I'm trying to build python 2.6 on a Mac OS X 10.3 system. This fails to 
build the multiprocessing extension due to a missing definition of "struct 
iovec".

It turns out that you have to include <sys/uio.h> to get that definition 
on OSX 10.3.

Adding '#include <sys/uio.h>' to multiprocessing.h enables building the 
extension on OSX 10.3, but is probably not the right solution.

What does sendmsg(2) say what to include? What about writev(2)? (the
latter, on Linux, says that sys/uio.h is indeed the correct header)

The manpage for writev(2) mentions #include <sys/uio.h>. Adding a #include 
for <sys/uio.h> is the right fix after all...

This issues was fixed on 2008-11-04 by Martin Loewis in r67098

Index: multiprocessing.h
===================================================================
--- multiprocessing.h	(revision 67097)
+++ multiprocessing.h	(revision 67098)
@@ -20,7 +20,9 @@
 #  define SEM_VALUE_MAX LONG_MAX
 #else
 #  include <fcntl.h>                 /* O_CREAT and O_EXCL */
+#  include <netinet/in.h>
 #  include <sys/socket.h>
+#  include <sys/uio.h>
 #  include <arpa/inet.h>             /* htonl() and ntohl() */
 #  if HAVE_SEM_OPEN
 #    include <semaphore.h>

Closing