diff --git a/src/dc_key.rs b/src/dc_key.rs
index 2e5427332..eb164e574 100644
--- a/src/dc_key.rs
+++ b/src/dc_key.rs
@@ -24,9 +24,17 @@ pub struct dc_key_t {
 
 #[no_mangle]
 #[inline]
+#[cfg(target_os = "macos")]
 pub unsafe extern "C" fn toupper(mut _c: libc::c_int) -> libc::c_int {
     return __toupper(_c);
 }
+
+#[no_mangle]
+#[inline]
+#[cfg(not(target_os = "macos"))]
+pub unsafe extern "C" fn toupper(mut _c: libc::c_int) -> libc::c_int {
+    return _toupper(_c);
+}
 #[no_mangle]
 pub unsafe extern "C" fn dc_key_new() -> *mut dc_key_t {
     let mut key: *mut dc_key_t = 0 as *mut dc_key_t;
diff --git a/src/dc_saxparser.rs b/src/dc_saxparser.rs
index 853ce3f70..582e86e97 100644
--- a/src/dc_saxparser.rs
+++ b/src/dc_saxparser.rs
@@ -31,14 +31,6 @@ pub type dc_saxparser_starttag_cb_t = Option<
 unsafe extern "C" fn isascii(mut _c: libc::c_int) -> libc::c_int {
     return (_c & !0x7fi32 == 0i32) as libc::c_int;
 }
-#[inline]
-unsafe extern "C" fn __istype(mut _c: __darwin_ct_rune_t, mut _f: libc::c_ulong) -> libc::c_int {
-    return if 0 != isascii(_c) {
-        (0 != _DefaultRuneLocale.__runetype[_c as usize] as libc::c_ulong & _f) as libc::c_int
-    } else {
-        (0 != __maskrune(_c, _f)) as libc::c_int
-    };
-}
 
 #[no_mangle]
 pub unsafe extern "C" fn dc_saxparser_init(
diff --git a/src/dc_strencode.rs b/src/dc_strencode.rs
index 2bdf1dd66..2eef05a2c 100644
--- a/src/dc_strencode.rs
+++ b/src/dc_strencode.rs
@@ -8,14 +8,7 @@ use crate::x::*;
 unsafe extern "C" fn isascii(mut _c: libc::c_int) -> libc::c_int {
     return (_c & !0x7fi32 == 0i32) as libc::c_int;
 }
-#[inline]
-unsafe extern "C" fn __istype(mut _c: __darwin_ct_rune_t, mut _f: libc::c_ulong) -> libc::c_int {
-    return if 0 != isascii(_c) {
-        (0 != _DefaultRuneLocale.__runetype[_c as usize] as libc::c_ulong & _f) as libc::c_int
-    } else {
-        (0 != __maskrune(_c, _f)) as libc::c_int
-    };
-}
+
 #[inline]
 unsafe extern "C" fn __isctype(
     mut _c: __darwin_ct_rune_t,
@@ -27,15 +20,33 @@ unsafe extern "C" fn __isctype(
         (0 != _DefaultRuneLocale.__runetype[_c as usize] as libc::c_ulong & _f) as libc::c_int
     };
 }
+
 #[no_mangle]
 #[inline]
-pub unsafe extern "C" fn isalnum(mut _c: libc::c_int) -> libc::c_int {
-    return __istype(_c, (0x100i64 | 0x400i64) as libc::c_ulong);
+pub fn isalnum(mut _c: libc::c_int) -> libc::c_int {
+    if _c < std::u8::MAX as libc::c_int {
+        (_c as u8 as char).is_ascii_alphanumeric() as libc::c_int
+    } else {
+        0
+    }
 }
+
+#[cfg(test)]
+#[test]
+fn test_isalnum() {
+    assert_eq!(isalnum(0), 0);
+    assert_eq!(isalnum('5' as libc::c_int), 1);
+    assert_eq!(isalnum('Q' as libc::c_int), 1);
+}
+
 #[no_mangle]
 #[inline]
-pub unsafe extern "C" fn isdigit(mut _c: libc::c_int) -> libc::c_int {
-    return __isctype(_c, 0x400i64 as libc::c_ulong);
+pub fn isdigit(mut _c: libc::c_int) -> libc::c_int {
+    if _c < std::u8::MAX as libc::c_int {
+        (_c as u8 as char).is_ascii_digit() as libc::c_int
+    } else {
+        0
+    }
 }
 
