CMSG(3) Linux Programmer's Manual CMSG(3)
NAME
CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access ancillary
data
SYNOPSIS
#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);
DESCRIPTION
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 op-
tions. 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 ancil-
lary data buffer associated with the passed msghdr. It returns NULL
if there isn't enough space for a cmsghdr in the buffer.
* 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.
When initializing a buffer that will contain a series of cmsghdr
structures (e.g., to be sent with sendmsg(2)), that buffer should
first be zero-initialized to ensure the correct operation of
CMSG_NXTHDR().
* 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 ex-
pression.
* CMSG_DATA() returns a pointer to the data portion of a cmsghdr. The
pointer returned cannot be assumed to be suitably aligned for ac-
cessing arbitrary payload data types. Applications should not cast
it to a pointer type matching the payload, but should instead use
memcpy(3) to copy data to or from a suitably declared object.
* CMSG_LEN() returns the value to store in the cmsg_len member of the
cmsghdr structure, taking into account any necessary alignment. It
takes the data length as an argument. This is a constant expres-
sion.
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).
CONFORMING TO
This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
the IPv6 advanced API described in RFC 2292 and SUSv2.
CMSG_FIRSTHDR(), CMSG_NXTHDR(), and CMSG_DATA() are specified in
POSIX.1-2008. CMSG_SPACE() and CMSG_LEN() will be included in the next
POSIX release (Issue 8).
CMSG_ALIGN() is a Linux extension.
NOTES
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 ex-
pressions (assuming their argument is constant), meaning that these
values can be used to declare the size of global variables. This may
not be portable, however.
EXAMPLES
This code looks for the IP_TTL option in a received ancillary buffer:
struct msghdr msgh;
struct cmsghdr *cmsg;
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) {
memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(int));
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 */
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);
memcpy(CMSG_DATA(cmsg), myfds, NUM_FD * sizeof(int));
SEE ALSO
recvmsg(2), sendmsg(2)
RFC 2292
COLOPHON
This page is part of release 5.07 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 2020-06-09 CMSG(3)