1CMSG(3) Linux Programmer's Manual CMSG(3)
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6 CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - Access ancillary
7 data
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10 #include <sys/socket.h>
11 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 struct cmsghdr {
20 socklen_t cmsg_len; /* data byte count, including header */
21 int cmsg_level; /* originating protocol */
22 int cmsg_type; /* protocol-specific type */
23 /* followed by unsigned char cmsg_data[]; */
24 };
25
27 These macros are used to create and access control messages (also
28 called ancillary data) that are not a part of the socket payload. This
29 control information may include the interface the packet was received
30 on, various rarely used header fields, an extended error description, a
31 set of file descriptors or Unix credentials. For instance, control
32 messages can be used to send additional header fields such as IP
33 options. Ancillary data is sent by calling sendmsg(2) and received by
34 calling recvmsg(2). See their manual pages for more information.
35 Ancillary data is a sequence of struct cmsghdr structures with appended
37 data. This sequence should only be accessed using the macros described
38 in this manual page and never directly. See the specific protocol man
39 pages for the available control message types. The maximum ancillary
40 buffer size allowed per socket can be set using /proc/sys/net/core/opt‐
41 mem_max; see socket(7).
42 CMSG_FIRSTHDR() returns a pointer to the first cmsghdr in the ancillary
44 data buffer associated with the passed msghdr.
45 CMSG_NXTHDR() returns the next valid cmsghdr after the passed cmsghdr.
47 It returns NULL when there isn't enough space left in the buffer.
48 CMSG_ALIGN(), given a length, returns it including the required align‐
50 ment. This is a constant expression.
51 CMSG_SPACE() returns the number of bytes an ancillary element with pay‐
53 load of the passed data length occupies. This is a constant expres‐
54 sion.
55 CMSG_DATA() returns a pointer to the data portion of a cmsghdr.
57 CMSG_LEN() returns the value to store in the cmsg_len member of the
59 cmsghdr structure, taking into account any necessary alignment. It
60 takes the data length as an argument. This is a constant expression.
61 To create ancillary data, first initialize the msg_controllen member of
63 the msghdr with the length of the control message buffer. Use
64 CMSG_FIRSTHDR() on the msghdr to get the first control message and
65 CMSG_NEXTHDR() to get all subsequent ones. In each control message,
66 initialize cmsg_len (with CMSG_LEN()), the other cmsghdr header fields,
67 and the data portion using CMSG_DATA(). Finally, the msg_controllen
68 field of the msghdr should be set to the sum of the CMSG_SPACE() of the
69 length of all control messages in the buffer. For more information on
70 the msghdr, see recvmsg(2).
71 When the control message buffer is too short to store all messages, the
73 MSG_CTRUNC flag is set in the msg_flags member of the msghdr.
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76 This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite,
77 the IPv6 advanced API described in RFC 2292 and the SUSv2.
78 CMSG_ALIGN() is a Linux extension.
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81 For portability, ancillary data should be accessed only using the
82 macros described here. CMSG_ALIGN() is a Linux extension and should be
83 not used in portable programs.
84 In Linux, CMSG_LEN(), CMSG_DATA(), and CMSG_ALIGN() are constant
86 expressions (assuming their argument is constant); this could be used
87 to declare the size of global variables. This may be not portable,
88 however.
89
91 This code looks for the IP_TTL option in a received ancillary buffer:
92 struct msghdr msgh;
94 struct cmsghdr *cmsg;
95 int *ttlptr;
96 int received_ttl;
97 /* Receive auxiliary data in msgh */
99 for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;
100 cmsg = CMSG_NXTHDR(&msgh,cmsg)) {
101 if (cmsg->cmsg_level == IPPROTO_IP
102 && cmsg->cmsg_type == IP_TTL) {
103 ttlptr = (int *) CMSG_DATA(cmsg);
104 received_ttl = *ttlptr;
105 break;
106 }
107 }
108 if (cmsg == NULL) {
109 /*
110 * Error: IP_TTL not enabled or small buffer
111 * or I/O error.
112 */
113 }
114 The code below passes an array of file descriptors over a Unix socket
116 using SCM_RIGHTS:
117 struct msghdr msg = {0};
119 struct cmsghdr *cmsg;
120 int myfds[NUM_FD]; /* Contains the file descriptors to pass. */
121 char buf[CMSG_SPACE(sizeof myfds)]; /* ancillary data buffer */
122 int *fdptr;
123 msg.msg_control = buf;
125 msg.msg_controllen = sizeof buf;
126 cmsg = CMSG_FIRSTHDR(&msg);
127 cmsg->cmsg_level = SOL_SOCKET;
128 cmsg->cmsg_type = SCM_RIGHTS;
129 cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
130 /* Initialize the payload: */
131 fdptr = (int *) CMSG_DATA(cmsg);
132 memcpy(fdptr, myfds, NUM_FD * sizeof(int));
133 /* Sum of the length of all control messages in the buffer: */
134 msg.msg_controllen = cmsg->cmsg_len;
135
137 recvmsg(2), sendmsg(2)
138 RFC 2292
140
142 This page is part of release 3.25 of the Linux man-pages project. A
143 description of the project, and information about reporting bugs, can
144 be found at http://www.kernel.org/doc/man-pages/.
145Linux 2008-11-20 CMSG(3)