NAME
CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - Access ancillary data
SYNOPSIS
#include <sys/socket.h>
I struct cmsghdr *CMSG_FIRSTHDR(struct msghdr * msgh );
I struct cmsghdr *CMSG_NXTHDR(struct msghdr * msgh , struct cmsghdr * cmsg );
I size_t CMSG_ALIGN(size_t length );
I size_t CMSG_SPACE(size_t length );
I size_t CMSG_LEN(size_t length );
I 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 maximum ancillary buffer size allowed per socket can be set using the
net.core.optmem_max
sysctl; see
socket(7).
R CMSG_FIRSTHDR ()
returns a pointer to the first
cmsghdr
in the ancillary
data buffer associated with the passed
R msghdr .
R CMSG_NXTHDR ()
returns the next valid
cmsghdr
after the passed
R cmsghdr .
It returns NULL when there isn't enough space left in the buffer.
R CMSG_ALIGN (),
given a length, returns it including the required alignment.
This is a
constant expression.
R 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
R 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
R 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
R CMSG_LEN ),
the other
cmsghdr
header fields, and the data portion using
R CMSG_DATA .
Finally, the
msg_controllen
field of the
msghdr
should be set to the sum of the
R CMSG_SPACE ()
of the length of
all control messages in the buffer.
For more information on the
R 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
R msghdr .
CONFORMING TO
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.
NOTES
For portability, ancillary data should be accessed only using the macros
described here.
R CMSG_ALIGN ()
is a Linux extension and should be not used in portable programs.
In Linux,
R CMSG_LEN ,
R CMSG_DATA ,
and
R 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.
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 == 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 socket using
R 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;
SEE ALSO
RFC 2292
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