sigpending.2: Add discussion of rt_sigpending(2)

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

man2/sigpending.2

@@ -28,7 +28,7 @@
 .\"
 .TH SIGPENDING 2 2013-12-11 "Linux" "Linux Programmer's Manual"
 .SH NAME
-sigpending \- examine pending signals
+sigpending, rt_sigpending \- examine pending signals
 .SH SYNOPSIS
 .B #include <signal.h>
 .sp
@@ -82,6 +82,34 @@ A child created via
 initially has an empty pending signal set;
 the pending signal set is preserved across an
 .BR execve (2).
+.\"
+.SS C library/kernel ABI differences
+The original Linux system call was named
+.BR sigpending ().
+However, with the addition of real-time signals in Linux 2.2,
+the fixed-size, 32-bit
+.IR sigset_t
+argument supported by that system call was no longer fit for purpose.
+Consequently, a new system call,
+.BR rt_sigpending (),
+was added to support an enlarged
+.IR sigset_t
+type.
+The new system call takes a second argument,
+.IR "size_t sigsetsize" ,
+which specifies the size in bytes of the signal set in
+.IR set .
+.\" This argument is currently required to be less than or equal to
+.\" .IR sizeof(sigset_t)
+.\" (or the error
+.\" .B EINVAL
+.\" results).
+The glibc
+.BR sigpending ()
+wrapper function hides these details from us, transparently calling
+.BR rt_sigpending ()
+when the kernel provides it.
+.\"
 .SH BUGS
 In versions of glibc up to and including 2.2.1,
 there is a bug in the wrapper function for