Be a bit more consistent about comparing to zero

* src/buffer.c (xclose, archive_is_dev, close_archive)
(write_fatal_details, init_volume_number)
(closeout_volume_number, new_volume, try_new_volume):
* src/checkpoint.c (format_checkpoint_string):
* src/compare.c (process_rawdata, diff_file, diff_dumpdir):
* src/create.c (create_archive, restore_parent_fd, dump_file0):
* src/delete.c (delete_archive_members):
* src/exclist.c (cvs_addfn):
* src/extract.c (set_mode, mark_after_links, delay_set_stat)
(repair_delayed_set_stat, make_directories, file_newer_p)
(maybe_recoverable, apply_nonancestor_delayed_set_stat)
(extract_dir, open_output_file, find_delayed_link_source)
(create_placeholder_file, extract_symlink, extract_node)
(extract_fifo, apply_delayed_link):
* src/incremen.c (update_parent_directory, scan_directory)
(read_obstack, read_incr_db_2, write_directory_file)
(try_purge_directory):
* src/map.c (map_read):
* src/misc.c (maybe_backup_file, undo_last_backup, chdir_do)
(tar_savedir):
* src/names.c (handle_file_selection_option, add_file_id)
(handle_option, read_next_name, add_hierarchy_to_namelist)
(collect_and_sort_names):
* src/system.c (run_decompress_program, dec_to_env, time_to_env)
(oct_to_env, str_to_env, chr_to_env, sys_exec_setmtime_script):
* src/tar.c (get_date_or_file, parse_default_options)
(decode_options, main):
* src/unlink.c (flush_deferred_unlinks):
* src/update.c (append_file):
* src/xattrs.c (xattrs__acls_set, xattrs_xattrs_set):
Prefer < 0 when looking at syscalls; prefer != 0 to nothing
when testing an integer in a boolean context.
This is for style, not substance; for example, it’s easier
to read ‘if (wordsplit (...) != WRDSE_OK) ...’ than
‘if (wordsplit (...)) ...’ if you don’t already know that
wordsplit returns an enum rather than bool.
* src/names.c (add_file_id, read_next_name, regex_usage_warning):
* src/transform.c (parse_xform_flags):
Return bool not int, possibly inverting sense so that true means OK.
All callers changed.
* src/tar.c (main): Report errno info if stdopen fails.

src/misc.c

@@ -765,7 +765,7 @@ maybe_backup_file (const char *file_name, bool this_is_the_archive)
   if (this_is_the_archive && _remdev (file_name))
     return true;
 
-  if (deref_stat (file_name, &file_stat) != 0)
+  if (deref_stat (file_name, &file_stat) < 0)
     {
       if (errno == ENOENT)
 	return true;
@@ -814,7 +814,7 @@ undo_last_backup (void)
   if (after_backup_name)
     {
       if (renameat (chdir_fd, after_backup_name, chdir_fd, before_backup_name)
-	  != 0)
+	  < 0)
 	{
 	  int e = errno;
 	  paxerror (e, _("%s: Cannot rename to %s"),
@@ -1023,7 +1023,7 @@ chdir_do (idx_t i)
 	  else
 	    {
 	      struct wd *stale = &wd[wdcache[CHDIR_CACHE_SIZE - 1]];
-	      if (close (stale->fd) != 0)
+	      if (close (stale->fd) < 0)
 		close_diag (stale->name);
 	      stale->fd = 0;
 	      wdcache[CHDIR_CACHE_SIZE - 1] = i;
@@ -1315,7 +1315,7 @@ tar_savedir (const char *name, int must_exist)
 	      && (ret = streamsavedir (dir, savedir_sort_order))))
     savedir_error (name);
 
-  if (dir ? closedir (dir) != 0 : 0 <= fd && close (fd) != 0)
+  if (dir ? closedir (dir) < 0 : 0 <= fd && close (fd) < 0)
     savedir_error (name);
 
   return ret;