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.
69
70       *  CMSG_LEN()  returns the value to store in the cmsg_len member of the
71          cmsghdr structure, taking into account any necessary alignment.   It
72          takes  the  data  length as an argument.  This is a constant expres‐
73          sion.
74
75       To create ancillary data, first initialize the msg_controllen member of
76       the  msghdr  with  the  length  of  the  control  message  buffer.  Use
77       CMSG_FIRSTHDR() on the msghdr to get  the  first  control  message  and
78       CMSG_NXTHDR()  to  get  all  subsequent ones.  In each control message,
79       initialize cmsg_len (with CMSG_LEN()), the other cmsghdr header fields,
80       and  the  data  portion using CMSG_DATA().  Finally, the msg_controllen
81       field of the msghdr should be set to the sum of the CMSG_SPACE() of the
82       length  of all control messages in the buffer.  For more information on
83       the msghdr, see recvmsg(2).
84

CONFORMING TO

86       This ancillary data model conforms to the POSIX.1g draft,  4.4BSD-Lite,
87       the IPv6 advanced API described in RFC 2292 and SUSv2.  CMSG_ALIGN() is
88       a Linux extension.
89

NOTES

91       For portability, ancillary data  should  be  accessed  using  only  the
92       macros  described  here.   CMSG_ALIGN() is a Linux extension and should
93       not be used in portable programs.
94
95       In  Linux,  CMSG_LEN(),  CMSG_DATA(),  and  CMSG_ALIGN()  are  constant
96       expressions  (assuming  their argument is constant), meaning that these
97       values can be used to declare the size of global variables.   This  may
98       not be portable, however.
99

EXAMPLE

101       This code looks for the IP_TTL option in a received ancillary buffer:
102
103           struct msghdr msgh;
104           struct cmsghdr *cmsg;
105           int *ttlptr;
106           int received_ttl;
107
108           /* Receive auxiliary data in msgh */
109
110           for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
111                   cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
112               if (cmsg->cmsg_level == IPPROTO_IP
113                       && cmsg->cmsg_type == IP_TTL) {
114                   ttlptr = (int *) CMSG_DATA(cmsg);
115                   received_ttl = *ttlptr;
116                   break;
117               }
118           }
119
120           if (cmsg == NULL) {
121               /* Error: IP_TTL not enabled or small buffer or I/O error */
122           }
123
124       The  code  below passes an array of file descriptors over a UNIX domain
125       socket using SCM_RIGHTS:
126
127           struct msghdr msg = { 0 };
128           struct cmsghdr *cmsg;
129           int myfds[NUM_FD];  /* Contains the file descriptors to pass */
130           char iobuf[1];
131           struct iovec io = {
132               .iov_base = iobuf,
133               .iov_len = sizeof(iobuf)
134           };
135           union {         /* Ancillary data buffer, wrapped in a union
136                              in order to ensure it is suitably aligned */
137               char buf[CMSG_SPACE(sizeof(myfds))];
138               struct cmsghdr align;
139           } u;
140
141           msg.msg_iov = &io;
142           msg.msg_iovlen = 1;
143           msg.msg_control = u.buf;
144           msg.msg_controllen = sizeof(u.buf);
145           cmsg = CMSG_FIRSTHDR(&msg);
146           cmsg->cmsg_level = SOL_SOCKET;
147           cmsg->cmsg_type = SCM_RIGHTS;
148           cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
149           memcpy(CMSG_DATA(cmsg), myfds, NUM_FD * sizeof(int));
150

SEE ALSO

152       recvmsg(2), sendmsg(2)
153
154       RFC 2292
155

COLOPHON

157       This page is part of release 5.02 of the Linux  man-pages  project.   A
158       description  of  the project, information about reporting bugs, and the
159       latest    version    of    this    page,    can     be     found     at
160       https://www.kernel.org/doc/man-pages/.
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164Linux                             2019-03-06                           CMSG(3)
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