[clang] [Clang][Docs] Documented sentinel attribute (PR #196088)

[Tony Guillot via cfe-commits]

https://github.com/to268 created https://github.com/llvm/llvm-project/pull/196088

The documentation of the sentinel attribute was missing, this PR documents the behavior of the sentinel attribute.

>From 5e9fccdc38bd681c064859ea8898c104a2bd5b2e Mon Sep 17 00:00:00 2001
From: Tony Guillot <tony.guillot at protonmail.com>
Date: Wed, 6 May 2026 16:48:40 +0200
Subject: [PATCH] Documented sentinel attribute

---
 clang/include/clang/Basic/Attr.td     |  2 +-
 clang/include/clang/Basic/AttrDocs.td | 28 +++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 04b7f420e1b32..3eeb58efa7e3b 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -3415,7 +3415,7 @@ def Sentinel : InheritableAttr {
   let Args = [DefaultIntArgument<"Sentinel", 0>,
               DefaultIntArgument<"NullPos", 0>];
 //  let Subjects = SubjectList<[Function, ObjCMethod, Block, Var]>;
-  let Documentation = [Undocumented];
+  let Documentation = [SentinelDocs];
 }
 
 def StdCall : DeclOrTypeAttr {
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index 67aef81dd3a81..d10b9539999e1 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -10000,3 +10000,31 @@ different languages to coexist on the same call stack while each interpreting
 exceptions according to their own rules.
   }];
 }
+
+def SentinelDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``sentinel`` attribute can only be applied to variadic functions to
+diagnose each function call that do not pass a sentinel value at the end of the
+argument list. It checks if a null pointer constant exists at the end of the
+argument list by default, or at the (N+1)th position starting from the end when
+an argument is provided.
+
+.. code-block:: c
+
+  #include <stddef.h>
+
+  void foo(const char*, ...) __attribute__((sentinel));
+  void bar(int, ...) __attribute__((sentinel(1)));
+
+  void example() {
+    foo("Example", (void*)0);
+    foo("Another", "example", NULL);
+    foo("Missing", "sentinel"); // Not OK
+
+    bar(1, 2, NULL, 3);         // OK: sentinel value at the 2nd to last positon
+    bar(1, 2, 3, nullptr, 4);   // OK: `nullptr` is valid in C23
+    bar(1, 2, 3, 4, NULL);      // Not OK
+  }
+  }];
+}