1CMSG(3)                    Linux Programmer's Manual                   CMSG(3)
2
3
4

NAME

6       CMSG_ALIGN,  CMSG_SPACE,  CMSG_NXTHDR, CMSG_FIRSTHDR - access ancillary
7       data
8

SYNOPSIS

10       #include <sys/socket.h>
11
12       struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);
13       struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh, struct cmsghdr *cmsg);
14       size_t CMSG_ALIGN(size_t length);
15       size_t CMSG_SPACE(size_t length);
16       size_t CMSG_LEN(size_t length);
17       unsigned char *CMSG_DATA(struct cmsghdr *cmsg);
18

DESCRIPTION

20       These macros are used to  create  and  access  control  messages  (also
21       called ancillary data) that are not a part of the socket payload.  This
22       control information may include the interface the packet  was  received
23       on, various rarely used header fields, an extended error description, a
24       set of file descriptors or UNIX  credentials.   For  instance,  control
25       messages  can  be  used  to  send  additional  header fields such as IP
26       options.  Ancillary data is sent by calling sendmsg(2) and received  by
27       calling recvmsg(2).  See their manual pages for more information.
28
29       Ancillary  data is a sequence of cmsghdr structures with appended data.
30       See the specific protocol man pages for the available  control  message
31       types.  The maximum ancillary buffer size allowed per socket can be set
32       using /proc/sys/net/core/optmem_max; see socket(7).
33
34       The cmsghdr structure is defined as follows:
35
36           struct cmsghdr {
37               size_t cmsg_len;    /* Data byte count, including header
38                                      (type is socklen_t in POSIX) */
39               int    cmsg_level;  /* Originating protocol */
40               int    cmsg_type;   /* Protocol-specific type */
41           /* followed by
42              unsigned char cmsg_data[]; */
43           };
44
45       The sequence of cmsghdr structures should never be  accessed  directly.
46       Instead, use only the following macros:
47
48       *  CMSG_FIRSTHDR() returns a pointer to the first cmsghdr in the ancil‐
49          lary data buffer associated with the passed msghdr.
50
51       *  CMSG_NXTHDR() returns the next valid cmsghdr after the  passed  cms‐
52          ghdr.   It  returns  NULL  when there isn't enough space left in the
53          buffer.
54
55       *  CMSG_ALIGN(), given a length,  returns  it  including  the  required
56          alignment.  This is a constant expression.
57
58       *  CMSG_SPACE()  returns  the number of bytes an ancillary element with
59          payload of the passed data length  occupies.   This  is  a  constant
60          expression.
61
62       *  CMSG_DATA() returns a pointer to the data portion of a cmsghdr.
63
64       *  CMSG_LEN()  returns the value to store in the cmsg_len member of the
65          cmsghdr structure, taking into account any necessary alignment.   It
66          takes  the  data  length as an argument.  This is a constant expres‐
67          sion.
68
69       To create ancillary data, first initialize the msg_controllen member of
70       the  msghdr  with  the  length  of  the  control  message  buffer.  Use
71       CMSG_FIRSTHDR() on the msghdr to get  the  first  control  message  and
72       CMSG_NXTHDR()  to  get  all  subsequent ones.  In each control message,
73       initialize cmsg_len (with CMSG_LEN()), the other cmsghdr header fields,
74       and  the  data  portion using CMSG_DATA().  Finally, the msg_controllen
75       field of the msghdr should be set to the sum of the CMSG_SPACE() of the
76       length  of all control messages in the buffer.  For more information on
77       the msghdr, see recvmsg(2).
78
79       When the control message buffer is too short to store all messages, the
80       MSG_CTRUNC flag is set in the msg_flags member of the msghdr.
81

CONFORMING TO

83       This  ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
84       the IPv6 advanced API described in RFC 2292 and SUSv2.  CMSG_ALIGN() is
85       a Linux extension.
86

NOTES

88       For  portability,  ancillary  data  should  be  accessed using only the
89       macros described here.  CMSG_ALIGN() is a Linux  extension  and  should
90       not be used in portable programs.
91
92       In  Linux,  CMSG_LEN(),  CMSG_DATA(),  and  CMSG_ALIGN()  are  constant
93       expressions (assuming their argument is constant); this could  be  used
94       to  declare  the  size  of global variables.  This may not be portable,
95       however.
96

EXAMPLE

98       This code looks for the IP_TTL option in a received ancillary buffer:
99
100           struct msghdr msgh;
101           struct cmsghdr *cmsg;
102           int *ttlptr;
103           int received_ttl;
104
105           /* Receive auxiliary data in msgh */
106
107           for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
108                   cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
109               if (cmsg->cmsg_level == IPPROTO_IP
110                       && cmsg->cmsg_type == IP_TTL) {
111                   ttlptr = (int *) CMSG_DATA(cmsg);
112                   received_ttl = *ttlptr;
113                   break;
114               }
115           }
116
117           if (cmsg == NULL) {
118               /* Error: IP_TTL not enabled or small buffer or I/O error */
119           }
120
121       The code below passes an array of file descriptors over a  UNIX  domain
122       socket using SCM_RIGHTS:
123
124           struct msghdr msg = { 0 };
125           struct cmsghdr *cmsg;
126           int myfds[NUM_FD];  /* Contains the file descriptors to pass */
127           int *fdptr;
128           char iobuf[1];
129           struct iovec io = {
130               .iov_base = iobuf,
131               .iov_len = sizeof(iobuf)
132           };
133           union {         /* Ancillary data buffer, wrapped in a union
134                              in order to ensure it is suitably aligned */
135               char buf[CMSG_SPACE(sizeof(myfds))];
136               struct cmsghdr align;
137           } u;
138
139           msg.msg_iov = &io;
140           msg.msg_iovlen = 1;
141           msg.msg_control = u.buf;
142           msg.msg_controllen = sizeof(u.buf);
143           cmsg = CMSG_FIRSTHDR(&msg);
144           cmsg->cmsg_level = SOL_SOCKET;
145           cmsg->cmsg_type = SCM_RIGHTS;
146           cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
147           fdptr = (int *) CMSG_DATA(cmsg);    /* Initialize the payload */
148           memcpy(fdptr, myfds, NUM_FD * sizeof(int));
149

SEE ALSO

151       recvmsg(2), sendmsg(2)
152
153       RFC 2292
154

COLOPHON

156       This  page  is  part of release 4.15 of the Linux man-pages project.  A
157       description of the project, information about reporting bugs,  and  the
158       latest     version     of     this    page,    can    be    found    at
159       https://www.kernel.org/doc/man-pages/.
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163Linux                             2017-09-15                           CMSG(3)
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