API_J_MR1__
 __BIONIC_FORTIFY_INLINE
 void* memcpy(void* const dst __pass_object_size0, const void* src, size_t copy_amount)
@@ -100,6 +102,9 @@
 }
 #endif /* __ANDROID_API__ >= __ANDROID_API_J_MR1__ */
 
+
+#if defined(__clang__)
+
 #if __ANDROID_API__ >= __ANDROID_API_M__
 __BIONIC_FORTIFY_INLINE
 void* memchr(const void* const s __pass_object_size, int c, size_t n) __overloadable {
@@ -226,6 +231,192 @@
 }
 #endif /* __ANDROID_API__ >= __ANDROID_API_J_MR2__ */
 
+#else // defined(__clang__)
+extern char* __strncpy_real(char*, const char*, size_t) __RENAME(strncpy);
+extern size_t __strlcpy_real(char*, const char*, size_t)
+    __RENAME(strlcpy);
+extern size_t __strlcat_real(char*, const char*, size_t)
+    __RENAME(strlcat);
+
+__errordecl(__memchr_buf_size_error, "memchr called with size bigger than buffer");
+__errordecl(__memrchr_buf_size_error, "memrchr called with size bigger than buffer");
+
+#if __ANDROID_API__ >= __ANDROID_API_M__
+__BIONIC_FORTIFY_INLINE
+void* memchr(const void* s __pass_object_size, int c, size_t n) {
+    size_t bos = __bos(s);
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __builtin_memchr(s, c, n);
+    }
+
+    if (__builtin_constant_p(n) && (n > bos)) {
+        __memchr_buf_size_error();
+    }
+
+    if (__builtin_constant_p(n) && (n <= bos)) {
+        return __builtin_memchr(s, c, n);
+    }
+
+    return __memchr_chk(s, c, n, bos);
+}
+
+__BIONIC_FORTIFY_INLINE
+void* __memrchr_fortify(const void* s, int c, size_t n) {
+    size_t bos = __bos(s);
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __memrchr_real(s, c, n);
+    }
+
+    if (__builtin_constant_p(n) && (n > bos)) {
+        __memrchr_buf_size_error();
+    }
+
+    if (__builtin_constant_p(n) && (n <= bos)) {
+        return __memrchr_real(s, c, n);
+    }
+
+    return __memrchr_chk(s, c, n, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_M__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_L__
+__BIONIC_FORTIFY_INLINE
+char* stpncpy(char* dst, const char* src, size_t n) {
+    size_t bos_dst = __bos(dst);
+    size_t bos_src = __bos(src);
+
+    if (bos_src == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __builtin___stpncpy_chk(dst, src, n, bos_dst);
+    }
+
+    if (__builtin_constant_p(n) && (n <= bos_src)) {
+        return __builtin___stpncpy_chk(dst, src, n, bos_dst);
+    }
+
+    size_t slen = __builtin_strlen(src);
+    if (__builtin_constant_p(slen)) {
+        return __builtin___stpncpy_chk(dst, src, n, bos_dst);
+    }
+
+    return __stpncpy_chk2(dst, src, n, bos_dst, bos_src);
+}
+
+__BIONIC_FORTIFY_INLINE
+char* strncpy(char* dst, const char* src, size_t n) {
+    size_t bos_dst = __bos(dst);
+    size_t bos_src = __bos(src);
+
+    if (bos_src == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __strncpy_real(dst, src, n);
+    }
+
+    if (__builtin_constant_p(n) && (n <= bos_src)) {
+        return __builtin___strncpy_chk(dst, src, n, bos_dst);
+    }
+
+    size_t slen = __builtin_strlen(src);
+    if (__builtin_constant_p(slen)) {
+        return __builtin___strncpy_chk(dst, src, n, bos_dst);
+    }
+
+    return __strncpy_chk2(dst, src, n, bos_dst, bos_src);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_L__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_J_MR1__
+__BIONIC_FORTIFY_INLINE
+size_t strlcpy(char* dst __pass_object_size, const char* src, size_t size) {
+    size_t bos = __bos(dst);
+
+    // Compiler doesn't know destination size. Don't call __strlcpy_chk
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __strlcpy_real(dst, src, size);
+    }
+
+    // Compiler can prove, at compile time, that the passed in size
+    // is always <= the actual object size. Don't call __strlcpy_chk
+    if (__builtin_constant_p(size) && (size <= bos)) {
+        return __strlcpy_real(dst, src, size);
+    }
+
+    return __strlcpy_chk(dst, src, size, bos);
+}
+
+__BIONIC_FORTIFY_INLINE
+size_t strlcat(char* dst, const char* src, size_t size) {
+    size_t bos = __bos(dst);
+
+    // Compiler doesn't know destination size. Don't call __strlcat_chk
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __strlcat_real(dst, src, size);
+    }
+
+    // Compiler can prove, at compile time, that the passed in size
+    // is always <= the actual object size. Don't call __strlcat_chk
+    if (__builtin_constant_p(size) && (size <= bos)) {
+        return __strlcat_real(dst, src, size);
+    }
+
+    return __strlcat_chk(dst, src, size, bos);
+}
+
+__BIONIC_FORTIFY_INLINE
+size_t strlen(const char* s) __overloadable {
+    size_t bos = __bos(s);
+
+    // Compiler doesn't know destination size. Don't call __strlen_chk
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __builtin_strlen(s);
+    }
+
+    size_t slen = __builtin_strlen(s);
+    if (__builtin_constant_p(slen)) {
+        return slen;
+    }
+
+    return __strlen_chk(s, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_J_MR1__ */
+
+#if  __ANDROID_API__ >= __ANDROID_API_J_MR2__
+__BIONIC_FORTIFY_INLINE
+char* strchr(const char* s, int c) {
+    size_t bos = __bos(s);
+
+    // Compiler doesn't know destination size. Don't call __strchr_chk
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __builtin_strchr(s, c);
+    }
+
+    size_t slen = __builtin_strlen(s);
+    if (__builtin_constant_p(slen) && (slen < bos)) {
+        return __builtin_strchr(s, c);
+    }
+
+    return __strchr_chk(s, c, bos);
+}
+
+__BIONIC_FORTIFY_INLINE
+char* strrchr(const char* s, int c) {
+    size_t bos = __bos(s);
+
+    // Compiler doesn't know destination size. Don't call __strrchr_chk
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __builtin_strrchr(s, c);
+    }
+
+    size_t slen = __builtin_strlen(s);
+    if (__builtin_constant_p(slen) && (slen < bos)) {
+        return __builtin_strrchr(s, c);
+    }
+
+    return __strrchr_chk(s, c, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_J_MR2__ */
+#endif /* defined(__clang__) */
+
 #if __ANDROID_API__ >= __ANDROID_API_M__
 #if defined(__cplusplus)
 extern "C++" {
diff --git a/libc/include/bits/fortify/unistd.h b/libc/include/bits/fortify/unistd.h
index a07907b..ea5c89d 100644
--- a/libc/include/bits/fortify/unistd.h
+++ b/libc/include/bits/fortify/unistd.h
@@ -59,6 +59,7 @@
 #define __PWRITE_PREFIX(x) __pwrite_ ## x
 #endif
 
