1CMSG(3)                    Library Functions Manual                    CMSG(3)
2
3
4

NAME

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

LIBRARY

10       Standard C library (libc, -lc)
11

SYNOPSIS

13       #include <sys/socket.h>
14
15       struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);
16       struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh,
17                                   struct cmsghdr *cmsg);
18       size_t CMSG_ALIGN(size_t length);
19       size_t CMSG_SPACE(size_t length);
20       size_t CMSG_LEN(size_t length);
21       unsigned char *CMSG_DATA(struct cmsghdr *cmsg);
22

DESCRIPTION

24       These macros are used to  create  and  access  control  messages  (also
25       called ancillary data) that are not a part of the socket payload.  This
26       control information may include the interface the packet  was  received
27       on, various rarely used header fields, an extended error description, a
28       set of file descriptors, or UNIX credentials.   For  instance,  control
29       messages  can  be  used to send additional header fields such as IP op‐
30       tions.  Ancillary data is sent by calling sendmsg(2)  and  received  by
31       calling recvmsg(2).  See their manual pages for more information.
32
33       Ancillary  data is a sequence of cmsghdr structures with appended data.
34       See the specific protocol man pages for the available  control  message
35       types.  The maximum ancillary buffer size allowed per socket can be set
36       using /proc/sys/net/core/optmem_max; see socket(7).
37
38       The cmsghdr structure is defined as follows:
39
40           struct cmsghdr {
41               size_t cmsg_len;    /* Data byte count, including header
42                                      (type is socklen_t in POSIX) */
43               int    cmsg_level;  /* Originating protocol */
44               int    cmsg_type;   /* Protocol-specific type */
45           /* followed by
46              unsigned char cmsg_data[]; */
47           };
48
49       The sequence of cmsghdr structures should never be  accessed  directly.
50       Instead, use only the following macros:
51
52       CMSG_FIRSTHDR()
53              returns  a  pointer  to  the first cmsghdr in the ancillary data
54              buffer associated with the passed msghdr.  It  returns  NULL  if
55              there isn't enough space for a cmsghdr in the buffer.
56
57       CMSG_NXTHDR()
58              returns the next valid cmsghdr after the passed cmsghdr.  It re‐
59              turns NULL when there isn't enough space left in the buffer.
60
61              When initializing a buffer that will contain a series of cmsghdr
62              structures  (e.g.,  to  be  sent  with  sendmsg(2)), that buffer
63              should first be zero-initialized to ensure the correct operation
64              of CMSG_NXTHDR().
65
66       CMSG_ALIGN(),
67              given  a  length,  returns  it including the required alignment.
68              This is a constant expression.
69
70       CMSG_SPACE()
71              returns the number of bytes an ancillary element with payload of
72              the passed data length occupies.  This is a constant expression.
73
74       CMSG_DATA()
75              returns a pointer to the data portion of a cmsghdr.  The pointer
76              returned cannot be assumed to be suitably aligned for  accessing
77              arbitrary  payload  data types.  Applications should not cast it
78              to a pointer type matching the payload, but should  instead  use
79              memcpy(3) to copy data to or from a suitably declared object.
80
81       CMSG_LEN()
82              returns the value to store in the cmsg_len member of the cmsghdr
83              structure, taking into  account  any  necessary  alignment.   It
84              takes  the  data  length as an argument.  This is a constant ex‐
85              pression.
86
87       To create ancillary data, first initialize the msg_controllen member of
88       the  msghdr  with  the  length  of  the  control  message  buffer.  Use
89       CMSG_FIRSTHDR() on the msghdr to get  the  first  control  message  and
90       CMSG_NXTHDR()  to  get  all  subsequent ones.  In each control message,
91       initialize cmsg_len (with CMSG_LEN()), the other cmsghdr header fields,
92       and  the  data  portion using CMSG_DATA().  Finally, the msg_controllen
93       field of the msghdr should be set to the sum of the CMSG_SPACE() of the
94       length  of all control messages in the buffer.  For more information on
95       the msghdr, see recvmsg(2).
96

VERSIONS

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

STANDARDS

107       CMSG_FIRSTHDR()
108       CMSG_NXTHDR()
109       CMSG_DATA()
110              POSIX.1-2008.
111
112       CMSG_SPACE()
113       CMSG_LEN()
114       CMSG_ALIGN()
115              Linux.
116

HISTORY

118       This ancillary data model conforms to the POSIX.1g draft,  4.4BSD-Lite,
119       the IPv6 advanced API described in RFC 2292 and SUSv2.
120
121       CMSG_SPACE()  and CMSG_LEN() will be included in the next POSIX release
122       (Issue 8).
123

EXAMPLES

125       This code looks for the IP_TTL option in a received ancillary buffer:
126
127           struct msghdr msgh;
128           struct cmsghdr *cmsg;
129           int received_ttl;
130
131           /* Receive auxiliary data in msgh */
132
133           for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
134                   cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
135               if (cmsg->cmsg_level == IPPROTO_IP
136                       && cmsg->cmsg_type == IP_TTL) {
137                   memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(received_ttl));
138                   break;
139               }
140           }
141
142           if (cmsg == NULL) {
143               /* Error: IP_TTL not enabled or small buffer or I/O error */
144           }
145
146       The code below passes an array of file descriptors over a  UNIX  domain
147       socket using SCM_RIGHTS:
148
149           struct msghdr msg = { 0 };
150           struct cmsghdr *cmsg;
151           int myfds[NUM_FD];  /* Contains the file descriptors to pass */
152           char iobuf[1];
153           struct iovec io = {
154               .iov_base = iobuf,
155               .iov_len = sizeof(iobuf)
156           };
157           union {         /* Ancillary data buffer, wrapped in a union
158                              in order to ensure it is suitably aligned */
159               char buf[CMSG_SPACE(sizeof(myfds))];
160               struct cmsghdr align;
161           } u;
162
163           msg.msg_iov = &io;
164           msg.msg_iovlen = 1;
165           msg.msg_control = u.buf;
166           msg.msg_controllen = sizeof(u.buf);
167           cmsg = CMSG_FIRSTHDR(&msg);
168           cmsg->cmsg_level = SOL_SOCKET;
169           cmsg->cmsg_type = SCM_RIGHTS;
170           cmsg->cmsg_len = CMSG_LEN(sizeof(myfds));
171           memcpy(CMSG_DATA(cmsg), myfds, sizeof(myfds));
172
173       For a complete code example that shows passing of file descriptors over
174       a UNIX domain socket, see seccomp_unotify(2).
175

SEE ALSO

177       recvmsg(2), sendmsg(2)
178
179       RFC 2292
180
181
182
183Linux man-pages 6.04              2023-03-30                           CMSG(3)
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