| NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON | The Linux Programming Interface |
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);
struct cmsghdr {
socklen_t cmsg_len; /* data byte count, including header */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by unsigned char cmsg_data[]; */
};
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 struct cmsghdr structures with appended data.
This sequence should only be accessed using the macros described in this
manual page and never directly. 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).
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 cmsghdr. 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 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_NEXTHDR() 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 the SUSv2. CMSG_ALIGN() is a
Linux extension.
For portability, ancillary data should be accessed only using the macros
described here. CMSG_ALIGN() is a Linux extension and should be not 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 be not 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. */
char buf[CMSG_SPACE(sizeof myfds)]; /* ancillary data buffer */
int *fdptr;
msg.msg_control = buf;
msg.msg_controllen = sizeof buf;
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
/* Initialize the payload: */
fdptr = (int *) CMSG_DATA(cmsg);
memcpy(fdptr, myfds, NUM_FD * sizeof(int));
/* Sum of the length of all control messages in the buffer: */
msg.msg_controllen = cmsg->cmsg_len;
recvmsg(2), sendmsg(2)
RFC 2292
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Linux 2008-11-20 CMSG(3)
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