+#if defined(__clang__)
 #define __error_if_overflows_ssizet(what, fn) \
     __clang_error_if((what) > SSIZE_MAX, "in call to '" #fn "', '" #what "' must be <= SSIZE_MAX")
 
@@ -206,6 +207,243 @@
 #undef __enable_if_no_overflow_ssizet
 #undef __error_if_overflows_objectsize
 #undef __error_if_overflows_ssizet
+#else /* defined(__clang__) */
+
+char* __getcwd_real(char*, size_t) __RENAME(getcwd);
+ssize_t __read_real(int, void*, size_t) __RENAME(read);
+ssize_t __write_real(int, const void*, size_t) __RENAME(write);
+ssize_t __readlink_real(const char*, char*, size_t) __RENAME(readlink);
+ssize_t __readlinkat_real(int dirfd, const char*, char*, size_t) __RENAME(readlinkat);
+
+__errordecl(__getcwd_dest_size_error, "getcwd called with size bigger than destination");
+__errordecl(__pread_dest_size_error, "pread called with size bigger than destination");
+__errordecl(__pread_count_toobig_error, "pread called with count > SSIZE_MAX");
+__errordecl(__pread64_dest_size_error, "pread64 called with size bigger than destination");
+__errordecl(__pread64_count_toobig_error, "pread64 called with count > SSIZE_MAX");
+__errordecl(__pwrite_dest_size_error, "pwrite called with size bigger than destination");
+__errordecl(__pwrite_count_toobig_error, "pwrite called with count > SSIZE_MAX");
+__errordecl(__pwrite64_dest_size_error, "pwrite64 called with size bigger than destination");
+__errordecl(__pwrite64_count_toobig_error, "pwrite64 called with count > SSIZE_MAX");
+__errordecl(__read_dest_size_error, "read called with size bigger than destination");
+__errordecl(__read_count_toobig_error, "read called with count > SSIZE_MAX");
+__errordecl(__write_dest_size_error, "write called with size bigger than destination");
+__errordecl(__write_count_toobig_error, "write called with count > SSIZE_MAX");
+__errordecl(__readlink_dest_size_error, "readlink called with size bigger than destination");
+__errordecl(__readlink_size_toobig_error, "readlink called with size > SSIZE_MAX");
+__errordecl(__readlinkat_dest_size_error, "readlinkat called with size bigger than destination");
+__errordecl(__readlinkat_size_toobig_error, "readlinkat called with size > SSIZE_MAX");
+
+#if __ANDROID_API__ >= __ANDROID_API_N__
+__BIONIC_FORTIFY_INLINE
+char* getcwd(char* buf, size_t size) __overloadable {
+    size_t bos = __bos(buf);
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __getcwd_real(buf, size);
+    }
+
+    if (__builtin_constant_p(size) && (size > bos)) {
+        __getcwd_dest_size_error();
+    }
+
+    if (__builtin_constant_p(size) && (size <= bos)) {
+        return __getcwd_real(buf, size);
+    }
+
+    return __getcwd_chk(buf, size, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_N__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_M__
+__BIONIC_FORTIFY_INLINE
+ssize_t pread(int fd, void* buf, size_t count, off_t offset) {
+    size_t bos = __bos0(buf);
+
+    if (__builtin_constant_p(count) && (count > SSIZE_MAX)) {
+        __PREAD_PREFIX(count_toobig_error)();
+    }
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __PREAD_PREFIX(real)(fd, buf, count, offset);
+    }
+
+    if (__builtin_constant_p(count) && (count > bos)) {
+        __PREAD_PREFIX(dest_size_error)();
+    }
+
+    if (__builtin_constant_p(count) && (count <= bos)) {
+        return __PREAD_PREFIX(real)(fd, buf, count, offset);
+    }
+
+    return __PREAD_PREFIX(chk)(fd, buf, count, offset, bos);
+}
+
+__BIONIC_FORTIFY_INLINE
+ssize_t pread64(int fd, void* buf, size_t count, off64_t offset) {
+    size_t bos = __bos0(buf);
+
+    if (__builtin_constant_p(count) && (count > SSIZE_MAX)) {
+        __pread64_count_toobig_error();
+    }
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __pread64_real(fd, buf, count, offset);
+    }
+
+    if (__builtin_constant_p(count) && (count > bos)) {
+        __pread64_dest_size_error();
+    }
+
+    if (__builtin_constant_p(count) && (count <= bos)) {
+        return __pread64_real(fd, buf, count, offset);
+    }
+
+    return __pread64_chk(fd, buf, count, offset, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_M__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_N__
+__BIONIC_FORTIFY_INLINE
+ssize_t pwrite(int fd, const void* buf, size_t count, off_t offset) {
+    size_t bos = __bos0(buf);
+
+    if (__builtin_constant_p(count) && (count > SSIZE_MAX)) {
+        __PWRITE_PREFIX(count_toobig_error)();
+    }
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __PWRITE_PREFIX(real)(fd, buf, count, offset);
+    }
+
+    if (__builtin_constant_p(count) && (count > bos)) {
