On 07/20/2017 01:49 AM, Sukadev Bhattiprolu wrote: > >>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. Thanks, Sukadev. Applied. Cheers, Michael > > 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); > -- 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@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
