From dfb1232ca856020e65d1a3407e1f094d59d62aa4 Mon Sep 17 00:00:00 2001
From: Michael Kerrisk <mtk.manpages@gmail.com>
Date: Mon, 10 Mar 2014 10:05:24 +0100
Subject: [PATCH] feature_test_macros.7: Improve discussion of _DEFAULT_SOURCE

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
---
 man7/feature_test_macros.7 | 45 ++++++++++++++++++++++++++++----------
 1 file changed, 34 insertions(+), 11 deletions(-)

diff --git a/man7/feature_test_macros.7 b/man7/feature_test_macros.7
index 468d8bed5..6d7a48f08 100644
--- a/man7/feature_test_macros.7
+++ b/man7/feature_test_macros.7
@@ -22,7 +22,7 @@
 .\" the source, must acknowledge the copyright and authors of this work.
 .\" %%%LICENSE_END
 .\"
-.TH FEATURE_TEST_MACROS 7 2014-01-06 "Linux" "Linux Programmer's Manual"
+.TH FEATURE_TEST_MACROS 7 2014-03-10 "Linux" "Linux Programmer's Manual"
 .SH NAME
 feature_test_macros \- feature test macros
 .SH SYNOPSIS
@@ -282,17 +282,24 @@ System V-derived definitions.
 .BR standards (7).)
 .TP
 .BR _DEFAULT_SOURCE " (since glibc 2.19)"
-Defining this macro provides an effect similar to
-the feature test macros that are defined by default; that is:
-
-    cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809
-
 This macro can be defined to ensure that the "default"
 definitions are provided even when the defaults would otherwise
 be disabled,
 as happens when individual macros are explicitly defined,
 or the compiler is invoked in one of its "standard" modes (e.g.,
 .IR "cc\ \-std=c99" ).
+Defining
+.B _DEFAULT_SOURCE
+without defining other individual macros
+or invoking the compiler in one of its "standard" modes has no effect.
+
+The "default" definitions comprise those required by POSIX.1-2008
+as well as various definitions derived from BSD and System V.
+On glibc 2.19 and earlier, these defaults were approximately equivalent
+to explicitly defining the following:
+
+    cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809
+
 .TP
 .BR _ATFILE_SOURCE " (since glibc 2.4)"
 Defining this macro with any value causes header files to expose
@@ -305,8 +312,6 @@ is defined with a value greater than or equal to 200809L.
 .TP
 .B _GNU_SOURCE
 Defining this macro (with any value) is equivalent to defining
-.BR _BSD_SOURCE ,
-.BR _SVID_SOURCE ,
 .BR _ATFILE_SOURCE ,
 .BR _LARGEFILE64_SOURCE ,
 .BR _ISOC99_SOURCE ,
@@ -323,7 +328,17 @@ with the value 700
 (600 in glibc versions before 2.10;
 500 in glibc versions before 2.2).
 In addition, various GNU-specific extensions are also exposed.
-Where standards conflict, BSD definitions are disfavored.
+
+Since glibc 2.19, defining
+.BR _GNU_SOURCE
+also has the effect of defining
+.BR _DEFAULT_SOURCE .
+In glibc versions before 2.20, defining
+.BR _GNU_SOURCE
+also had the effect of defining
+.BR _BSD_SOURCE
+and
+.BR _SVID_SOURCE .
 .TP
 .B _REENTRANT
 Defining this macro exposes definitions of certain reentrant functions.
@@ -387,6 +402,8 @@ If no feature test macros are explicitly defined,
 then the following feature test macros are defined by default:
 .BR _BSD_SOURCE ,
 .BR _SVID_SOURCE ,
+.BR _DEFAULT_SOURCE
+(since glibc 2.19),
 .BR _POSIX_SOURCE ,
 and
 .BR _POSIX_C_SOURCE =200809L
@@ -405,9 +422,10 @@ If any of
 or
 .B _SVID_SOURCE
 is explicitly defined, then
-.BR _BSD_SOURCE
+.BR _BSD_SOURCE ,
+.BR _SVID_SOURCE ,
 and
-.B _SVID_SOURCE
+.BR _DEFAULT_SOURCE
 are not defined by default.
 
 If
@@ -483,6 +501,7 @@ but is employed on some other implementations.
 
 .BR _BSD_SOURCE ,
 .BR _SVID_SOURCE ,
+.BR _DEFAULT_SOURCE ,
 .BR _ATFILE_SOURCE ,
 .BR _GNU_SOURCE ,
 .BR _FORTIFY_SOURCE ,
@@ -596,6 +615,10 @@ main(int argc, char *argv[])
     printf("_SVID_SOURCE defined\\n");
 #endif
 
+#ifdef _DEFAULT_SOURCE
+    printf("_DEFAULT_SOURCE defined\\n");
+#endif
+
 #ifdef _ATFILE_SOURCE
     printf("_ATFILE_SOURCE defined\\n");
 #endif