Add mempcpy(3)

We'll use it for implementing stpecpy(), and may be interesting to have
it around.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

configure.ac

@@ -47,7 +47,7 @@ AC_CHECK_HEADER([shadow.h],,[AC_MSG_ERROR([You need a libc with shadow.h])])
 
 AC_CHECK_FUNCS(arc4random_buf futimes \
 	getentropy getrandom getspnam getusershell \
-	initgroups lckpwdf lutimes \
+	initgroups lckpwdf lutimes mempcpy \
 	setgroups updwtmp updwtmpx innetgr \
 	getspnam_r \
 	memset_explicit explicit_bzero stpeprintf)

lib/mempcpy.h

@@ -0,0 +1,31 @@
+/*
+ * SPDX-FileCopyrightText:  2023, Alejandro Colomar <alx@kernel.org>
+ *
+ * SPDX-License-Identifier:  BSD-3-Clause
+ */
+
+
+#ifndef SHADOW_INCLUDE_LIB_MEMPCPY_H_
+#define SHADOW_INCLUDE_LIB_MEMPCPY_H_
+
+
+#include <config.h>
+
+#if !defined(HAVE_MEMPCPY)
+
+#include <stddef.h>
+#include <string.h>
+
+
+inline void *mempcpy(void *restrict dst, const void *restrict src, size_t n);
+
+
+inline void *
+mempcpy(void *restrict dst, const void *restrict src, size_t n)
+{
+	return memcpy(dst, src, n) + n;
+}
+
+
+#endif  // !HAVE_MEMPCPY
+#endif  // include guard

libmisc/Makefile.am

@@ -44,6 +44,7 @@ libmisc_la_SOURCES = \
 	list.c log.c \
 	loginprompt.c \
 	mail.c \
+	mempcpy.c \
 	motd.c \
 	myname.c \
 	obscure.c \

libmisc/mempcpy.c

@@ -0,0 +1,23 @@
+/*
+ * SPDX-FileCopyrightText:  2023, Alejandro Colomar <alx@kernel.org>
+ *
+ * SPDX-License-Identifier:  BSD-3-Clause
+ */
+
+
+#include <config.h>
+
+#if !defined(HAVE_MEMPCPY)
+
+#ident "$Id$"
+
+#include "mempcpy.h"
+
+#include <stddef.h>
+
+
+extern inline void *mempcpy(void *restrict dst, const void *restrict src,
+    size_t n);
+
+
+#endif