+        __PWRITE_PREFIX(dest_size_error)();
+    }
+
+    if (__builtin_constant_p(count) && (count <= bos)) {
+        return __PWRITE_PREFIX(real)(fd, buf, count, offset);
+    }
+
+    return __PWRITE_PREFIX(chk)(fd, buf, count, offset, bos);
+}
+
+__BIONIC_FORTIFY_INLINE
+ssize_t pwrite64(int fd, const void* buf, size_t count, off64_t offset) {
+    size_t bos = __bos0(buf);
+
+    if (__builtin_constant_p(count) && (count > SSIZE_MAX)) {
+        __pwrite64_count_toobig_error();
+    }
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __pwrite64_real(fd, buf, count, offset);
+    }
+
+    if (__builtin_constant_p(count) && (count > bos)) {
+        __pwrite64_dest_size_error();
+    }
+
+    if (__builtin_constant_p(count) && (count <= bos)) {
+        return __pwrite64_real(fd, buf, count, offset);
+    }
+
+    return __pwrite64_chk(fd, buf, count, offset, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_N__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_L__
+__BIONIC_FORTIFY_INLINE
+ssize_t read(int fd, void* buf, size_t count) {
+    size_t bos = __bos0(buf);
+
+    if (__builtin_constant_p(count) && (count > SSIZE_MAX)) {
+        __read_count_toobig_error();
+    }
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __read_real(fd, buf, count);
+    }
+
+    if (__builtin_constant_p(count) && (count > bos)) {
+        __read_dest_size_error();
+    }
+
+    if (__builtin_constant_p(count) && (count <= bos)) {
+        return __read_real(fd, buf, count);
+    }
+
+    return __read_chk(fd, buf, count, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_L__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_N__
+__BIONIC_FORTIFY_INLINE
+ssize_t write(int fd, const void* buf, size_t count) {
+    size_t bos = __bos0(buf);
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __write_real(fd, buf, count);
+    }
+
+    if (__builtin_constant_p(count) && (count > bos)) {
+        __write_dest_size_error();
+    }
+
+    if (__builtin_constant_p(count) && (count <= bos)) {
+        return __write_real(fd, buf, count);
+    }
+
+    return __write_chk(fd, buf, count, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_N__ */
+
+#if __ANDROID_API__ >= __ANDROID_API_M__
+__BIONIC_FORTIFY_INLINE
+ssize_t readlink(const char* path, char* buf, size_t size) {
+    size_t bos = __bos(buf);
+
+    if (__builtin_constant_p(size) && (size > SSIZE_MAX)) {
+        __readlink_size_toobig_error();
+    }
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __readlink_real(path, buf, size);
+    }
+
+    if (__builtin_constant_p(size) && (size > bos)) {
+        __readlink_dest_size_error();
+    }
+
+    if (__builtin_constant_p(size) && (size <= bos)) {
+        return __readlink_real(path, buf, size);
+    }
+
+    return __readlink_chk(path, buf, size, bos);
+}
+
+__BIONIC_FORTIFY_INLINE
+ssize_t readlinkat(int dirfd, const char* path, char* buf, size_t size) {
+    size_t bos = __bos(buf);
+
+    if (__builtin_constant_p(size) && (size > SSIZE_MAX)) {
+        __readlinkat_size_toobig_error();
+    }
+
+    if (bos == __BIONIC_FORTIFY_UNKNOWN_SIZE) {
+        return __readlinkat_real(dirfd, path, buf, size);
+    }
+
+    if (__builtin_constant_p(size) && (size > bos)) {
+        __readlinkat_dest_size_error();
+    }
+
+    if (__builtin_constant_p(size) && (size <= bos)) {
+        return __readlinkat_real(dirfd, path, buf, size);
+    }
+
+    return __readlinkat_chk(dirfd, path, buf, size, bos);
+}
+#endif /* __ANDROID_API__ >= __ANDROID_API_M__ */
+#endif /* defined(__clang__) */
 #undef __PREAD_PREFIX
 #undef __PWRITE_PREFIX
 #endif /* defined(__BIONIC_FORTIFY) */
diff --git a/libc/include/bits/ioctl.h b/libc/include/bits/ioctl.h
index 745bbf0..3357c1b 100644
--- a/libc/include/bits/ioctl.h
+++ b/libc/include/bits/ioctl.h
@@ -51,7 +51,8 @@
  * FIXME: __has_extension is more or less a clang version check. Remove it when
  * we don't need to support old clang code.
  */
-#if __has_extension(overloadable_unmarked) && !defined(BIONIC_IOCTL_NO_SIGNEDNESS_OVERLOAD)
+#if defined(__clang__) && __has_extension(overloadable_unmarked) && \
+  !defined(BIONIC_IOCTL_NO_SIGNEDNESS_OVERLOAD)
 /* enable_if(1) just exists to break overloading ties. */
 int ioctl(int __fd, unsigned __request, ...) __overloadable __enable_if(1, "") __RENAME(ioctl);
 #endif
diff --git a/libc/include/complex.h b/libc/include/complex.h
index 50da330..3fe8f35 100644
--- a/libc/include/complex.h
+++ b/libc/include/complex.h
@@ -44,9 +44,15 @@
 #define	I		_Complex_I
 
