Implement C23 memalignment

Add the C23 memalignment function (query the alignment of a pointer)
to glibc.

Given how simple this operation is, it would make sense for compilers
to inline calls to this function, but I'm treating that as a compiler
matter (compilers should add it as a built-in function) rather than
adding an inline version to glibc headers (although such an inline
version would be reasonable as well).  I've filed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122117 for this feature
in GCC.

Tested for x86_64 and x86.

NEWS

@@ -11,6 +11,8 @@ Major new features:
 
 * The ISO C23 memset_explicit function has been added.
 
+* The ISO C23 memalignment function has been added.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * Support for dumped heaps has been removed - malloc_set_state() now always

manual/memory.texi

@@ -1135,6 +1135,61 @@ The @code{valloc} function is obsolete and @code{aligned_alloc} or
 @code{posix_memalign} should be used instead.
 @end deftypefun
 
+You can determine the alignment of a pointer with the
+@code{memalignment} function.
+
+@deftypefun size_t memalignment (void *@var{p})
+@standards{C23, stdlib.h}
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
+
+This function, defined in C23, returns the alignment of @var{p}, as a
+power of two.  If @var{p} is a null pointer, it returns zero.  C23
+requires @var{p} to be a valid pointer to an object or a null pointer;
+as a GNU extension, @theglibc{} supports this function on arbitrary
+bit patterns of pointer type.
+
+This function was added to the C23 standard to support unconventional
+platforms where a pointer's low-order bits are unrelated to alignment.
+For conventional platforms, one can instead cast the pointer to
+@code{uintptr_t} and then test the low order bits:
+this is portable to pre-C23 and is typically a bit faster.
+
+For example, if you want to read an @code{int}
+addressed by possibly-misaligned pointer @code{p},
+the following pre-C23 code works on all conventional platforms:
+
+@smallexample
+int i;
+if (((uintptr_t) p & (alignof (int) - 1)) != 0)
+  memcpy (&i, p, sizeof i);
+else
+  i = *p;
+@end smallexample
+
+However, it might not work on unconventional platforms, where one
+would need something like the following C23 code:
+
+@smallexample
+int i;
+if (memalignment (p) < alignof (int))
+  memcpy (&i, p, sizeof i);
+else
+  i = *p;
+@end smallexample
+
+However, for this particular case, performance does not improve if
+different code is used for aligned and unaligned pointers,
+and the following code is preferable:
+
+@smallexample
+int i;
+memcpy (&i, p, sizeof i);
+@end smallexample
+
+The compiler will generate the most efficient way to access unaligned
+data for the architecture, optimizing away the @code{memcpy} call.
+@end deftypefun
+
 @node Malloc Tunable Parameters
 @subsubsection Malloc Tunable Parameters
 

stdlib/Makefile

@@ -97,6 +97,7 @@ routines := \
   mblen \
   mbstowcs \
   mbtowc \
+  memalignment \
   mrand48 \
   mrand48_r \
   nrand48 \
@@ -299,6 +300,7 @@ tests := \
   tst-makecontext-align \
   tst-makecontext2 \
   tst-makecontext3 \
+  tst-memalignment \
   tst-on_exit \
   tst-qsort \
   tst-qsort2 \
@@ -441,6 +443,8 @@ CFLAGS-tst-stdc_bit_width.c += -fno-builtin
 CFLAGS-tst-stdc_bit_floor.c += -fno-builtin
 CFLAGS-tst-stdc_bit_ceil.c += -fno-builtin
 
+CFLAGS-tst-memalignment.c += -fno-builtin
+
 ifeq ($(have-cxx-thread_local),yes)
 CFLAGS-tst-quick_exit.o = -std=c++11
 LDLIBS-tst-quick_exit = -lstdc++

stdlib/Versions

@@ -229,6 +229,9 @@ libc {
     ulabs;
     ullabs;
   }
+  GLIBC_2.43 {
+    memalignment;
+  }
   GLIBC_PRIVATE {
     # functions which have an additional interface since they are
     # are cancelable.

