@item
POSIX does not specify whether @code{setjmp} saves the signal mask in the
@code{jmp_buf}. It does on BSD platforms, and on glibc platforms when
-@code{_BSD_SOURCE} is defined; in this case @code{setjmp} behaves like
-@code{sigsetjmp(.,1)}, and functions @code{_setjmp} and @code{_longjmp} are
-available that don't save or restore the signal mask. On System V platforms
-(excluding HP-UX), and on glibc platforms by default, @code{setjmp} doesn't
-save the signal mask.
+@code{_BSD_SOURCE} (and/or @code{_DEFAULT_SOURCE} on glibc >= 2.19) is defined;
+in this case @code{setjmp} behaves like @code{sigsetjmp(.,1)}, and functions
+@code{_setjmp} and @code{_longjmp} are available that don't save or restore
+the signal mask. On System V platforms (excluding HP-UX), and on glibc
+platforms by default, @code{setjmp} doesn't save the signal mask.
@end itemize