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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 creden-
tials. 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 mes-
sage types. The maximum 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 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 struc-
ture, 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 con-
trol 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 == 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
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 pro-
grams.
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 vari-
ables. This may be not portable, however.
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.
SEE ALSO
recvmsg(2), sendmsg(2)
RFC 2292
Linux Man Page 1998-10-02 CMSG(3)
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