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diff --git a/sys/etc/propen.x b/sys/etc/propen.x
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+# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+
+include	<knet.h>
+
+# PROPEN -- Open a connected subprocess.  Call PROPCPR to spawn the child
+# process and connect the input and output IPC channels to FIO file
+# descriptors, then pass the environment list and current working directory
+# to the child.  The child is left waiting for the next command from the
+# parent, which is eventually written to the OUT channel by the parent.
+#
+# N.B.: If the child cannot process a SET or CHDIR command it is will take
+# a panic exit, raising the X_IPC exception in the parent.  This is necessary
+# to avoid filling the IN (childs out) IPC channel, which would cause deadlock
+# if the parent were to fill the other channel with SET commands.  Output of
+# SET commands w/o handshaking is desirable to minimize context switches and
+# IPC records and hence speed up process startup.
+
+int procedure propen (process, in, out)
+
+char	process[ARB]		# filename of executable file
+int	in, out			# input, output file descriptors to child
+
+int	pid, print_redefined_variables, status
+pointer	sp, cwd
+int	propcpr()
+data	print_redefined_variables /NO/
+errchk	propcpr, envlist, putline, putci
+
+begin
+	call smark (sp)
+	call salloc (cwd, SZ_PATHNAME, TY_CHAR)
+
+	# Connect the subprocess with read and write IPC channels.
+
+	pid = propcpr (process, in, out)
+
+	# Pass the environment list to the child, omitting all but the most
+	# recent definitions of each variable.  The list is passed in the
+	# opposite order from which it was redefined, but it does not matter
+	# since there is only one entry for each variable.
+
+	call envlist (out, "set ", print_redefined_variables)
+
+	# Set the current working directory in the child, in case the OS does
+	# not do so, and to save the child the need to ask the kernel for the
+	# cwd name, an expensive operation on some systems.
+
+	call zfgcwd (Memc[cwd], SZ_PATHNAME, status)
+	call strupk (Memc[cwd], Memc[cwd], SZ_PATHNAME)
+	call putline (out, "chdir ")
+	call putline (out, Memc[cwd])
+	call putci (out, '\n')
+
+	# The command "_go_" must be sent to the child to signal that process
+	# startup is completed.  The process STDOUT and STDERR are redirected
+	# into the nullfile during startup, hence we will see no output if
+	# this command is not sent.
+
+	call putline (out, "_go_\n")
+
+	# Flush the output so the child can munch on this while we go off and
+	# do something else.
+	call flush (out)
+
+	call sfree (sp)
+	return (pid)
+end