makecontext.3: Add text on use of pointer arguments to makecontext()

Passing pointer arguments to makecontext() is possible,
but only on some architectures, and with no guarantees
of portability.

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=504699

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Reported-by: Paul Evans <leonerd@leonerd.org.uk>

man3/makecontext.3

@@ -22,7 +22,7 @@
 .\" the source, must acknowledge the copyright and authors of this work.
 .\" 2006-08-02, mtk, Added example program
 .\"
-.TH MAKECONTEXT 3 2008-08-06 "GNU" "Linux Programmer's Manual"
+.TH MAKECONTEXT 3 2008-11-07 "GNU" "Linux Programmer's Manual"
 .SH NAME
 makecontext, swapcontext \- manipulate user context
 .SH SYNOPSIS
@@ -113,6 +113,22 @@ to be used as the stack, regardless of the direction of growth of
 the stack.
 Thus, it is not necessary for the user program to
 worry about this direction.
+
+On architectures where
+.I int
+and pointer types are the same size
+(e.g., x86-32, where both types are 32 bits),
+you may be able to get away with passing pointers as arguments to
+.BR makecontext ()
+following
+.IR argc .
+However, doing this is not guaranteed to be portable,
+is undefined according to the standards,
+and won't work on architectures where pointers are larger than
+.IR int s.
+Nevertheless, starting with version 2.8 glibc makes some changes to
+.BR makecontext (3),
+to permit this on some 64-bit architectures (e.g., x86-64).
 .SH EXAMPLE
 .PP
 The example program below demonstrates the use of