cmsg.3: Add a scatter/gather buffer to sample code

From b2800c8d5dcd8ede89ebccb25f1a9b087dc2684b Mon Sep 17 00:00:00 2001

Quoted 2 lines

From b2800c8d5dcd8ede89ebccb25f1a9b087dc2684b Mon Sep 17 00:00:00 2001

From: Sukadev Bhattiprolu <[email protected]>
Date: Wed, 19 Jul 2017 22:47:14 -0400
Subject: [PATCH 1/1] cmsg.3: Add a scatter/gather buffer to sample code

Add a simple scatter/gather buffer to the cmsg sample code.

It appears that the scatter/gather buffer is required even though it may not be used. In my testing (on Fedora 24 4.8.7-200.fc24.x86_64 as well as a 4.11 based kernel) return value of sendmsg() was the number of bytes in the scatter/gather buffer (1 with the patch below).

Without the iovec buffer, sendmsg() returns 0 which means no bytes were transferred and the corresponding recvmsg() blocks indefinitely.

Signed-off-by Sukadev Bhattiprolu <[email protected]>

---
A sample test case is at:
https://github.com/sukadev/linux-tests/blob/master/share-fds/share-fds.c

 man3/cmsg.3 | 7 +++++++
 1 file changed, 7 insertions(+)

man3/cmsg.3 +7 -0

index 69af571..2ec87ad 100644
@@ -200,12 +200,19 @@ struct msghdr msg = { 0 };
 struct cmsghdr *cmsg;
 int myfds[NUM_FD];  /* Contains the file descriptors to pass */
 int *fdptr;
+char iobuf[1];
+struct iovec io = {
+    .iov_base = iobuf,
+    .iov_len = sizeof(iobuf)
+};
 union {         /* Ancillary data buffer, wrapped in a union
                    in order to ensure it is suitably aligned */
     char buf[CMSG_SPACE(sizeof(myfds))];
     struct cmsghdr align;
 } u;
 
+msg.msg_iov = &io;
+msg.msg_iovlen = 1;
 msg.msg_control = u.buf;
 msg.msg_controllen = sizeof(u.buf);
 cmsg = CMSG_FIRSTHDR(&msg);