>From b2800c8d5dcd8ede89ebccb25f1a9b087dc2684b Mon Sep 17 00:00:00 2001
From: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
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 <sukadev@xxxxxxxxxxxxxxxxxx>
---
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(+)
diff --git a/man3/cmsg.3 b/man3/cmsg.3
index 69af571..2ec87ad 100644
--- a/man3/cmsg.3
+++ b/man3/cmsg.3
@@ -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);
--
1.8.3.1
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