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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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CMSG(3) Library Functions Manual CMSG(3)
CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access
ancillary data
Standard C library (libc, -lc)
#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. 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 cmsghdr.
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 expression.
CMSG_DATA()
returns a pointer to the data portion of a cmsghdr. The
pointer returned cannot be assumed to be suitably aligned
for accessing 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 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).
For portability, ancillary data should be accessed using only the
macros described here.
In Linux, CMSG_LEN(), CMSG_DATA(), and CMSG_ALIGN() are constant
expressions (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.
CMSG_FIRSTHDR()
CMSG_NXTHDR()
CMSG_DATA()
POSIX.1-2008.
CMSG_SPACE()
CMSG_LEN()
CMSG_ALIGN()
Linux.
This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-
Lite, the IPv6 advanced API described in RFC 2292 and SUSv2.
CMSG_SPACE() and CMSG_LEN() will be included in the next POSIX
release (Issue 8).
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(received_ttl));
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(myfds));
memcpy(CMSG_DATA(cmsg), myfds, sizeof(myfds));
For a complete code example that shows passing of file descriptors
over a UNIX domain socket, see seccomp_unotify(2).
recvmsg(2), sendmsg(2)
RFC 2292
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
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⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2025-02-02. If you discover any rendering problems in this HTML
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man-pages@man7.org
Linux man-pages 6.10 2024-07-23 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), sctp(7), socket(7), udp(7), unix(7), lslocks(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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