strlcpy.h: Add STRLCPY() macro

It wraps strlcpy(3bsd) so that it performs some steps that one might
forget, or might be prone to accidents:

-  It calculates the size of the destination buffer, and makes sure it's
   an array (otherwise, using sizeof(dst) would be very bad).

-  It calculates if there's truncation, returning an easy-to-use value.

BTW, this macro doesn't have any issues of double evaluation, because
sizeof() doesn't evaluate its argument (unless it's a VLA, but then
the static_assert(3) within SIZEOF_ARRAY() makes sure VLAs are not
allowed).

Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/Makefile.am

@@ -140,6 +140,7 @@ libshadow_la_SOURCES = \
 	stpecpy.h \
 	stpeprintf.c \
 	stpeprintf.h \
+	strlcpy.h \
 	strtoday.c \
 	sub.c \
 	subordinateio.h \

lib/strlcpy.h

@@ -0,0 +1,56 @@
+/*
+ * SPDX-FileCopyrightText: 2023, Alejandro Colomar <alx@kernel.org>
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+
+#ifndef SHADOW_INCLUDE_LIB_STRLCPY_H_
+#define SHADOW_INCLUDE_LIB_STRLCPY_H_
+
+
+#include <config.h>
+
+#include <stddef.h>
+#include <string.h>
+
+#include "sizeof.h"
+
+
+/*
+ * SYNOPSIS
+ *	int STRLCPY(char dst[restrict], const char *restrict src);
+ *
+ * ARGUMENTS
+ *	dst	Destination buffer where to copy a string.
+ *	src	Source string to be copied into dst.
+ *
+ * DESCRIPTION
+ *	This macro copies the string pointed to by src, into a string
+ *	at the buffer pointed to by dst.  If the destination buffer,
+ *	isn't large enough to hold the copy, the resulting string is
+ *	truncated.  The size of the buffer is calculated internally via
+ *	SIZEOF_ARRAY().
+ *
+ * RETURN VALUE
+ *	-1	If this call truncated the resulting string.
+ *
+ *	strlen(dst)
+ *		On success.
+ *
+ * ERRORS
+ *	This function doesn't set errno.
+ */
+
+
+#define STRLCPY(dst, src)                                                     \
+({                                                                            \
+	size_t  sz_, len_;                                                    \
+                                                                              \
+	sz_ = SIZEOF_ARRAY(dst);                                              \
+	len_ = strlcpy(dst, src, sz_);                                        \
+                                                                              \
+	(len_ >= sz_) ? -1 : len_;                                            \
+})
+
+
+#endif  // include guard