c-family: Allow arguments with NULLPTR_TYPE as sentinels [PR114780]

While in C++ the ellipsis argument conversions include
"An argument that has type cv std::nullptr_t is converted to type void*"
in C23 a nullptr_t argument is not promoted in any way, but va_arg
description says:
"the type of the next argument is nullptr_t and type is a pointer type that has the same
representation and alignment requirements as a pointer to a character type."
So, while in C++ check_function_sentinel will never see NULLPTR_TYPE, for
C23 it can see that and currently we incorrectly warn about those.

The only question is whether we should warn on any argument with
nullptr_t type or just about nullptr (nullptr_t argument with integer_zerop
value).  Through undefined behavior guess one could pass non-NULL pointer
that way, say by union { void *p; nullptr_t q; } u; u.p = &whatever;
and pass u.q to ..., but valid code should always pass something that will
read as (char *) 0 when read using va_arg (ap, char *), so I think it is
better not to warn rather than warn in those cases.

Note, clang seems to pass (void *)0 rather than expression of nullptr_t
type to ellipsis in C23 mode as if it did the C++ ellipsis argument
conversions, in that case guess not warning about that would be even safer,
but what GCC does I think follows the spec more closely, even when in a
valid program one shouldn't be able to observe the difference.

2024-04-20  Jakub Jelinek  <jakub@redhat.com>

	PR c/114780
	* c-common.cc (check_function_sentinel): Allow as sentinel any
	argument of NULLPTR_TYPE.

	* gcc.dg/format/sentinel-2.c: New test.

diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index 6fa8243b02b5eda3c427cc210c58e3ed677d8460..01e3d247fc281e3c5ef9e2d9fb20235cf99a7c87 100644 (file)
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -5783,6 +5783,7 @@ check_function_sentinel (const_tree fntype, int nargs, tree *argarray)
       sentinel = fold_for_warn (argarray[nargs - 1 - pos]);
       if ((!POINTER_TYPE_P (TREE_TYPE (sentinel))
           || !integer_zerop (sentinel))
+         && TREE_CODE (TREE_TYPE (sentinel)) != NULLPTR_TYPE
          /* Although __null (in C++) is only an integer we allow it
             nevertheless, as we are guaranteed that it's exactly
             as wide as a pointer, and we don't want to force

diff --git a/gcc/testsuite/gcc.dg/format/sentinel-2.c b/gcc/testsuite/gcc.dg/format/sentinel-2.c
new file mode 100644 (file)
index 0000000..4c29f6f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/format/sentinel-2.c
@@ -0,0 +1,21 @@
+/* PR c/114780 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -Wformat" } */
+
+#include <stddef.h>
+
+[[gnu::sentinel]] void foo (int, ...);
+[[gnu::sentinel]] void bar (...);
+
+void
+baz (nullptr_t p)
+{
+  foo (1, 2, nullptr);
+  foo (3, 4, 5, p);
+  bar (nullptr);
+  bar (p);
+  foo (6, 7, 0);       // { dg-warning "missing sentinel in function call" }
+  bar (0);             // { dg-warning "missing sentinel in function call" }
+  foo (8, 9, NULL);
+  bar (NULL);
+}