1CMSG(3) Linux Programmer's Manual CMSG(3)
2
6 CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access ancillary
7 data
8
10 #include <sys/socket.h>
11 struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);
13 struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh,
14 struct cmsghdr *cmsg);
15 size_t CMSG_ALIGN(size_t length);
16 size_t CMSG_SPACE(size_t length);
17 size_t CMSG_LEN(size_t length);
18 unsigned char *CMSG_DATA(struct cmsghdr *cmsg);
19
21 These macros are used to create and access control messages (also
22 called ancillary data) that are not a part of the socket payload. This
23 control information may include the interface the packet was received
24 on, various rarely used header fields, an extended error description, a
25 set of file descriptors, or UNIX credentials. For instance, control
26 messages can be used to send additional header fields such as IP op‐
27 tions. Ancillary data is sent by calling sendmsg(2) and received by
28 calling recvmsg(2). See their manual pages for more information.
29 Ancillary data is a sequence of cmsghdr structures with appended data.
31 See the specific protocol man pages for the available control message
32 types. The maximum ancillary buffer size allowed per socket can be set
33 using /proc/sys/net/core/optmem_max; see socket(7).
34 The cmsghdr structure is defined as follows:
36 struct cmsghdr {
38 size_t cmsg_len; /* Data byte count, including header
39 (type is socklen_t in POSIX) */
40 int cmsg_level; /* Originating protocol */
41 int cmsg_type; /* Protocol-specific type */
42 /* followed by
43 unsigned char cmsg_data[]; */
44 };
45 The sequence of cmsghdr structures should never be accessed directly.
47 Instead, use only the following macros:
48 * CMSG_FIRSTHDR() returns a pointer to the first cmsghdr in the ancil‐
50 lary data buffer associated with the passed msghdr. It returns NULL
51 if there isn't enough space for a cmsghdr in the buffer.
52 * CMSG_NXTHDR() returns the next valid cmsghdr after the passed cms‐
54 ghdr. It returns NULL when there isn't enough space left in the
55 buffer.
56 When initializing a buffer that will contain a series of cmsghdr
58 structures (e.g., to be sent with sendmsg(2)), that buffer should
59 first be zero-initialized to ensure the correct operation of
60 CMSG_NXTHDR().
61 * CMSG_ALIGN(), given a length, returns it including the required
63 alignment. This is a constant expression.
64 * CMSG_SPACE() returns the number of bytes an ancillary element with
66 payload of the passed data length occupies. This is a constant ex‐
67 pression.
68 * CMSG_DATA() returns a pointer to the data portion of a cmsghdr. The
70 pointer returned cannot be assumed to be suitably aligned for ac‐
71 cessing arbitrary payload data types. Applications should not cast
72 it to a pointer type matching the payload, but should instead use
73 memcpy(3) to copy data to or from a suitably declared object.
74 * CMSG_LEN() returns the value to store in the cmsg_len member of the
76 cmsghdr structure, taking into account any necessary alignment. It
77 takes the data length as an argument. This is a constant expres‐
78 sion.
79 To create ancillary data, first initialize the msg_controllen member of
81 the msghdr with the length of the control message buffer. Use
82 CMSG_FIRSTHDR() on the msghdr to get the first control message and
83 CMSG_NXTHDR() to get all subsequent ones. In each control message,
84 initialize cmsg_len (with CMSG_LEN()), the other cmsghdr header fields,
85 and the data portion using CMSG_DATA(). Finally, the msg_controllen
86 field of the msghdr should be set to the sum of the CMSG_SPACE() of the
87 length of all control messages in the buffer. For more information on
88 the msghdr, see recvmsg(2).
89
91 This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
92 the IPv6 advanced API described in RFC 2292 and SUSv2.
93 CMSG_FIRSTHDR(), CMSG_NXTHDR(), and CMSG_DATA() are specified in
94 POSIX.1-2008. CMSG_SPACE() and CMSG_LEN() will be included in the next
95 POSIX release (Issue 8).
96 CMSG_ALIGN() is a Linux extension.
98
100 For portability, ancillary data should be accessed using only the
101 macros described here. CMSG_ALIGN() is a Linux extension and should
102 not be used in portable programs.
103 In Linux, CMSG_LEN(), CMSG_DATA(), and CMSG_ALIGN() are constant ex‐
105 pressions (assuming their argument is constant), meaning that these
106 values can be used to declare the size of global variables. This may
107 not be portable, however.
108
110 This code looks for the IP_TTL option in a received ancillary buffer:
111 struct msghdr msgh;
113 struct cmsghdr *cmsg;
114 int received_ttl;
115 /* Receive auxiliary data in msgh */
117 for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
119 cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
120 if (cmsg->cmsg_level == IPPROTO_IP
121 && cmsg->cmsg_type == IP_TTL) {
122 memcpy(&receive_ttl, CMSG_DATA(cmsg), sizeof(received_ttl));
123 break;
124 }
125 }
126 if (cmsg == NULL) {
128 /* Error: IP_TTL not enabled or small buffer or I/O error */
129 }
130 The code below passes an array of file descriptors over a UNIX domain
132 socket using SCM_RIGHTS:
133 struct msghdr msg = { 0 };
135 struct cmsghdr *cmsg;
136 int myfds[NUM_FD]; /* Contains the file descriptors to pass */
137 char iobuf[1];
138 struct iovec io = {
139 .iov_base = iobuf,
140 .iov_len = sizeof(iobuf)
141 };
142 union { /* Ancillary data buffer, wrapped in a union
143 in order to ensure it is suitably aligned */
144 char buf[CMSG_SPACE(sizeof(myfds))];
145 struct cmsghdr align;
146 } u;
147 msg.msg_iov = &io;
149 msg.msg_iovlen = 1;
150 msg.msg_control = u.buf;
151 msg.msg_controllen = sizeof(u.buf);
152 cmsg = CMSG_FIRSTHDR(&msg);
153 cmsg->cmsg_level = SOL_SOCKET;
154 cmsg->cmsg_type = SCM_RIGHTS;
155 cmsg->cmsg_len = CMSG_LEN(sizeof(myfds));
156 memcpy(CMSG_DATA(cmsg), myfds, sizeof(myfds));
157 For a complete code example that shows passing of file descriptors over
159 a UNIX domain socket, see seccomp_unotify(2).
160
162 recvmsg(2), sendmsg(2)
163 RFC 2292
165
167 This page is part of release 5.13 of the Linux man-pages project. A
168 description of the project, information about reporting bugs, and the
169 latest version of this page, can be found at
170 https://www.kernel.org/doc/man-pages/.
171Linux 2021-03-22 CMSG(3)