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Date: Mon, 04 Sep 2017 09:10:00 -0000
To: libc-alpha@sourceware.org
Subject: [PATCH] math.h: Warn about an already-defined log macro
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Message-Id: <20170904090959.276DE439942E3@oldenburg.str.redhat.com>
From: fweimer@redhat.com (Florian Weimer)
X-SW-Source: 2017-09/txt/msg00155.txt.bz2

This is a common programming error, and the cause of the problem is not
always obvious.

<tgmath.h> defines a log macro, but it includes <math.h> before that, so
that is compatible with the warning.

2017-09-04  Florian Weimer  <fweimer@redhat.com>

	* math/math.h: Issue warning if log is defined.

diff --git a/math/math.h b/math/math.h
index 547b36d10e..6c2ad97fb8 100644
--- a/math/math.h
+++ b/math/math.h
@@ -26,6 +26,11 @@
 #define __GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION
 #include <bits/libc-header-start.h>
 
+#if defined log && defined __GNUC__
+# warning A macro called log was already defined when <math.h> was included.
+# warning This will cause compilation problems.
+#endif
+
 __BEGIN_DECLS
 
 /* Get definitions of __intmax_t and __uintmax_t.  */