Port to strict C99 struct hack

Portability bug caught by GCC 13 -fstrict-flex-arrays.
* gnulib.modules: Add flexmember.
* src/create.c (struct link):
* src/exclist.c (struct excfile):
* src/extract.c (struct delayed_link, struct string_list):
Include <flexmember.h>.  Use FLEXIBLE_ARRAY_MEMBER, for
portability to strict C99 or later.  All storage
allocations changed to use FLEXNSIZEOF.

gnulib.modules

@@ -41,6 +41,7 @@ fdopendir
 fdutimensat
 file-has-acl
 fileblocks
+flexmember
 fnmatch-gnu
 fprintftime
 free-posix

src/create.c

@@ -22,6 +22,7 @@
 #include <system.h>
 
 #include <areadlink.h>
+#include <flexmember.h>
 #include <quotearg.h>
 
 #include "common.h"
@@ -36,7 +37,7 @@ struct link
     dev_t dev;
     ino_t ino;
     nlink_t nlink;
-    char name[1];
+    char name[FLEXIBLE_ARRAY_MEMBER];
   };
 
 struct exclusion_tag
@@ -1518,8 +1519,7 @@ file_count_links (struct tar_stat_info *st)
 						   absolute_names_option));
       transform_name (&linkname, XFORM_LINK);
 
-      lp = xmalloc (offsetof (struct link, name)
-				 + strlen (linkname) + 1);
+      lp = xmalloc (FLEXNSIZEOF (struct link, name, strlen (linkname) + 1));
       lp->ino = st->stat.st_ino;
       lp->dev = st->stat.st_dev;
       lp->nlink = st->stat.st_nlink;

src/exclist.c

@@ -19,6 +19,7 @@
 */
 #include <system.h>
 #include <quotearg.h>
+#include <flexmember.h>
 #include <fnmatch.h>
 #include <wordsplit.h>
 #include "common.h"
@@ -40,7 +41,7 @@ struct excfile
 {
   struct excfile *next;
   int flags;
-  char name[1];
+  char name[FLEXIBLE_ARRAY_MEMBER];
 };
 
 static struct excfile *excfile_head, *excfile_tail;
@@ -48,7 +49,8 @@ static struct excfile *excfile_head, *excfile_tail;
 void
 excfile_add (const char *name, int flags)
 {
-  struct excfile *p = xmalloc (sizeof (*p) + strlen (name));
+  struct excfile *p = xmalloc (FLEXNSIZEOF (struct excfile, name,
+					    strlen (name) + 1));
   p->next = NULL;
   p->flags = flags;
   strcpy (p->name, name);

src/extract.c

@@ -22,6 +22,7 @@
 #include <system.h>
 #include <quotearg.h>
 #include <errno.h>
+#include <flexmember.h>
 #include <hash.h>
 #include <priv-set.h>
 #include <root-uid.h>
@@ -180,7 +181,7 @@ struct delayed_link
     struct xattr_map xattr_map;
 
     /* The desired target of the desired link.  */
-    char target[1];
+    char target[FLEXIBLE_ARRAY_MEMBER];
   };
 
 static Hash_table *delayed_link_table;
@@ -188,7 +189,7 @@ static Hash_table *delayed_link_table;
 struct string_list
   {
     struct string_list *next;
-    char string[1];
+    char string[FLEXIBLE_ARRAY_MEMBER];
   };
 
 static size_t
@@ -1134,7 +1135,7 @@ extract_dir (char *file_name, int typeflag)
 	      status = 0;
 	      break;
 	    }
-	  
+
 	  errno = EEXIST;
 	}
 
@@ -1470,8 +1471,8 @@ create_placeholder_file (char *file_name, bool is_symlink, bool *interdir_made,
 	  p->mtime = current_stat_info.mtime;
 	}
       p->change_dir = chdir_current;
-      p->sources = xmalloc (offsetof (struct string_list, string)
-			     + strlen (file_name) + 1);
+      p->sources = xmalloc (FLEXNSIZEOF (struct string_list, string,
+					 strlen (file_name) + 1));
       p->sources->next = 0;
       strcpy (p->sources->string, file_name);
       p->cntx_name = NULL;
@@ -1534,8 +1535,9 @@ extract_link (char *file_name, int typeflag)
 	      if (ds && ds->change_dir == chdir_current
 		  && BIRTHTIME_EQ (ds->birthtime, get_stat_birthtime (&st1)))
 		{
-		  struct string_list *p =  xmalloc (offsetof (struct string_list, string)
-						    + strlen (file_name) + 1);
+		  struct string_list *p
+		    = xmalloc (FLEXNSIZEOF (struct string_list,
+					    string, strlen (file_name) + 1));
 		  strcpy (p->string, file_name);
 		  p->next = ds->sources;
 		  ds->sources = p;