From 8e062105593bee87b89ca99778b30e5695d398d7 Mon Sep 17 00:00:00 2001
From: Muhammad Bassiouni <60100307+bassiounix@users.noreply.github.com>
Date: Wed, 18 Mar 2026 23:58:27 +0200
Subject: [PATCH] [libc][annex_k] Add errno_t (#163094)

RFC
https://discourse.llvm.org/t/rfc-bounds-checking-interfaces-for-llvm-libc/87685

Add `errno_t` type required by Annex K interface in LLVM libc.
---
 libc/hdr/types/CMakeLists.txt               |  8 ++++++
 libc/hdr/types/errno_t.h                    | 30 +++++++++++++++++++++
 libc/include/CMakeLists.txt                 |  1 +
 libc/include/errno.yaml                     |  3 ++-
 libc/include/llvm-libc-types/CMakeLists.txt |  1 +
 libc/include/llvm-libc-types/errno_t.h      | 26 ++++++++++++++++++
 6 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 libc/hdr/types/errno_t.h
 create mode 100644 libc/include/llvm-libc-types/errno_t.h

diff --git a/libc/hdr/types/CMakeLists.txt b/libc/hdr/types/CMakeLists.txt
index dec73b0665f0..ba99a02a1011 100644
--- a/libc/hdr/types/CMakeLists.txt
+++ b/libc/hdr/types/CMakeLists.txt
@@ -162,6 +162,14 @@ add_proxy_header_library(
     libc.include.fcntl
 )
 
+add_proxy_header_library(
+  errno_t
+  HDRS
+    errno_t.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.errno_t
+)
+
 add_proxy_header_library(
   fenv_t
   HDRS
diff --git a/libc/hdr/types/errno_t.h b/libc/hdr/types/errno_t.h
new file mode 100644
index 000000000000..d83692a6585d
--- /dev/null
+++ b/libc/hdr/types/errno_t.h
@@ -0,0 +1,30 @@
+//===-- Proxy for errno_t -------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_HDR_TYPES_ERRNO_T_H
+#define LLVM_LIBC_HDR_TYPES_ERRNO_T_H
+
+#ifndef __STDC_WANT_LIB_EXT1__
+#define __STDC_WANT_LIB_EXT1__ 1
+#endif
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/errno_t.h"
+
+#else // Overlay mode
+
+#include <errno.h>
+
+#endif // LIBC_FULL_BUILD
+
+#ifdef __STDC_WANT_LIB_EXT1__
+#undef __STDC_WANT_LIB_EXT1__
+#endif
+
+#endif // LLVM_LIBC_HDR_TYPES_ERRNO_T_H
diff --git a/libc/include/CMakeLists.txt b/libc/include/CMakeLists.txt
index b2ebd035872a..9eae20c820e9 100644
--- a/libc/include/CMakeLists.txt
+++ b/libc/include/CMakeLists.txt
@@ -318,6 +318,7 @@ add_header_macro(
   DEPENDS
     .llvm-libc-macros.generic_error_number_macros
     .llvm-libc-macros.error_number_macros
+    .llvm-libc-types.errno_t
 )
 
 add_header_macro(
diff --git a/libc/include/errno.yaml b/libc/include/errno.yaml
index 188a9fa1211a..86afd2f03b20 100644
--- a/libc/include/errno.yaml
+++ b/libc/include/errno.yaml
@@ -5,7 +5,8 @@ standards:
   - Linux
   - POSIX
 macros: []
-types: []
+types:
+  - type_name: errno_t
 enums: []
 objects: []
 functions: []
diff --git a/libc/include/llvm-libc-types/CMakeLists.txt b/libc/include/llvm-libc-types/CMakeLists.txt
index 5d82660fd676..09f9d759f0dd 100644
--- a/libc/include/llvm-libc-types/CMakeLists.txt
+++ b/libc/include/llvm-libc-types/CMakeLists.txt
@@ -29,6 +29,7 @@ add_header(double_t HDR double_t.h)
 add_header(DIR HDR DIR.h)
 add_header(dev_t HDR dev_t.h)
 add_header(div_t HDR div_t.h)
+add_header(errno_t HDR errno_t.h DEPENDS libc.include.llvm-libc-macros.annex_k_macros)
 add_header(ldiv_t HDR ldiv_t.h)
 add_header(lldiv_t HDR lldiv_t.h)
 add_header(FILE HDR FILE.h)
diff --git a/libc/include/llvm-libc-types/errno_t.h b/libc/include/llvm-libc-types/errno_t.h
new file mode 100644
index 000000000000..4ce694959ef2
--- /dev/null
+++ b/libc/include/llvm-libc-types/errno_t.h
@@ -0,0 +1,26 @@
+//===-- Definition of type errno_t ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_INCLUDE_LLVM_LIBC_TYPES_ERRNO_T_H
+#define LLVM_LIBC_INCLUDE_LLVM_LIBC_TYPES_ERRNO_T_H
+
+#include "../llvm-libc-macros/annex-k-macros.h"
+
+// LIBC_HAS_ANNEX_K is used to check whether C11 Annex K (the optional
+// “Bounds-checking interfaces”) is enabled in this libc implementation. Annex K
+// introduces additional types and functions, including `errno_t` (a typedef
+// used by the *_s functions). Since `errno_t` is *not defined* in the standard
+// library unless Annex K is enabled, we must guard any code that uses it with
+// LIBC_HAS_ANNEX_K.
+#ifdef LIBC_HAS_ANNEX_K
+
+typedef int errno_t;
+
+#endif // LIBC_HAS_ANNEX_K
+
+#endif // LLVM_LIBC_INCLUDE_LLVM_LIBC_TYPES_ERRNO_T_H