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. 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).
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), meaning that these
values can 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 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));
recvmsg(2),
sendmsg(2)
RFC 2292