CMSG

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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;   /* data byte count, including header */
        int         cmsg_level; /* originating protocol */
        int         cmsg_type;  /* protocol-specific type */
/* followed by      unsigned charcmsg_data[]; */
};

ÀâÌÀ

Êä½õ¥Ç¡¼¥¿¤Ï struct cmsghdr ¹½Â¤ÂΤΥ·¡¼¥±¥ó¥¹¤ËÄɲåǡ¼¥¿¤¬Éղ䵤줿¤â¤Î¤Ç¤¢¤ë¡£ ¤³¤Î¥·¡¼¥±¥ó¥¹¤Ë¤Ï¤³¤Î¥Þ¥Ë¥å¥¢¥ë¥Ú¡¼¥¸¤Ë½ñ¤«¤ì¤Æ¤¤¤ë ¥Þ¥¯¥í¤ò»È¤Ã¤Æ¥¢¥¯¥»¥¹¤¹¤Ù¤­¤Ç¡¢Ä¾ÀÜ¥¢¥¯¥»¥¹¤¹¤Ù¤­¤Ç¤Ï¤Ê¤¤¡£ »ÈÍѲÄǽ¤ÊÀ©¸æ¥á¥Ã¥»¡¼¥¸¤Î¥¿¥¤¥×¤Ë¤Ä¤¤¤Æ¤Ï¡¢ ¤½¤ì¤¾¤ì¤Î¥×¥í¥È¥³¥ë¤Î¥Þ¥Ë¥å¥¢¥ë¥Ú¡¼¥¸¤ò»²¾È¤Î¤³¤È¡£ ÀܳËè¤ÎºÇÂçÊä½õÍѥХåե¡¥µ¥¤¥º¤Ï net.core.optmem_max sysctl ¤ò»È¤Ã¤ÆÀßÄê¤Ç¤­¤ë¡£ socket(7) ¤ò»²¾È¡£

CMSG_FIRSTHDR ¤Ï¡¢ÅϤ·¤¿ msghdr ¤Ë´ØÏ¢¤·¤¿Êä½õ¥Ç¡¼¥¿¥Ð¥Ã¥Õ¥¡Ãæ¤Î¡¢ºÇ½é¤Î cmsghdr ¤Ø¤Î¥Ý¥¤¥ó¥¿¤òÊÖ¤¹¡£

CMSG_NXTHDR ¤Ï¡¢ÅϤ·¤¿ cmsghdr ¤Î¼¡¤Ë¤¯¤ë (Í­¸ú¤Ê) cmsghdr ¤òÊÖ¤¹¡£ ¥Ð¥Ã¥Õ¥¡¤Ë½½Ê¬¤Ê¶õ¤­¤¬Ìµ¤¤¾ì¹ç¡¢ NULL ¤òÊÖ¤¹¡£

CMSG_ALIGN ¤ËŤµ¤òÍ¿¤¨¤ë¤È¡¢É¬Íפʥ¢¥é¥¤¥ó¥á¥ó¥È¤ò²ÃÌ£¤·¤¿Ä¹¤µ¤òÊÖ¤·¤Æ¤¯¤ë¡£ ¤³¤ì¤ÏÄê¿ô¼°¤Ç¤¢¤ë¡£

CMSG_SPACE ¤Ï¡¢Í¿¤¨¤¿¥Ç¡¼¥¿Ä¹¤¬Àê¤á¤ë¤Î¤ËɬÍפÊÊä½õÍ×ÁÇ (ancillary element) ¤Î ¥Ð¥¤¥È¿ô¤òÊÖ¤¹¡£¤³¤ì¤ÏÄê¿ô¼°¤Ç¤¢¤ë¡£

CMSG_DATA ¤Ï¡¢ cmsghdr ¤Î¥Ç¡¼¥¿Éôʬ¤Ø¤Î¥Ý¥¤¥ó¥¿¤òÊÖ¤¹¡£

CMSG_LEN ¤Ï¡¢ cmsghdr ¹½Â¤ÂΤΠcmsg_len ¥á¥ó¥Ð¤Ë¥Ç¡¼¥¿¤ò³ÊǼ¤¹¤ëºÝ¤ËɬÍפÊÃͤòÊÖ¤¹¡£¥¢¥é¥¤¥ó¥á¥ó¥È¤â¹Íθ¤ËÆþ¤ì ¤é¤ì¤ë¡£ °ú¿ô¤È¤·¤Æ¥Ç¡¼¥¿Ä¹¤ò¤È¤ë¡£¤³¤ì¤ÏÄê¿ô¼°¤Ç¤¢¤ë¡£

