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diff --git a/lib/filenamecat-lgpl.c b/lib/filenamecat-lgpl.c
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+/* Concatenate two arbitrary file names.
+
+ Copyright (C) 1996-2007, 2009-2020 Free Software Foundation, Inc.
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program 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 General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <https://www.gnu.org/licenses/>. */
+
+/* Written by Jim Meyering. */
+
+#include <config.h>
+
+/* Specification. */
+#include "filenamecat.h"
+
+#include <stdlib.h>
+#include <string.h>
+
+#include "basename-lgpl.h"
+#include "filename.h"
+
+#if ! HAVE_MEMPCPY && ! defined mempcpy
+# define mempcpy(D, S, N) ((void *) ((char *) memcpy (D, S, N) + (N)))
+#endif
+
+/* Concatenate two file name components, DIR and BASE, in
+ newly-allocated storage and return the result.
+ The resulting file name F is such that the commands "ls F" and "(cd
+ DIR; ls ./BASE)" refer to the same file. If necessary, put
+ a separator between DIR and BASE in the result. Typically this
+ separator is "/", but in rare cases it might be ".".
+ In any case, if BASE_IN_RESULT is non-NULL, set
+ *BASE_IN_RESULT to point to the copy of BASE at the end of the
+ returned concatenation.
+
+ If malloc fails, return NULL with errno set. */
+
+char *
+mfile_name_concat (char const *dir, char const *base, char **base_in_result)
+{
+ char const *dirbase = last_component (dir);
+ size_t dirbaselen = base_len (dirbase);
+ size_t dirlen = dirbase - dir + dirbaselen;
+ size_t baselen = strlen (base);
+ char sep = '\0';
+ if (dirbaselen)
+ {
+ /* DIR is not a file system root, so separate with / if needed. */
+ if (! ISSLASH (dir[dirlen - 1]) && ! ISSLASH (*base))
+ sep = '/';
+ }
+ else if (ISSLASH (*base))
+ {
+ /* DIR is a file system root and BASE begins with a slash, so
+ separate with ".". For example, if DIR is "/" and BASE is
+ "/foo" then return "/./foo", as "//foo" would be wrong on
+ some POSIX systems. A fancier algorithm could omit "." in
+ some cases but is not worth the trouble. */
+ sep = '.';
+ }
+
+ char *p_concat = malloc (dirlen + (sep != '\0') + baselen + 1);
+ if (p_concat == NULL)
+ return NULL;
+
+ {
+ char *p;
+
+ p = mempcpy (p_concat, dir, dirlen);
+ *p = sep;
+ p += sep != '\0';
+
+ if (base_in_result)
+ *base_in_result = p;
+
+ p = mempcpy (p, base, baselen);
+ *p = '\0';
+ }
+
+ return p_concat;
+}