cmsg(3) - Linux manual page (original) (raw)
CMSG(3) Library Functions Manual CMSG(3)
NAME top
CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access
ancillary dataLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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 top
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)](../man2/sendmsg.2.html) and received by calling [recvmsg(2)](../man2/recvmsg.2.html). 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/optmemmax_; see
[socket(7)](../man7/socket.7.html).
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)](../man2/sendmsg.2.html)), 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)](../man3/memcpy.3.html) to copy data to or from a
suitably declared object.
**CMSG_LEN**()
returns the value to store in the _cmsglen_ 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 _msgcontrollen_
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 _cmsglen_ (with **CMSG_LEN**()), the
other _cmsghdr_ header fields, and the data portion using
**CMSG_DATA**(). Finally, the _msgcontrollen_ 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)](../man2/recvmsg.2.html).VERSIONS top
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.STANDARDS top
**CMSG_FIRSTHDR**()
**CMSG_NXTHDR**()
**CMSG_DATA**()
POSIX.1-2008.
**CMSG_SPACE**()
**CMSG_LEN**()
**CMSG_ALIGN**()
Linux.HISTORY top
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).EXAMPLES top
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)](../man2/seccomp%5Funotify.2.html).SEE ALSO top
[recvmsg(2)](../man2/recvmsg.2.html), [sendmsg(2)](../man2/sendmsg.2.html)
RFC 2292COLOPHON top
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man-pages@man7.orgLinux 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)