From 9b45269cc2d360a87ace6cbd0e7a1509ca3fa647 Mon Sep 17 00:00:00 2001
From: Keith Packard <keithp@keithp.com>
Date: Thu, 4 Oct 2012 15:59:48 -0700
Subject: [PATCH] Example file descriptor passing code

Signed-off-by: Keith Packard <keithp@keithp.com>
---
 Makefile    |   4 ++
 fdpassing.c | 142 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 146 insertions(+)
 create mode 100644 Makefile
 create mode 100644 fdpassing.c

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..e208bc1
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,4 @@
+CFLAGS=-O0 -g
+
+fdpassing: fdpassing.o
+	$(CC) -o $@ fdpassing.o
\ No newline at end of file
diff --git a/fdpassing.c b/fdpassing.c
new file mode 100644
index 0000000..cbfd8dd
--- /dev/null
+++ b/fdpassing.c
@@ -0,0 +1,142 @@
+/*
+ * Copyright © 2012 Keith Packard <keithp@keithp.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ */
+
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <signal.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+void
+child(int sock)
+{
+	ssize_t		size;
+	struct msghdr	msg;
+	struct iovec	iov;
+	char		buf[16];
+	union {
+		struct cmsghdr	cmsghdr;
+		char		control[CMSG_SPACE(sizeof (int))];
+	} cmsgu;
+	struct cmsghdr	*cmsg;
+
+	int		fd;
+
+	iov.iov_base = buf;
+	iov.iov_len = sizeof(buf);
+
+	msg.msg_name = NULL;
+	msg.msg_namelen = 0;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsgu.control;
+	msg.msg_controllen = sizeof(cmsgu.control);
+
+	size = recvmsg (sock, &msg, 0);
+	if (size < 0) {
+		perror("recvmsg");
+		exit(1);
+	}
+
+	cmsg = CMSG_FIRSTHDR(&msg);
+	if (cmsg &&
+	    cmsg->cmsg_len == CMSG_LEN(sizeof(int))) {
+		if (cmsg->cmsg_level != SOL_SOCKET) {
+			fprintf (stderr, "invalid cmsg_level %d\n",
+				 cmsg->cmsg_level);
+			exit(1);
+		}
+		if (cmsg->cmsg_type != SCM_RIGHTS) {
+			fprintf (stderr, "invalid cmsg_type %d\n",
+				 cmsg->cmsg_type);
+			exit(1);
+		}
+
+		fd = *((int *) CMSG_DATA(cmsg));
+		printf ("received fd %d\n", fd);
+		write(fd, "hello, world\n", 13);
+	} else {
+		fd = -1;
+	}
+	exit(0);
+}
+
+void
+parent(int sock)
+{
+	ssize_t		size;
+	struct msghdr	msg;
+	struct iovec	iov;
+	char		buf[16];
+	union {
+		struct cmsghdr	cmsghdr;
+		char		control[CMSG_SPACE(sizeof (int))];
+	} cmsgu;
+	struct cmsghdr	*cmsg;
+
+	int		fd;
+
+	iov.iov_base = "1";
+	iov.iov_len = 1;
+
+	msg.msg_name = NULL;
+	msg.msg_namelen = 0;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsgu.control;
+	msg.msg_controllen = sizeof(cmsgu.control);
+
+	cmsg = CMSG_FIRSTHDR(&msg);
+	cmsg->cmsg_len = CMSG_LEN(sizeof (int));
+	cmsg->cmsg_level = SOL_SOCKET;
+	cmsg->cmsg_type = SCM_RIGHTS;
+
+	fd = creat("test-file", 0666);
+	printf ("passing fd %d\n", fd);
+	*((int *) CMSG_DATA(cmsg)) = fd;
+
+	size = sendmsg(sock, &msg, 0);
+
+	if (size < 0)
+		perror ("sendmsg");
+}
+
+int
+main(int argc, char **argv)
+{
+	int	sv[2];
+	int	pid;
+
+	if (socketpair(AF_LOCAL, SOCK_STREAM, 0, sv) < 0) {
+		perror("socketpair");
+		exit(1);
+	}
+	switch ((pid = fork())) {
+	case 0:
+		child(sv[1]);
+		break;
+	case -1:
+		perror("fork");
+		exit(1);
+	default:
+		parent(sv[0]);
+		sleep(1);
+		kill(pid, SIGTERM);
+		break;
+	}
+}