diff --git a/include/stddef.h b/include/stddef.h
deleted file mode 100644
index 7397dc006d..0000000000
--- a/include/stddef.h
+++ /dev/null
@@ -1,19 +0,0 @@
-#ifndef _STDDEF_H
-#define _STDDEF_H
-
-// wchar_t should come before NULL and size_t
-// according to comment in wchar cbindgen
-#include <bits/wchar-t.h>
-#include <bits/size-t.h>
-#include <bits/null.h>
-
-#ifndef __PTRDIFF_TYPE__
-#define __PTRDIFF_TYPE__ long int
-#endif
-typedef __PTRDIFF_TYPE__ ptrdiff_t;
-
-typedef struct { long long __ll; long double __ld; } max_align_t;
-
-#define offsetof(type, member) __builtin_offsetof(type, member)
-
-#endif /* _STDDEF_H */
diff --git a/src/header/bits_null/cbindgen.toml b/src/header/bits_null/cbindgen.toml
index f0f1e8a10e..16e065a090 100644
--- a/src/header/bits_null/cbindgen.toml
+++ b/src/header/bits_null/cbindgen.toml
@@ -10,6 +10,9 @@ include_guard = "_RELIBC_BITS_NULL_T_H"
 language = "C"
 no_includes = true
 after_includes = """
+// Null pointer constant.
+// Any pointer object whose representation has all bits set to zero, perhaps by
+// memset() to 0 or by calloc(), shall be treated as a null pointer.
 #ifdef __cplusplus
 #if __cplusplus >= 201103L
   #define NULL nullptr
diff --git a/src/header/bits_null/mod.rs b/src/header/bits_null/mod.rs
index aa36f0cf83..92f520a41e 100644
--- a/src/header/bits_null/mod.rs
+++ b/src/header/bits_null/mod.rs
@@ -1 +1,5 @@
-// NULL is defined in cbindgen
+//! `NULL` from `stddef.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html>.
+//!
+//! Implemented via ifdefs and defines in cbindgen.
diff --git a/src/header/bits_size-t/mod.rs b/src/header/bits_size-t/mod.rs
index 6fefaa468d..4bde8f9acf 100644
--- a/src/header/bits_size-t/mod.rs
+++ b/src/header/bits_size-t/mod.rs
@@ -1,3 +1,7 @@
+//! `size_t` from `stddef.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html>.
+
 use crate::platform::types::c_ulong;
 
 /// Unsigned integer type of the result of the sizeof operator.
diff --git a/src/header/bits_wchar-t/cbindgen.toml b/src/header/bits_wchar-t/cbindgen.toml
index 397c3fde12..1d25b19d39 100644
--- a/src/header/bits_wchar-t/cbindgen.toml
+++ b/src/header/bits_wchar-t/cbindgen.toml
@@ -3,6 +3,9 @@
 # This type is split out to prevent importing all of stddef.h into inttypes.h
 include_guard = "_RELIBC_BITS_WCHAR_T_H"
 after_includes = """
+// Integer type whose range of values can represent distinct codes for all
+// members of the largest extended character set specified among the supported
+// locales; the null character shall have the code value zero.
 #ifndef _WCHAR_T
 #define _WCHAR_T
 #ifndef __cplusplus
diff --git a/src/header/bits_wchar-t/mod.rs b/src/header/bits_wchar-t/mod.rs
index 755d43c0cd..d299be4a82 100644
--- a/src/header/bits_wchar-t/mod.rs
+++ b/src/header/bits_wchar-t/mod.rs
@@ -1 +1,5 @@
-// `wchar_t` is defined in cbindgen
+//! `wchar_t` from `stddef.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html>.
+//!
+//! Implemented via ifdefs and defines in cbindgen.
diff --git a/src/header/mod.rs b/src/header/mod.rs
index 08a8fdd610..5dbf59b857 100644
--- a/src/header/mod.rs
+++ b/src/header/mod.rs
@@ -136,7 +136,7 @@ pub mod spawn;
 pub mod stdarg;
 // stdatomic.h implemented in C
 // stdbool.h implemented in C
-// stddef.h implemented in C
+pub mod stddef;
 // stdint.h implemented in C
 pub mod stdio;
 pub mod stdlib;
diff --git a/src/header/stddef/cbindgen.toml b/src/header/stddef/cbindgen.toml
new file mode 100644
index 0000000000..ab10db8119
--- /dev/null
+++ b/src/header/stddef/cbindgen.toml
@@ -0,0 +1,29 @@
+# POSIX header spec: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html
+#
+# There are no spec quotations relating to includes
+include_guard = "_RELIBC_STDDEF_H"
+after_includes = """
+// wchar_t should come before NULL and size_t
+// according to comment in wchar cbindgen
+#include <bits/wchar-t.h>
+#include <bits/size-t.h>
+#include <bits/null.h>
+
+#ifndef __PTRDIFF_TYPE__
+#define __PTRDIFF_TYPE__ long int
+#endif
+// Signed integer type of the result of subtracting two pointers.
+typedef __PTRDIFF_TYPE__ ptrdiff_t;
+
+// Object type whose alignment is the greatest fundamental alignment.
+typedef struct { long long __ll; long double __ld; } max_align_t;
+
+// Integer constant expression of type size_t, the value of which is the offset
+// in bytes to the structure member from the beginning of its structure.
+#define offsetof(type, member) __builtin_offsetof(type, member)
+"""
+language = "C"
+no_includes = true
+cpp_compat = true
+[enum]
+prefix_with_name = true
diff --git a/src/header/stddef/mod.rs b/src/header/stddef/mod.rs
new file mode 100644
index 0000000000..50fd1de6a0
--- /dev/null
+++ b/src/header/stddef/mod.rs
@@ -0,0 +1,5 @@
+//! `stddef.h` implementation.
+//!
+//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stddef.h.html>.
+//!
+//! Implemented via includes, typedefs and defines in cbindgen.