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NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
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CMSG(3) Linux Programmer's Manual CMSG(3)
CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access ancillary
data
#include <sys/socket.h>
struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);
struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh, struct cmsghdr
*cmsg);
size_t CMSG_ALIGN(size_t length);
size_t CMSG_SPACE(size_t length);
size_t CMSG_LEN(size_t length);
unsigned char *CMSG_DATA(struct cmsghdr *cmsg);
These macros are used to create and access control messages (also
called ancillary data) that are not a part of the socket payload.
This control information may include the interface the packet was
received on, various rarely used header fields, an extended error
description, a set of file descriptors or UNIX credentials. For
instance, control messages can be used to send additional header
fields such as IP options. Ancillary data is sent by calling
sendmsg(2) and received by calling recvmsg(2). See their manual
pages for more information.
Ancillary data is a sequence of cmsghdr structures with appended
data. See the specific protocol man pages for the available control
message types. The maximum ancillary buffer size allowed per socket
can be set using /proc/sys/net/core/optmem_max; see socket(7).
The cmsghdr structure is defined as follows:
struct cmsghdr {
size_t cmsg_len; /* Data byte count, including header
(type is socklen_t in POSIX) */
int cmsg_level; /* Originating protocol */
int cmsg_type; /* Protocol-specific type */
/* followed by
unsigned char cmsg_data[]; */
};
The sequence of cmsghdr structures should never be accessed directly.
Instead, use only the following macros:
* CMSG_FIRSTHDR() returns a pointer to the first cmsghdr in the
ancillary data buffer associated with the passed msghdr.
* CMSG_NXTHDR() returns the next valid cmsghdr after the passed cms‐
ghdr. It returns NULL when there isn't enough space left in the
buffer.
* CMSG_ALIGN(), given a length, returns it including the required
alignment. This is a constant expression.
* CMSG_SPACE() returns the number of bytes an ancillary element with
payload of the passed data length occupies. This is a constant
expression.
* CMSG_DATA() returns a pointer to the data portion of a cmsghdr.
* CMSG_LEN() returns the value to store in the cmsg_len member of
the cmsghdr structure, taking into account any necessary align‐
ment. It takes the data length as an argument. This is a con‐
stant expression.
To create ancillary data, first initialize the msg_controllen member
of the msghdr with the length of the control message buffer. Use
CMSG_FIRSTHDR() on the msghdr to get the first control message and
CMSG_NXTHDR() to get all subsequent ones. In each control message,
initialize cmsg_len (with CMSG_LEN()), the other cmsghdr header
fields, and the data portion using CMSG_DATA(). Finally, the
msg_controllen field of the msghdr should be set to the sum of the
CMSG_SPACE() of the length of all control messages in the buffer.
For more information on the msghdr, see recvmsg(2).
When the control message buffer is too short to store all messages,
the MSG_CTRUNC flag is set in the msg_flags member of the msghdr.
This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-
Lite, the IPv6 advanced API described in RFC 2292 and SUSv2.
CMSG_ALIGN() is a Linux extension.
For portability, ancillary data should be accessed using only the
macros described here. CMSG_ALIGN() is a Linux extension and should
not be used in portable programs.
In Linux, CMSG_LEN(), CMSG_DATA(), and CMSG_ALIGN() are constant
expressions (assuming their argument is constant); this could be used
to declare the size of global variables. This may not be portable,
however.
This code looks for the IP_TTL option in a received ancillary buffer:
struct msghdr msgh;
struct cmsghdr *cmsg;
int *ttlptr;
int received_ttl;
/* Receive auxiliary data in msgh */
for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
if (cmsg->cmsg_level == IPPROTO_IP
&& cmsg->cmsg_type == IP_TTL) {
ttlptr = (int *) CMSG_DATA(cmsg);
received_ttl = *ttlptr;
break;
}
}
if (cmsg == NULL) {
/* Error: IP_TTL not enabled or small buffer or I/O error */
}
The code below passes an array of file descriptors over a UNIX domain
socket using SCM_RIGHTS:
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);
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
fdptr = (int *) CMSG_DATA(cmsg); /* Initialize the payload */
memcpy(fdptr, myfds, NUM_FD * sizeof(int));
recvmsg(2), sendmsg(2)
RFC 2292
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 CMSG(3)
Pages that refer to this page: memfd_create(2), recv(2), send(2), netlink(3), ipv6(7), netlink(7), packet(7), rtnetlink(7), socket(7), unix(7)
Copyright and license for this manual page
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