 #[no_mangle]
diff --git a/src/types.rs b/src/types.rs
index 27bda01c6..e367d97d1 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -1311,29 +1311,31 @@ pub struct _dc_sqlite3 {
 pub unsafe extern "C" fn isascii(mut _c: libc::c_int) -> libc::c_int {
     return (_c & !0x7fi32 == 0i32) as libc::c_int;
 }
-#[inline]
-pub unsafe extern "C" fn __istype(
-    mut _c: __darwin_ct_rune_t,
-    mut _f: libc::c_ulong,
-) -> libc::c_int {
-    return if 0 != isascii(_c) {
-        (0 != _DefaultRuneLocale.__runetype[_c as usize] as libc::c_ulong & _f) as libc::c_int
-    } else {
-        (0 != __maskrune(_c, _f)) as libc::c_int
-    };
-}
+
 #[no_mangle]
 #[inline]
 pub unsafe extern "C" fn isspace(mut _c: libc::c_int) -> libc::c_int {
-    return __istype(_c, 0x4000i64 as libc::c_ulong);
+    if _c < std::u8::MAX as libc::c_int {
+        ((_c as u8 as char) == ' ') as libc::c_int
+    } else {
+        0
+    }
 }
 
 #[no_mangle]
 #[inline]
+#[cfg(target_os = "macos")]
 pub unsafe extern "C" fn tolower(mut _c: libc::c_int) -> libc::c_int {
     return __tolower(_c);
 }
 
+#[no_mangle]
+#[inline]
+#[cfg(not(target_os = "macos"))]
+pub unsafe extern "C" fn tolower(mut _c: libc::c_int) -> libc::c_int {
+    return _tolower(_c);
+}
+
 #[inline]
 pub unsafe extern "C" fn carray_count(mut array: *mut carray) -> libc::c_uint {
     return (*array).len;
diff --git a/src/x.rs b/src/x.rs
index 78e0d6685..501224b5a 100644
--- a/src/x.rs
+++ b/src/x.rs
@@ -91,7 +91,10 @@ extern "C" {
         result: *mut *mut libc::c_char,
         result_len: *mut size_t,
     ) -> libc::c_int;
+    #[cfg(target_os = "macos")]
     pub fn __toupper(_: __darwin_ct_rune_t) -> __darwin_ct_rune_t;
+    #[cfg(not(target_os = "macos"))]
+    pub fn _toupper(_: __darwin_ct_rune_t) -> __darwin_ct_rune_t;
     pub fn memcmp(_: *const libc::c_void, _: *const libc::c_void, _: libc::c_ulong) -> libc::c_int;
     pub fn encode_base64(in_0: *const libc::c_char, len: libc::c_int) -> *mut libc::c_char;
     pub fn mmap_string_new(init: *const libc::c_char) -> *mut MMAPString;
@@ -279,8 +282,10 @@ extern "C" {
     pub fn mailmime_substitute(old_mime: *mut mailmime, new_mime: *mut mailmime) -> libc::c_int;
     pub fn mailprivacy_prepare_mime(mime: *mut mailmime);
     pub fn atol(_: *const libc::c_char) -> libc::c_long;
-    pub fn __maskrune(_: __darwin_ct_rune_t, _: libc::c_ulong) -> libc::c_int;
+    #[cfg(target_os = "macos")]
     pub fn __tolower(_: __darwin_ct_rune_t) -> __darwin_ct_rune_t;
+    #[cfg(not(target_os = "macos"))]
+    pub fn _tolower(_: __darwin_ct_rune_t) -> __darwin_ct_rune_t;
     pub fn mmap_string_append(string: *mut MMAPString, val: *const libc::c_char)
         -> *mut MMAPString;
     pub fn mmap_string_append_len(