 #if __STDC_VERSION__ >= 201112L
+#ifdef __clang__
 #define	CMPLX(x, y)	((double complex){ x, y })
 #define	CMPLXF(x, y)	((float complex){ x, y })
 #define	CMPLXL(x, y)	((long double complex){ x, y })
+#else
+#define	CMPLX(x, y)	__builtin_complex((double)(x), (double)(y))
+#define	CMPLXF(x, y)	__builtin_complex((float)(x), (float)(y))
+#define	CMPLXL(x, y)	__builtin_complex((long double)(x), (long double)(y))
+#endif
 #endif
 
 __BEGIN_DECLS
diff --git a/libc/include/malloc.h b/libc/include/malloc.h
index f7adfbb..3a678a9 100644
--- a/libc/include/malloc.h
+++ b/libc/include/malloc.h
@@ -23,8 +23,8 @@
 
 __BEGIN_DECLS
 
-// Remove this workaround once b/37423073 is fixed.
-#if !__has_attribute(alloc_size)
+// Remove the workaround once b/37423073 is fixed.
+#if defined(__clang__) && !__has_attribute(alloc_size)
 #define __BIONIC_ALLOC_SIZE(...)
 #else
 #define __BIONIC_ALLOC_SIZE(...) __attribute__((__alloc_size__(__VA_ARGS__)))
diff --git a/libc/include/stdatomic.h b/libc/include/stdatomic.h
index 3c61334..013407e 100644
--- a/libc/include/stdatomic.h
+++ b/libc/include/stdatomic.h
@@ -33,7 +33,11 @@
 #include <sys/cdefs.h>
 
