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       struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);
12       struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh ,
13                                   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.  It returns NULL
50          if there isn't enough space for a cmsghdr in the buffer.
51
52       *  CMSG_NXTHDR()  returns  the next valid cmsghdr after the passed cms‐
53          ghdr.  It returns NULL when there isn't enough  space  left  in  the
54          buffer.
55
56          When  initializing  a  buffer  that will contain a series of cmsghdr
57          structures (e.g., to be sent with sendmsg(2)),  that  buffer  should
58          first  be  zero-initialized  to  ensure  the  correct  operation  of
59          CMSG_NXTHDR().
60
61       *  CMSG_ALIGN(), given a length,  returns  it  including  the  required
62          alignment.  This is a constant expression.
63
64       *  CMSG_SPACE()  returns  the number of bytes an ancillary element with
65          payload of the passed data length  occupies.   This  is  a  constant
66          expression.
67
68       *  CMSG_DATA() returns a pointer to the data portion of a cmsghdr.  The
69          pointer returned cannot  be  assumed  to  be  suitably  aligned  for
70          accessing  arbitrary  payload  data  types.  Applications should not
71          cast it to a pointer type matching the payload, but  should  instead
72          use memcpy(3) to copy data to or from a suitably declared object.
73
74       *  CMSG_LEN()  returns the value to store in the cmsg_len member of the
75          cmsghdr structure, taking into account any necessary alignment.   It
76          takes  the  data  length as an argument.  This is a constant expres‐
77          sion.
78
79       To create ancillary data, first initialize the msg_controllen member of
80       the  msghdr  with  the  length  of  the  control  message  buffer.  Use
81       CMSG_FIRSTHDR() on the msghdr to get  the  first  control  message  and
82       CMSG_NXTHDR()  to  get  all  subsequent ones.  In each control message,
83       initialize cmsg_len (with CMSG_LEN()), the other cmsghdr header fields,
84       and  the  data  portion using CMSG_DATA().  Finally, the msg_controllen
85       field of the msghdr should be set to the sum of the CMSG_SPACE() of the
86       length  of all control messages in the buffer.  For more information on
87       the msghdr, see recvmsg(2).
88

CONFORMING TO

90       This ancillary data model conforms to the POSIX.1g draft,  4.4BSD-Lite,
91       the    IPv6   advanced   API   described   in   RFC 2292   and   SUSv2.
92       CMSG_FIRSTHDR(),  CMSG_NXTHDR(),  and  CMSG_DATA()  are  specified   in
93       POSIX.1-2008.  CMSG_SPACE() and CMSG_LEN() will be included in the next
94       POSIX release (Issue 8).
95
96       CMSG_ALIGN() is a Linux extension.
97

NOTES

99       For portability, ancillary data  should  be  accessed  using  only  the
100       macros  described  here.   CMSG_ALIGN() is a Linux extension and should
101       not be used in portable programs.
102
103       In  Linux,  CMSG_LEN(),  CMSG_DATA(),  and  CMSG_ALIGN()  are  constant
104       expressions  (assuming  their argument is constant), meaning that these
105       values can be used to declare the size of global variables.   This  may
106       not be portable, however.
107

EXAMPLES

109       This code looks for the IP_TTL option in a received ancillary buffer:
110
111           struct msghdr msgh;
112           struct cmsghdr *cmsg;
113           int received_ttl;
114
115           /* Receive auxiliary data in msgh */
116
117           for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
118                   cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
119               if (cmsg->cmsg_level == IPPROTO_IP
120                       && cmsg->cmsg_type == IP_TTL) {
121                   memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(int));
122                   break;
123               }
124           }
125
126           if (cmsg == NULL) {
127               /* Error: IP_TTL not enabled or small buffer or I/O error */
128           }
129
130       The  code  below passes an array of file descriptors over a UNIX domain
131       socket using SCM_RIGHTS:
132
133           struct msghdr msg = { 0 };
134           struct cmsghdr *cmsg;
135           int myfds[NUM_FD];  /* Contains the file descriptors to pass */
136           char iobuf[1];
137           struct iovec io = {
138               .iov_base = iobuf,
139               .iov_len = sizeof(iobuf)
140           };
141           union {         /* Ancillary data buffer, wrapped in a union
142                              in order to ensure it is suitably aligned */
143               char buf[CMSG_SPACE(sizeof(myfds))];
144               struct cmsghdr align;
145           } u;
146
147           msg.msg_iov = &io;
148           msg.msg_iovlen = 1;
149           msg.msg_control = u.buf;
150           msg.msg_controllen = sizeof(u.buf);
151           cmsg = CMSG_FIRSTHDR(&msg);
152           cmsg->cmsg_level = SOL_SOCKET;
153           cmsg->cmsg_type = SCM_RIGHTS;
154           cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
155           memcpy(CMSG_DATA(cmsg), myfds, NUM_FD * sizeof(int));
156

SEE ALSO

158       recvmsg(2), sendmsg(2)
159
160       RFC 2292
161

COLOPHON

163       This page is part of release 5.07 of the Linux  man-pages  project.   A
164       description  of  the project, information about reporting bugs, and the
165       latest    version    of    this    page,    can     be     found     at
166       https://www.kernel.org/doc/man-pages/.
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170Linux                             2020-06-09                           CMSG(3)
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