Re: [PATCH 1/1] cmsg.3: Add a scatter/gather buffer to sample code
From: Michael Kerrisk (man-pages) <hidden>
Date: 2017-08-16 00:30:11
Possibly related (same subject, not in this thread)
- 2017-07-19 · [PATCH 1/1] cmsg.3: Add a scatter/gather buffer to sample code · Sukadev Bhattiprolu <hidden>
On 07/20/2017 01:49 AM, Sukadev Bhattiprolu wrote:
quoted
From b2800c8d5dcd8ede89ebccb25f1a9b087dc2684b Mon Sep 17 00:00:00 2001From: Sukadev Bhattiprolu <redacted> 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.
Thanks, Sukadev. Applied. Cheers, Michael
quoted hunk
Signed-off-by: Sukadev Bhattiprolu <redacted> --- 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);
-- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html