 #if defined(__cplusplus) && __cplusplus >= 201103L && defined(_USING_LIBCXX)
-# if __has_feature(cxx_atomic)
+# ifdef __clang__
+#  if __has_feature(cxx_atomic)
+#   define _STDATOMIC_HAVE_ATOMIC
+#  endif
+# else /* gcc */
 #  define _STDATOMIC_HAVE_ATOMIC
 # endif
 #endif
@@ -146,6 +150,20 @@
 # include <uchar.h>  /* For char16_t and char32_t.              */
 #endif
 
+
+#ifdef __clang__
+# if __has_extension(c_atomic) || __has_extension(cxx_atomic)
+#  define       __CLANG_ATOMICS
+# else
+#  error "stdatomic.h does not support your compiler"
+# endif
+# if __has_builtin(__sync_swap)
+#  define __HAS_BUILTIN_SYNC_SWAP
+# endif
+#else
+# define __GNUC_ATOMICS
+#endif
+
 /*
  * 7.17.1 Atomic lock-free macros.
  */
@@ -185,8 +203,13 @@
  * 7.17.2 Initialization.
  */
 
+#if defined(__CLANG_ATOMICS)
 #define	ATOMIC_VAR_INIT(value)		(value)
 #define	atomic_init(obj, value)		__c11_atomic_init(obj, value)
+#else
+#define	ATOMIC_VAR_INIT(value)		{ .__val = (value) }
+#define	atomic_init(obj, value)		((void)((obj)->__val = (value)))
+#endif
 
 /*
  * Clang and recent GCC both provide predefined macros for the memory
@@ -235,24 +258,63 @@
  * 7.17.4 Fences.
  */
 
-static __inline void atomic_thread_fence(memory_order __order __attribute__((unused))) {
+static __inline void
+atomic_thread_fence(memory_order __order __attribute__((unused)))
+{
+
+#ifdef __CLANG_ATOMICS
 	__c11_atomic_thread_fence(__order);
+#elif defined(__GNUC_ATOMICS)
+	__atomic_thread_fence(__order);
+#else
+	__sync_synchronize();
+#endif
 }
 
-static __inline void atomic_signal_fence(memory_order __order __attribute__((unused))) {
+static __inline void
+atomic_signal_fence(memory_order __order __attribute__((unused)))
+{
+
+#ifdef __CLANG_ATOMICS
 	__c11_atomic_signal_fence(__order);
+#elif defined(__GNUC_ATOMICS)
+	__atomic_signal_fence(__order);
+#else
+	__asm volatile ("" ::: "memory");
+#endif
 }
 
 /*
  * 7.17.5 Lock-free property.
  */
 
-#define	atomic_is_lock_free(obj) __c11_atomic_is_lock_free(sizeof(*(obj)))
+#if defined(_KERNEL)
+/* Atomics in kernelspace are always lock-free. */
+#define	atomic_is_lock_free(obj) \
+	((void)(obj), (_Bool)1)
+#elif defined(__CLANG_ATOMICS)
+#define	atomic_is_lock_free(obj) \
+	__c11_atomic_is_lock_free(sizeof(*(obj)))
+#elif defined(__GNUC_ATOMICS)
+#define	atomic_is_lock_free(obj) \
+	__atomic_is_lock_free(sizeof((obj)->__val), &(obj)->__val)
+#else
+#define	atomic_is_lock_free(obj) \
+	((void)(obj), sizeof((obj)->__val) <= sizeof(void *))
+#endif
 
 /*
  * 7.17.6 Atomic integer types.
  */
 
+#ifndef __CLANG_ATOMICS
+/*
+ * No native support for _Atomic(). Place object in structure to prevent
+ * most forms of direct non-atomic access.
+ */
+#define _Atomic(T)              struct { T volatile __val; }
+#endif
+
 typedef _Atomic(bool)			atomic_bool;
 typedef _Atomic(char)			atomic_char;
 typedef _Atomic(signed char)		atomic_schar;
@@ -301,6 +363,7 @@
  * Compiler-specific operations.
  */
 
