lib/libc: implement C23 memalignment()

This new function computes the alignment of a pointer.
It is part of ISO/IEC 9899:2024, the new C standard.
If the pointer is a null pointer, null is returned.
I have tried to write an implementation that can cope
with traditional address-based architectures, even if
size_t and uintptr_t are of different length.  Adjustments
may be needed for CHERI though.

A man page is provided, too.  No unit test for now.

Reviewed by:	kib, imp, ziaee (manpages), pauamma@gundo.com
Approved by:	markj (mentor)
MFC after:	1 month
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D53673

(cherry picked from commit 6c57e368eb1777f6097158eeca2fcc175d068dba)

1 files changed, 28 insertions, 0 deletions

diff --git a/lib/libc/stdlib/memalignment.c b/lib/libc/stdlib/memalignment.c
new file mode 100644
index 000000000000..771ddc2f5253
--- /dev/null
+++ b/lib/libc/stdlib/memalignment.c
@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+
+size_t
+memalignment(const void *p)
+{
+	uintptr_t align;
+
+	if (p == NULL)
+		return (0);
+
+	align = (uintptr_t)p;
+	align &= -align;
+
+#if UINTPTR_MAX > SIZE_MAX
+	/* if alignment overflows size_t, return maximum possible */
+	if (align > SIZE_MAX)
+		align = SIZE_MAX - SIZE_MAX/2;
+#endif
+
+	return (align);
+}