diff --git a/src/header/ctype/mod.rs b/src/header/ctype/mod.rs
index c9a58dfd59..bc82fbfdb6 100644
--- a/src/header/ctype/mod.rs
+++ b/src/header/ctype/mod.rs
@@ -1,52 +1,73 @@
-//! ctype implementation for Redox, following http://pubs.opengroup.org/onlinepubs/7908799/xsh/ctype.h.html
+//! `ctype.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/ctype.h.html>.
+
+// TODO: set this for entire crate when possible
+#![deny(unsafe_op_in_unsafe_fn)]
+
+// TODO: *_l functions
 
 use crate::platform::types::*;
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isalnum.html>.
 #[no_mangle]
 pub extern "C" fn isalnum(c: c_int) -> c_int {
     c_int::from(isdigit(c) != 0 || isalpha(c) != 0)
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isalpha.html>.
 #[no_mangle]
 pub extern "C" fn isalpha(c: c_int) -> c_int {
     c_int::from(islower(c) != 0 || isupper(c) != 0)
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9699919799/functions/isascii.html>.
+///
+/// The `isascii()` function was marked obsolescent in the Open Group Base
+/// Specifications Issue 7, and removed in Issue 8.
+#[deprecated]
 #[no_mangle]
 pub extern "C" fn isascii(c: c_int) -> c_int {
     c_int::from((c & !0x7f) == 0)
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isblank.html>.
 #[no_mangle]
 pub extern "C" fn isblank(c: c_int) -> c_int {
     c_int::from(c == c_int::from(b' ') || c == c_int::from(b'\t'))
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/iscntrl.html>.
 #[no_mangle]
 pub extern "C" fn iscntrl(c: c_int) -> c_int {
     c_int::from((c >= 0x00 && c <= 0x1f) || c == 0x7f)
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isdigit.html>.
 #[no_mangle]
 pub extern "C" fn isdigit(c: c_int) -> c_int {
     c_int::from(c >= c_int::from(b'0') && c <= c_int::from(b'9'))
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isgraph.html>.
 #[no_mangle]
 pub extern "C" fn isgraph(c: c_int) -> c_int {
     c_int::from(c >= 0x21 && c <= 0x7e)
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/islower.html>.
 #[no_mangle]
 pub extern "C" fn islower(c: c_int) -> c_int {
     c_int::from(c >= c_int::from(b'a') && c <= c_int::from(b'z'))
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isprint.html>.
 #[no_mangle]
 pub extern "C" fn isprint(c: c_int) -> c_int {
     c_int::from(c >= 0x20 && c < 0x7f)
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/ispunct.html>.
 #[no_mangle]
 pub extern "C" fn ispunct(c: c_int) -> c_int {
     c_int::from(
@@ -57,6 +78,7 @@ pub extern "C" fn ispunct(c: c_int) -> c_int {
     )
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isspace.html>.
 #[no_mangle]
 pub extern "C" fn isspace(c: c_int) -> c_int {
     c_int::from(
@@ -69,23 +91,29 @@ pub extern "C" fn isspace(c: c_int) -> c_int {
     )
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isupper.html>.
 #[no_mangle]
 pub extern "C" fn isupper(c: c_int) -> c_int {
     c_int::from(c >= c_int::from(b'A') && c <= c_int::from(b'Z'))
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/isxdigit.html>.
 #[no_mangle]
 pub extern "C" fn isxdigit(c: c_int) -> c_int {
     c_int::from(isdigit(c) != 0 || (c | 32 >= c_int::from(b'a') && c | 32 <= c_int::from(b'f')))
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9699919799/functions/toascii.html>.
+///
+/// The `toascii()` function was marked obsolescent in the Open Group Base
+/// Specifications Issue 7, and removed in Issue 8.
+#[deprecated]
 #[no_mangle]
-/// The comment in musl:
-/// "nonsense function that should NEVER be used!"
 pub extern "C" fn toascii(c: c_int) -> c_int {
     c & 0x7f
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/tolower.html>.
 #[no_mangle]
 pub extern "C" fn tolower(c: c_int) -> c_int {
     if isupper(c) != 0 {
@@ -95,6 +123,7 @@ pub extern "C" fn tolower(c: c_int) -> c_int {
     }
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/toupper.html>.
 #[no_mangle]
 pub extern "C" fn toupper(c: c_int) -> c_int {
     if islower(c) != 0 {