sigaltstack.2: Add use os sigaction() to example code

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

man2/sigaltstack.2

@@ -288,7 +288,13 @@ different struct, and had the major disadvantage that the caller
 had to know the direction of stack growth.
 .SH EXAMPLE
 The following code segment demonstrates the use of
-.BR sigaltstack ():
+.BR sigaltstack ()
+(and
+.BR sigaction (2))
+to install an alternate signal stack that is employed by a handler
+for the
+.BR SIGSEGV
+signal:
 .PP
 .in +4n
 .EX
@@ -306,6 +312,14 @@ if (sigaltstack(&ss, NULL) == \-1)
     perror("sigaltstack");
     exit(EXIT_FAILURE);
 }
+
+sa.sa_flags = SA_ONSTACK;
+sa.sa_handler = handler();      /* Address of a signal handler */
+sigemptyset(&sa.sa_mask);
+if (sigaction(SIGSEGV, &sa, NULL) == -1) {
+    perror("sigaction");
+    exit(EXIT_FAILURE);
+}
 .EE
 .in
 .SH BUGS