stdlib/memalignment.c

@@ -0,0 +1,27 @@
+/* Return the alignment of a pointer.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdlib.h>
+
+/* Return the alignment of P.  */
+size_t
+memalignment (const void *p)
+{
+  size_t i = (size_t) p;
+  return i & -i;
+}

stdlib/stdlib.h

@@ -1164,6 +1164,9 @@ extern int ttyslot (void) __THROW;
 /* Call function __FUNC exactly once, even if invoked from several threads.
    All calls must be made with the same __FLAGS object.  */
 extern void call_once (once_flag *__flag, void (*__func)(void));
+
+/* Return the alignment of P.  */
+extern size_t memalignment (const void *__p);
 #endif
 
 #include <bits/stdlib-float.h>

stdlib/tst-memalignment.c

@@ -0,0 +1,43 @@
+/* Test memalignment.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdlib.h>
+#include <array_length.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+  void *null = NULL;
+  TEST_COMPARE (memalignment (NULL), 0);
+  TEST_COMPARE (memalignment (null), 0);
+  char ca[256];
+  array_foreach (p, ca)
+    TEST_VERIFY (memalignment (p) >= 1);
+  TEST_VERIFY (memalignment (&ca[0]) == 1 || memalignment (&ca[1]) == 1);
+  TEST_VERIFY (memalignment (&ca[0]) == 2 || memalignment (&ca[1]) == 2
+	       || memalignment (&ca[2]) == 2 || memalignment (&ca[3]) == 2);
+  long long int lla[256];
+  array_foreach (p, lla)
+    TEST_VERIFY (memalignment (p) >= _Alignof (long long int));
+  TEST_VERIFY (memalignment (&lla[0]) <= sizeof (long long int)
+	       || memalignment (&lla[1]) <= sizeof (long long int));
+  return 0;
+}
+
+#include <support/test-driver.c>

sysdeps/mach/hurd/i386/libc.abilist

@@ -2663,6 +2663,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.43 pthread_cancel F
 GLIBC_2.43 pthread_clockjoin_np F

sysdeps/mach/hurd/x86_64/libc.abilist

@@ -2344,6 +2344,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.43 pthread_cancel F
 GLIBC_2.43 pthread_clockjoin_np F

sysdeps/unix/sysv/linux/aarch64/libc.abilist

@@ -2768,4 +2768,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/alpha/libc.abilist

@@ -3115,6 +3115,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/arc/libc.abilist

@@ -2529,4 +2529,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/arm/be/libc.abilist

@@ -2821,6 +2821,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/arm/le/libc.abilist

@@ -2818,6 +2818,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/csky/libc.abilist

@@ -2805,4 +2805,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/hppa/libc.abilist

@@ -2842,6 +2842,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/i386/libc.abilist

@@ -3025,6 +3025,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/loongarch/lp64/libc.abilist

@@ -2289,4 +2289,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist

@@ -2801,6 +2801,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist

@@ -2968,6 +2968,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/microblaze/be/libc.abilist

@@ -2854,4 +2854,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/microblaze/le/libc.abilist

@@ -2851,4 +2851,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist

@@ -2931,6 +2931,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist

@@ -2929,6 +2929,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist

@@ -2937,6 +2937,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist

@@ -2839,6 +2839,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/or1k/libc.abilist

@@ -2279,4 +2279,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist

@@ -3158,6 +3158,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist

@@ -3203,6 +3203,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist

@@ -2912,6 +2912,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist

@@ -2988,4 +2988,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/riscv/rv32/libc.abilist

@@ -2532,4 +2532,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist

@@ -2732,4 +2732,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F

sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist

@@ -3156,6 +3156,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist

@@ -2949,6 +2949,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/sh/be/libc.abilist

@@ -2848,6 +2848,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/sh/le/libc.abilist

@@ -2845,6 +2845,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist

@@ -3179,6 +3179,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist

@@ -2815,6 +2815,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/x86_64/64/libc.abilist

@@ -2764,6 +2764,7 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F
 GLIBC_2.5 __readlinkat_chk F
 GLIBC_2.5 inet6_opt_append F

sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist

@@ -2783,4 +2783,5 @@ GLIBC_2.42 uimaxabs F
 GLIBC_2.42 ulabs F
 GLIBC_2.42 ullabs F
 GLIBC_2.43 __memset_explicit_chk F
+GLIBC_2.43 memalignment F
 GLIBC_2.43 memset_explicit F