setjmp.3: Improve description of pre-2.19 glibc default behavior

Make it clearer that the default FTM settings result in
BSD semantics for setjmp().

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man3/setjmp.3 b/man3/setjmp.3
index 9a18554a482bcd134440e5165edcbe5f7a783a6f..85d019c86104d720f280d1205a1348e8c62ee6e1 100644 (file)
--- a/man3/setjmp.3
+++ b/man3/setjmp.3
@@ -172,12 +172,12 @@ In 4.3BSD it will, and there
 is a function
 .B _setjmp
 that will not.
+The behavior under Linux depends on the glibc version
+and the setting of feature text macros.
 On Linux with glibc versions before 2.19,
-.BR setjmp ()
-follows the System V behavior by default,
-but the BSD behavior is provided if the
+the BSD behavior is provided if (as per the feature test macro defaults) the
 .BR _BSD_SOURCE
-feature test macro is defined and none of
+feature test macro is (explicitly or implicitly) defined and none of
 .BR _POSIX_SOURCE ,
 .BR _POSIX_C_SOURCE ,
 .BR _XOPEN_SOURCE ,
@@ -185,7 +185,8 @@ feature test macro is defined and none of
 .BR _GNU_SOURCE ,
 or
 .B _SVID_SOURCE
-is defined.
+is explicitly defined;
+otherwise, the System V behavior is provided.
 Since glibc 2.19,
 .IR <setjmp.h>
 exposes only the System V version of