cmsg(3) - linux man page

Name

CMSG_ALIGN, CMSG_SPACE, CMSG_NXTHDR, CMSG_FIRSTHDR - access ancillary data

Synopsis

#include <sys/socket.h>

struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh, struct cmsghdr *cmsg);size_t CMSG_ALIGN(size_t length);size_t CMSG_SPACE(size_t length);size_t CMSG_LEN(size_t length);unsigned char *CMSG_DATA(struct cmsghdr *cmsg);

struct cmsghdr {    socklen_t cmsg_len;    /*數據字節數，包含頭部*/    int       cmsg_level;  /* originating PRotocol */    int       cmsg_type;   /* protocol-specific type */    /* followed by unsigned char cmsg_data[]; */};
Description
這些宏用于創建和訪問控制消息(也被稱為附加數據)--不是socket playload部分。這些控制信息包含接受報文的網絡接口，各種很少用的Header字段，一個擴展的錯誤描敘，文件描敘的集合，或者Unix UNIXcredentials。舉個例子，控制消息可以用來發送附加Header 字段比如IP 選項。通過調用sendmsg或者recvmsg 發送或者接受輔助數據。詳細信息可以參考man。
Ancillary data is a sequence of struct cmsghdr structures with appended data. This sequence should only be accessed using the macros described inthis manual page and never directly. See the specific protocol man pages for the available control message types. The maximum ancillary buffer size allowed persocket can be set using /proc/sys/net/core/optmem_max; see socket(7).
CMSG_FIRSTHDR() returns a pointer to the first cmsghdr in the ancillary data buffer associated with the passedmsghdr.
CMSG_NXTHDR() returns the next valid cmsghdr after the passedcmsghdr. It returns NULL when there isn't enough space left in thebuffer.
CMSG_ALIGN(), given a length, returns it including the required alignment. This is a constant expression.
CMSG_SPACE() returns the number of bytes an ancillary element with payload of the passed data length occupies. This is a constant expression.
CMSG_DATA() returns a pointer to the data portion of a cmsghdr.
CMSG_LEN() returns the value to store in the cmsg_len member of thecmsghdr structure, taking into account any necessary alignment. Ittakes the data length as an argument. This is a constant expression.
To create ancillary data, first initialize the msg_controllen member of themsghdr with the length of the control message buffer. UseCMSG_FIRSTHDR() on themsghdr to get the first control message and CMSG_NXTHDR() to get all subsequent ones. In each control message,initializecmsg_len (with CMSG_LEN()), the other cmsghdr header fields, and the data portion usingCMSG_DATA(). Finally, themsg_controllen field of the msghdr should be set to the sum of the CMSG_SPACE() of the length of all control messages in the buffer. Formore information on themsghdr, see recvmsg(2).
When the control message buffer is too short to store all messages, the MSG_CTRUNC flag is set in the msg_flags member of themsghdr.
Conforming To
This ancillary data model conforms to the POSIX.1g draft, 4.4BSD-Lite, the IPv6 advanced API described in RFC 2292 and the SUSv2.CMSG_ALIGN() is aLinux extension.
Notes
For portability, ancillary data should be accessed only using the macros described here.CMSG_ALIGN() is a Linux extension and should be not used inportable programs.
In Linux, CMSG_LEN(), CMSG_DATA(), and CMSG_ALIGN() are constant expressions (assuming their argument is constant); this could be usedto declare the size of global variables. This may be not portable, however.
Example
This code looks for the IP_TTL option in a received ancillary buffer:
struct msghdr msgh;struct cmsghdr *cmsg;int *ttlptr;int received_ttl;/* Receive auxiliary data in msgh */for (cmsg = CMSG_FIRSTHDR(&msgh); cmsg != NULL;        cmsg = CMSG_NXTHDR(&msgh,cmsg)) {    if (cmsg->cmsg_level == IPPROTO_IP            && cmsg->cmsg_type == IP_TTL) {        ttlptr = (int *) CMSG_DATA(cmsg);        received_ttl = *ttlptr;        break;    }}if (cmsg == NULL) {    /*     * Error: IP_TTL not enabled or small buffer     * or I/O error.     */}The code below passes an array of file descriptors over a UNIX domain socket usingSCM_RIGHTS:
struct msghdr msg = {0};struct cmsghdr *cmsg;int myfds[NUM_FD]; /* Contains the file descriptors to pass. */char buf[CMSG_SPACE(sizeof myfds)];  /* ancillary data buffer */int *fdptr;msg.msg_control = buf;msg.msg_controllen = sizeof buf;cmsg = CMSG_FIRSTHDR(&msg);cmsg->cmsg_level = SOL_SOCKET;cmsg->cmsg_type = SCM_RIGHTS;cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);/* Initialize the payload: */fdptr = (int *) CMSG_DATA(cmsg);memcpy(fdptr, myfds, NUM_FD * sizeof(int));/* Sum of the length of all control messages in the buffer: */msg.msg_controllen = cmsg->cmsg_len;
See Also
recvmsg(2),sendmsg(2)
RFC 2292
Referenced By
ipv6(7),netlink(3),netlink(7),recv(2),rtnetlink(7),send(2),unix(7)