+#if defined(__CLANG_ATOMICS)
 #define	atomic_compare_exchange_strong_explicit(object, expected,	\
     desired, success, failure)						\
 	__c11_atomic_compare_exchange_strong(object, expected, desired,	\
@@ -325,11 +388,91 @@
 	__c11_atomic_load(object, order)
 #define	atomic_store_explicit(object, desired, order)			\
 	__c11_atomic_store(object, desired, order)
+#elif defined(__GNUC_ATOMICS)
+#define	atomic_compare_exchange_strong_explicit(object, expected,	\
+    desired, success, failure)						\
+	__atomic_compare_exchange_n(&(object)->__val, expected,		\
+	    desired, 0, success, failure)
+#define	atomic_compare_exchange_weak_explicit(object, expected,		\
+    desired, success, failure)						\
+	__atomic_compare_exchange_n(&(object)->__val, expected,		\
+	    desired, 1, success, failure)
+#define	atomic_exchange_explicit(object, desired, order)		\
+	__atomic_exchange_n(&(object)->__val, desired, order)
+#define	atomic_fetch_add_explicit(object, operand, order)		\
+	__atomic_fetch_add(&(object)->__val, operand, order)
+#define	atomic_fetch_and_explicit(object, operand, order)		\
+	__atomic_fetch_and(&(object)->__val, operand, order)
+#define	atomic_fetch_or_explicit(object, operand, order)		\
+	__atomic_fetch_or(&(object)->__val, operand, order)
+#define	atomic_fetch_sub_explicit(object, operand, order)		\
+	__atomic_fetch_sub(&(object)->__val, operand, order)
+#define	atomic_fetch_xor_explicit(object, operand, order)		\
+	__atomic_fetch_xor(&(object)->__val, operand, order)
+#define	atomic_load_explicit(object, order)				\
+	__atomic_load_n(&(object)->__val, order)
+#define	atomic_store_explicit(object, desired, order)			\
+	__atomic_store_n(&(object)->__val, desired, order)
+#else
+#define	__atomic_apply_stride(object, operand) \
+	(((__typeof__((object)->__val))0) + (operand))
+#define	atomic_compare_exchange_strong_explicit(object, expected,	\
+    desired, success, failure)	__extension__ ({			\
+	__typeof__(expected) __ep = (expected);				\
+	__typeof__(*__ep) __e = *__ep;					\
+	(void)(success); (void)(failure);				\
+	(bool)((*__ep = __sync_val_compare_and_swap(&(object)->__val,	\
+	    __e, desired)) == __e);					\
+})
+#define	atomic_compare_exchange_weak_explicit(object, expected,		\
+    desired, success, failure)						\
+	atomic_compare_exchange_strong_explicit(object, expected,	\
+		desired, success, failure)
+#ifdef __HAS_BUILTIN_SYNC_SWAP
+/* Clang provides a full-barrier atomic exchange - use it if available. */
+#define	atomic_exchange_explicit(object, desired, order)		\
+	((void)(order), __sync_swap(&(object)->__val, desired))
+#else
+/*
+ * __sync_lock_test_and_set() is only an acquire barrier in theory (although in
+ * practice it is usually a full barrier) so we need an explicit barrier before
+ * it.
+ */
+#define	atomic_exchange_explicit(object, desired, order)		\
+__extension__ ({							\
+	__typeof__(object) __o = (object);				\
+	__typeof__(desired) __d = (desired);				\
+	(void)(order);							\
+	__sync_synchronize();						\
+	__sync_lock_test_and_set(&(__o)->__val, __d);			\
+})
+#endif
+#define	atomic_fetch_add_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_add(&(object)->__val,		\
+	    __atomic_apply_stride(object, operand)))
+#define	atomic_fetch_and_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_and(&(object)->__val, operand))
+#define	atomic_fetch_or_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_or(&(object)->__val, operand))
+#define	atomic_fetch_sub_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_sub(&(object)->__val,		\
+	    __atomic_apply_stride(object, operand)))
+#define	atomic_fetch_xor_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_xor(&(object)->__val, operand))
+#define	atomic_load_explicit(object, order)				\
+	((void)(order), __sync_fetch_and_add(&(object)->__val, 0))
+#define	atomic_store_explicit(object, desired, order)			\
+	((void)atomic_exchange_explicit(object, desired, order))
+#endif
 
 /*
  * Convenience functions.
+ *
+ * Don't provide these in kernel space. In kernel space, we should be
+ * disciplined enough to always provide explicit barriers.
  */
 
+#ifndef _KERNEL
 #define	atomic_compare_exchange_strong(object, expected, desired)	\
 	atomic_compare_exchange_strong_explicit(object, expected,	\
 	    desired, memory_order_seq_cst, memory_order_seq_cst)
@@ -352,6 +495,7 @@
 	atomic_load_explicit(object, memory_order_seq_cst)
 #define	atomic_store(object, desired)					\
 	atomic_store_explicit(object, desired, memory_order_seq_cst)
+#endif /* !_KERNEL */
 