Êä½õ¥Ç¡¼¥¿¤òºî¤ë¤¿¤á¤Ë¤ÏºÇ½é¤Ë msghdr ¤Î¥á¥ó¥Ð¡¼ msg_controllen ¤ò¡¢À©¸æ¥á¥Ã¥»¡¼¥¸¥Ð¥Ã¥Õ¥¡¤ÎŤµ¤Ç½é´ü²½¤¹¤ë¡£ CMSG_FIRSTHDR ¤ò msghdr ¤ËÍѤ¤¤ë¤ÈºÇ½é¤ÎÀ©¸æ¥á¥Ã¥»¡¼¥¸¤¬ÆÀ¤é¤ì¡¢ CMSG_NEXTHDR ¤ò»È¤¦¤È¼¡¤ÎÀ©¸æ¥á¥Ã¥»¡¼¥¸¤¬ÆÀ¤é¤ì¤ë¡£ ¤½¤ì¤¾¤ì¤ÎÀ©¸æ¥á¥Ã¥»¡¼¥¸¤Ç¤Ï¡¢ cmsg_len ¤ò½é´ü²½¤¹¤ë ( CMSG_LEN ¤ò»È¤¦)¡£ ¤½¤Î¾¤Î cmsghdr ¥Ø¥Ã¥À¡¼¥Õ¥£¡¼¥ë¥É¡¢¤½¤·¤Æ¥Ç¡¼¥¿Éôʬ¤ËÂФ·¤Æ¤â CMSG_LEN ¤ò»È¤Ã¤Æ½é´ü²½¤ò¤¹¤ë¡£ ºÇ¸å¤Ë msghdr ¤Î msg_controllen ¥Õ¥£¡¼¥ë¥É¤Ë¡¢¥Ð¥Ã¥Õ¥¡Ãæ¤ÎÀ©¸æ¥á¥Ã¥»¡¼¥¸¤ÎŤµ¤Î CMSG_SPACE ¤Î¹ç·×¤¬¥»¥Ã¥È¤µ¤ì¤ë¡£ msghdr ¤Ë¤Ä¤¤¤Æ¤Î¾ÜºÙ¤Ï recvmsg(2) ¤ò»²¾È¡£

À©¸æ¥á¥Ã¥»¡¼¥¸¥Ð¥Ã¥Õ¥¡¤¬Á´¤Æ¤Î¥á¥Ã¥»¡¼¥¸¤òǼ¤á¤ë¤Î¤Ë¤Ïû¤¹¤®¤ë¾ì¹ç¡¢ msghdr ¤Î msg_flags ¥á¥ó¥Ð¡¼¤Ë MSG_CTRUNC ¥Õ¥é¥°¤¬¥»¥Ã¥È¤µ¤ì¤ë¡£  

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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 == SOL_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.
         */ 
} 

°Ê²¼¤Î¥³¡¼¥É¤Ï¡¢ SCM_RIGHTS ¤ò»È¤¤¡¢¥Õ¥¡¥¤¥ë¥Ç¥¹¥¯¥ê¥×¥¿¤ÎÇÛÎó¤ò Unix¥½¥±¥Ã¥È¤òÄ̤·¤ÆÁ÷¤ë¤â¤Î¤Ç¤¢¤ë¡£

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;

Ãí°Õ

Linux ¤Ç¤Ï CMSG_LEN, CMSG_DATA, CMSG_ALIGN ¤ÏÄê¿ô¼°¤Ç¤¢¤ë (¤½¤ì¤é¤Î°ú¿ô¤¬Äê¿ô¤È¤ß¤Ê¤µ¤ì¤ë)¡£ ¤³¤Î¤³¤È¤Ï¡¢¹­°èÊÑ¿ô¤Î¥µ¥¤¥º¤òÀë¸À¤¹¤ë¤Î¤Ë»È¤¨¤ë¡£ ¤·¤«¤·°Ü¿¢À­¤Ï¤Ê¤¯¤Ê¤ë¤À¤í¤¦¡£  

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RFC 2292

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