cmsg
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);
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[]; */
};
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 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 maxi-
mum ancillary buffer size allowed per socket can be set using the
net.core.optmem_max sysctl; 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 align-
ment. This is a constant expression.
CMSG_SPACE returns the number of bytes an ancillary element with pay-
load of the passed data length occupies. This is a constant expres-
sion.
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 cms-
ghdr 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.
EXAMPLE
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 == SOL_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 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;
NOTES
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.
CONFORMS TO
This ancillary data model conforms to the POSIX.1003.1g draft, 4.4BSD-
Lite, the IPv6 advanced API described in RFC2292 and the Single Unix
specification v2. CMSG_ALIGN is a Linux extension.
SEE ALSO
sendmsg(2), recvmsg(2)
RFC 2292
Linux Man Page 1998-10-02 CMSG(3)