 /*
  * 7.17.8 Atomic flag type and operations.
@@ -366,21 +510,36 @@
 
 #define	ATOMIC_FLAG_INIT		{ ATOMIC_VAR_INIT(false) }
 
-static __inline bool atomic_flag_test_and_set_explicit(volatile atomic_flag *__object, memory_order __order) {
+static __inline bool
+atomic_flag_test_and_set_explicit(volatile atomic_flag *__object,
+    memory_order __order)
+{
 	return (atomic_exchange_explicit(&__object->__flag, 1, __order));
 }
 
-static __inline void atomic_flag_clear_explicit(volatile atomic_flag *__object, memory_order __order) {
+static __inline void
+atomic_flag_clear_explicit(volatile atomic_flag *__object, memory_order __order)
+{
+
 	atomic_store_explicit(&__object->__flag, 0, __order);
 }
 
-static __inline bool atomic_flag_test_and_set(volatile atomic_flag *__object) {
-	return (atomic_flag_test_and_set_explicit(__object, memory_order_seq_cst));
+#ifndef _KERNEL
+static __inline bool
+atomic_flag_test_and_set(volatile atomic_flag *__object)
+{
+
+	return (atomic_flag_test_and_set_explicit(__object,
+	    memory_order_seq_cst));
 }
 
-static __inline void atomic_flag_clear(volatile atomic_flag *__object) {
+static __inline void
+atomic_flag_clear(volatile atomic_flag *__object)
+{
+
 	atomic_flag_clear_explicit(__object, memory_order_seq_cst);
 }
+#endif /* !_KERNEL */
 
 #endif /* <atomic> unavailable */
 
diff --git a/libc/include/string.h b/libc/include/string.h
index 25f6673..54d5e1c 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -144,7 +144,7 @@
 #endif
 
 /* Const-correct overloads. Placed after FORTIFY so we call those functions, if possible. */
-#if defined(__cplusplus)
+#if defined(__cplusplus) && defined(__clang__)
 /*
  * Use two enable_ifs so these overloads don't conflict with + are preferred over libcxx's. This can
  * be reduced to 1 after libcxx recognizes that we have const-correct overloads.
diff --git a/libc/include/sys/cdefs.h b/libc/include/sys/cdefs.h
index af2d03d..201f40a 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -155,12 +155,25 @@
 
 #define __wur __attribute__((__warn_unused_result__))
 
-#define __errorattr(msg) __attribute__((unavailable(msg)))
-#define __warnattr(msg) __attribute__((deprecated(msg)))
-#define __warnattr_real(msg) __attribute__((deprecated(msg)))
-#define __enable_if(cond, msg) __attribute__((enable_if(cond, msg)))
-#define __clang_error_if(cond, msg) __attribute__((diagnose_if(cond, msg, "error")))
-#define __clang_warning_if(cond, msg) __attribute__((diagnose_if(cond, msg, "warning")))
+#ifdef __clang__
+#  define __errorattr(msg) __attribute__((unavailable(msg)))
+#  define __warnattr(msg) __attribute__((deprecated(msg)))
+#  define __warnattr_real(msg) __attribute__((deprecated(msg)))
+#  define __enable_if(cond, msg) __attribute__((enable_if(cond, msg)))
+#  define __clang_error_if(cond, msg) __attribute__((diagnose_if(cond, msg, "error")))
+#  define __clang_warning_if(cond, msg) __attribute__((diagnose_if(cond, msg, "warning")))
+#else
+#  define __errorattr(msg) __attribute__((__error__(msg)))
+#  define __warnattr(msg) __attribute__((__warning__(msg)))
+#  define __warnattr_real __warnattr
+/* enable_if doesn't exist on other compilers; give an error if it's used. */
+/* diagnose_if doesn't exist either, but it's often tagged on non-clang-specific functions */
+#  define __clang_error_if(cond, msg)
+#  define __clang_warning_if(cond, msg)
+
+/* errordecls really don't work as well in clang as they do in GCC. */
+#  define __errordecl(name, msg) extern void name(void) __errorattr(msg)
+#endif
 
 #if defined(ANDROID_STRICT)
 /*
@@ -261,13 +274,17 @@
 #define __BIONIC_FORTIFY_UNKNOWN_SIZE ((size_t) -1)
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0
+#  if defined(__clang__)
 /*
  * FORTIFY's _chk functions effectively disable ASAN's stdlib interceptors.
  * Additionally, the static analyzer/clang-tidy try to pattern match some
  * standard library functions, and FORTIFY sometimes interferes with this. So,
  * we turn FORTIFY off in both cases.
  */
-#  if !__has_feature(address_sanitizer) && !defined(__clang_analyzer__)
+#    if !__has_feature(address_sanitizer) && !defined(__clang_analyzer__)
+#      define __BIONIC_FORTIFY 1
+#    endif
+#  elif defined(__OPTIMIZE__) && __OPTIMIZE__ > 0
 #    define __BIONIC_FORTIFY 1
 #  endif
 #endif
@@ -289,27 +306,40 @@
 
