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From: Alex Yursha <alexyursha@gmail.com>
Date: Mon, 19 Jun 2017 15:12:29 -0700
Message-ID: 
 <CACWrwPFN_kmfxoT+mfC6+oGb_7CWMsSUwFQ90sgpvEdp3+g+ow@mail.gmail.com>
Subject: features.h - confusing branch logic in preprocessor directives
To: libc-alpha@sourceware.org

Hi all,

There seem to be some tech debt in `features.h` on glibc master branch
when falling back to _DEFAULT_SOURCE. See the patch below:

1. Comment redundantly mentions _GNU_SOURCE but it is not tested anywhere.
2. Testing for _DEFAULT_SOURCE and redefining it if its already
defined makes no sense.

index 972cbd2..aa1bad8 100644

--- include/features.h
+++ include/features.h
@@ -209,13 +209,11 @@
 # define _ATFILE_SOURCE  1
 #endif

-/* If nothing (other than _GNU_SOURCE and _DEFAULT_SOURCE) is defined,
-   define _DEFAULT_SOURCE.  */
-#if (defined _DEFAULT_SOURCE         \
-     || (!defined __STRICT_ANSI__        \
+/* If nothing is defined, define _DEFAULT_SOURCE.  */
+#if (!defined __STRICT_ANSI__        \
   && !defined _ISOC99_SOURCE       \
   && !defined _POSIX_SOURCE && !defined _POSIX_C_SOURCE  \
-  && !defined _XOPEN_SOURCE))
+  && !defined _XOPEN_SOURCE)
 # undef  _DEFAULT_SOURCE
 # define _DEFAULT_SOURCE 1
 #endif