select.2: Add further details on pselect6() system call that underlies pselect()

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

man2/select.2

@@ -412,8 +412,16 @@ when
 is defined.
 Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
 .SS "Linux Notes"
-The Linux
+The
 .BR pselect ()
+interface described in this page is implemented by glibc.
+The underlying Linux system call is named
+.BR pselect6 ().
+This system call has somewhat different behavior from the gibc
+wrapper function.
+
+The Linux
+.BR pselect6 ()
 system call modifies its
 .I timeout
 argument.
@@ -426,6 +434,27 @@ function does not modify its
 .I timeout
 argument;
 this is the behavior required by POSIX.1-2001.
+
+The final argument of the
+.BR pselect6()
+system call is not a 
+.I "sigset_t\ *"
+pointer, but is instead a structure of the form:
+.in +4
+.nf
+
+struct {
+    const sigset_t *ss;     /* Pointer to signal set */
+    size_t          ss_len; /* Size (in bytes) of object pointed
+                               to by 'ss' */
+};
+
+.fi
+.in
+This allows the system call to obtain both
+a pointer to the signal set and its size,
+while allowing for the fact that most architectures
+support a maximum of 6 arguments to a system call.
 .SH BUGS
 Glibc 2.0 provided a version of
 .BR pselect ()