 #if defined(__BIONIC_FORTIFY)
 #  define __bos0(s) __bosn((s), 0)
-#  define __pass_object_size_n(n) __attribute__((pass_object_size(n)))
+#  if defined(__clang__)
+#    define __pass_object_size_n(n) __attribute__((pass_object_size(n)))
 /*
  * FORTIFY'ed functions all have either enable_if or pass_object_size, which
  * makes taking their address impossible. Saying (&read)(foo, bar, baz); will
  * therefore call the unFORTIFYed version of read.
  */
-#  define __call_bypassing_fortify(fn) (&fn)
+#    define __call_bypassing_fortify(fn) (&fn)
 /*
  * Because clang-FORTIFY uses overloads, we can't mark functions as `extern
  * inline` without making them available externally.
  */
-#  define __BIONIC_FORTIFY_INLINE static __inline__ __always_inline
+#    define __BIONIC_FORTIFY_INLINE static __inline__ __always_inline
 /*
  * We should use __BIONIC_FORTIFY_VARIADIC instead of __BIONIC_FORTIFY_INLINE
  * for variadic functions because compilers cannot inline them.
  * The __always_inline attribute is useless, misleading, and could trigger
  * clang compiler bug to incorrectly inline variadic functions.
  */
-#  define __BIONIC_FORTIFY_VARIADIC static __inline__
+#    define __BIONIC_FORTIFY_VARIADIC static __inline__
 /* Error functions don't have bodies, so they can just be static. */
-#  define __BIONIC_ERROR_FUNCTION_VISIBILITY static
+#    define __BIONIC_ERROR_FUNCTION_VISIBILITY static
+#  else
+/*
+ * Where they can, GCC and clang-style FORTIFY share implementations.
+ * So, make these nops in GCC.
+ */
+#    define __pass_object_size_n(n)
+#    define __call_bypassing_fortify(fn) (fn)
+/* __BIONIC_FORTIFY_NONSTATIC_INLINE is pointless in GCC's FORTIFY */
+#    define __BIONIC_FORTIFY_INLINE extern __inline__ __always_inline __attribute__((gnu_inline)) __attribute__((__artificial__))
+/* __always_inline is probably okay and ignored by gcc in __BIONIC_FORTIFY_VARIADIC */
+#    define __BIONIC_FORTIFY_VARIADIC __BIONIC_FORTIFY_INLINE
+#  endif
 #else
 /* Further increase sharing for some inline functions */
 #  define __pass_object_size_n(n)
@@ -321,7 +351,11 @@
 #  define __BIONIC_INCLUDE_FORTIFY_HEADERS 1
 #endif
 
-#define __overloadable __attribute__((overloadable))
+#if defined(__clang__)
+#  define __overloadable __attribute__((overloadable))
+#else
+#  define __overloadable
+#endif
 
 /* Used to tag non-static symbols that are private and never exposed by the shared library. */
 #define __LIBC_HIDDEN__ __attribute__((visibility("hidden")))
@@ -354,6 +388,7 @@
 }
 #endif
 
+#if defined(__clang__)
 /*
  * Used when we need to check for overflow when multiplying x and y. This
  * should only be used where __size_mul_overflow can not work, because it makes
@@ -362,5 +397,6 @@
  * __size_mul_overflow.
  */
 #define __unsafe_check_mul_overflow(x, y) ((__SIZE_TYPE__)-1 / (x) < (y))
+#endif
 
 #endif /* !_SYS_CDEFS_H_ */
diff --git a/libc/include/sys/mman.h b/libc/include/sys/mman.h
index 4073fbf..5a7e3c0 100644
--- a/libc/include/sys/mman.h
+++ b/libc/include/sys/mman.h
@@ -56,7 +56,14 @@
  * preserve the old behavior for GCC and emit a useful diagnostic.
  */
 #if defined(__USE_FILE_OFFSET64)
-void* mmap(void* __addr, size_t __size, int __prot, int __flags, int __fd, off_t __offset) __RENAME(mmap64);
+void* mmap(void* __addr, size_t __size, int __prot, int __flags, int __fd, off_t __offset)
+#  if !defined(__clang__) && __ANDROID_API__ < __ANDROID_API_L__
+      __attribute__((error("mmap is not available with _FILE_OFFSET_BITS=64 when using GCC until "
+                           "android-21. Either raise your minSdkVersion, disable "
+                           "_FILE_OFFSET_BITS=64, or switch to Clang.")));
+#  else
+    __RENAME(mmap64);
+#  endif
 #else
 void* mmap(void* __addr, size_t __size, int __prot, int __flags, int __fd, off_t __offset);
 #endif