sigprocmask.2: Add discussion of rt_sigprocmask(2)

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

man2/sigprocmask.2

@@ -28,7 +28,7 @@
 .\"
 .TH SIGPROCMASK 2 2013-04-19 "Linux" "Linux Programmer's Manual"
 .SH NAME
-sigprocmask \- examine and change blocked signals
+sigprocmask, rt_sigprocmask \- examine and change blocked signals
 .SH SYNOPSIS
 .B #include <signal.h>
 .sp
@@ -141,6 +141,36 @@ or
 See
 .BR sigsetops (3)
 for details on manipulating signal sets.
+.\"
+.SS C library/kernel ABI differences
+The original Linux system call was named
+.BR sigprocmask ().
+However, with the addition of real-time signals in Linux 2.2,
+the fixed-size, 32-bit
+.IR sigset_t
+type supported by that system call was no longer fit for purpose.
+Consequently, a new system call,
+.BR rt_sigprocmask (),
+was added to support an enlarged
+.IR sigset_t
+type.
+The new system call takes a fourth argument,
+.IR "size_t sigsetsize" ,
+which specifies the size in bytes of the signal sets in
+.IR set
+and
+.IR oldset .
+This argument is currently required to have the value
+.IR sizeof(sigset_t)
+(or the error
+.B EINVAL
+results).
+The glibc
+.BR sigprocmask ()
+wrapper function hides these details from us, transparently calling
+.BR rt_sigprocmask ()
+when the kernel provides it.
+.\"
 .SH SEE ALSO
 .BR kill (2),
 .BR pause (2),