* [PATCH 1/1] cmsg.3: Add a scatter/gather buffer to sample code
@ 2017-07-19 23:49 Sukadev Bhattiprolu
[not found] ` <20170719234955.GA24570-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
0 siblings, 1 reply; 2+ messages in thread
From: Sukadev Bhattiprolu @ 2017-07-19 23:49 UTC (permalink / raw)
To: mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w
Cc: linux-man-u79uwXL29TY76Z2rM5mHXA,
pradeeps-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8
>From b2800c8d5dcd8ede89ebccb25f1a9b087dc2684b Mon Sep 17 00:00:00 2001
From: Sukadev Bhattiprolu <sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
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-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
---
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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* Re: [PATCH 1/1] cmsg.3: Add a scatter/gather buffer to sample code
[not found] ` <20170719234955.GA24570-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
@ 2017-08-16 0:30 ` Michael Kerrisk (man-pages)
0 siblings, 0 replies; 2+ messages in thread
From: Michael Kerrisk (man-pages) @ 2017-08-16 0:30 UTC (permalink / raw)
To: Sukadev Bhattiprolu
Cc: mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w,
linux-man-u79uwXL29TY76Z2rM5mHXA,
pradeeps-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8
On 07/20/2017 01:49 AM, Sukadev Bhattiprolu wrote:
>
>>From b2800c8d5dcd8ede89ebccb25f1a9b087dc2684b Mon Sep 17 00:00:00 2001
> From: Sukadev Bhattiprolu <sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
> 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
>
> Signed-off-by: Sukadev Bhattiprolu <sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
> ---
> 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/
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2017-07-19 23:49 [PATCH 1/1] cmsg.3: Add a scatter/gather buffer to sample code Sukadev Bhattiprolu
[not found] ` <20170719234955.GA24570-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
2017-08-16 0:30 ` Michael Kerrisk (man-pages)
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