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and adjust it to our liking.

This commit is contained in:
christos 2008-06-20 14:35:10 +00:00
parent 0a69cbb3fd
commit 6fe0bc9474
1 changed files with 24 additions and 11 deletions
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35
share/man/man3/CMSG_DATA.3
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@ -1,7 +1,8 @@
.\" $NetBSD: CMSG_DATA.3,v 1.2 2008/06/20 14:35:10 christos Exp $
.\" $OpenBSD: CMSG_DATA.3,v 1.5 2008/03/24 16:11:07 deraadt Exp $
.\" Written by Jared Yanovich <jaredy@openbsd.org>
.\" Public domain, July 3, 2005
.Dd $Mdocdate: March 13 2008 $
.Dd June 20, 2008
.Dt CMSG_DATA 3
.Os
.Sh NAME
@ -66,6 +67,7 @@ This value is what is normally stored in the
of each control message.
This routine accounts for any alignment constraints on the beginning of
ancillary data.
This macro might not evaluate to a compile-time constant.
.It Fn CMSG_NXTHDR mhdr cmsg
This routine returns the location of the control message following
.Fa cmsg
@ -84,6 +86,7 @@ This value is what is normally stored in
.Fa msg_msgcontrollen .
This routine accounts for any alignment constraints on the beginning of
ancillary data as well as any needed to pad the next control message.
This macro might not evaluate to a compile-time constant.
.El
.Sh EXAMPLES
The following example constructs a control message containing a file
@ -91,14 +94,21 @@ descriptor and passes it over a socket:
.Bd -literal -offset indent
struct msghdr msg;
struct cmsghdr *cmsg;
/* We use a union to make sure hdr is aligned */
union {
struct cmsghdr hdr;
unsigned char buf[CMSG_SPACE(sizeof(int))];
} cmsgbuf;
} *cmsgbuf;
memset(&msg, 0, sizeof(msg));
msg.msg_control = &cmsgbuf.buf;
msg.msg_controllen = sizeof(cmsgbuf.buf);
/*
* We allocate in the heap instead of the stack to avoid C99
* variable stack allocation, which breaks gcc -fstack-protector.
*/
if ((cmsgbuf = malloc(sizeof(*cmsgbuf))) == NULL)
err(1, "malloc");
(void)memset(&msg, 0, sizeof(msg));
msg.msg_control = cmsgbuf->buf;
msg.msg_controllen = sizeof(cmsgbuf->buf);
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_len = CMSG_LEN(sizeof(int));
@ -108,6 +118,7 @@ cmsg->cmsg_type = SCM_RIGHTS;
if (sendmsg(s, &msg, 0) == -1)
err(1, "sendmsg");
free(cmsgbuf);
.Ed
.Pp
And an example that receives and decomposes the control message:
@ -117,15 +128,17 @@ struct cmsghdr *cmsg;
union {
struct cmsghdr hdr;
unsigned char buf[CMSG_SPACE(sizeof(int))];
} cmsgbuf;
} *cmsgbuf;
memset(&msg, 0, sizeof(msg));
msg.msg_control = &cmsgbuf.buf;
msg.msg_controllen = sizeof(cmsgbuf.buf);
if ((cmsgbuf = malloc(sizeof(*cmsgbuf))) == NULL)
err(1, "malloc");
(void)memset(&msg, 0, sizeof(msg));
msg.msg_control = cmsgbuf->buf;
msg.msg_controllen = sizeof(cmsgbuf->buf);
if (recvmsg(s, &msg, 0) == -1)
err(1, "recvmsg");
if ((msg.msg_flags & MSG_TRUNC) || (msg.msg_flags & MSG_CTRUNC))
if ((msg.msg_flags & (MSG_TRUNC|MSG_CTRUNC))
errx(1, "control message truncated");
for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
cmsg = CMSG_NXTHDR(&msg, cmsg)) {
@ -136,6 +149,7 @@ for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
/* Do something with the descriptor. */
}
}
free(cmsgbuf);
.Ed
.Sh SEE ALSO
.Xr recvmsg 2 ,
@ -144,4 +158,3 @@ for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
.Sh HISTORY
The control message API first appeared in
.Bx 4.2 .