继 记一次传递文件句柄引发的血案 之后，这个 demo 又引发了一次血案，现录如下。
这次我是在 linux 上测试文件句柄的传递，linux 上并没有 STREAMS 系统，
因此是采用 unix domain socket 的 sendmsg/recvmsg 中控制消息部分来传递句柄的。
代码的主要修改部分集中于发送 fd 与接收 fd 处，一开始代码是这样的，运行良好。
1 #define MAXLINE 128 2 #define RIGHTSLEN CMSG_LEN(sizeof(int)) 3 #define CREDSLEN CMSG_LEN(sizeof(struct CREDSTRUCT)) 4 #define CONTROLLEN (RIGHTSLEN+CREDSLEN) 5 6 int send_fd (int fd, int fd_to_send) 7 { 8 struct iovec iov[1]; 9 struct msghdr msg; 10 struct cmsghdr *cmptr = NULL; 11 char buf[2]; 12 13 iov[0].iov_base = buf; 14 iov[0].iov_len = 2; 15 16 msg.msg_iov = iov; 17 msg.msg_iovlen = 1; 18 msg.msg_name = NULL; 19 msg.msg_namelen = 0; 20 msg.msg_flags = 0; 21 22 if (fd_to_send < 0) { 23 msg.msg_control = NULL; 24 msg.msg_controllen = 0; 25 buf[1] = -fd_to_send; 26 if (buf[1] == 0) 27 buf[1] = 1; 28 } else { 29 if ((cmptr = malloc(CONTROLLEN)) == NULL) { 30 fprintf (stderr, "malloc memory failed "); 31 return -1; 32 } 33 34 msg.msg_control = cmptr; 35 msg.msg_controllen = CONTROLLEN; 36 37 cmptr->cmsg_level = SOL_SOCKET; 38 cmptr->cmsg_type = SCM_RIGHTS; 39 cmptr->cmsg_len = CONTROLLEN; 40 41 *(int *) CMSG_DATA(cmptr) = fd_to_send; 42 buf[1] = 0; 43 } 44 45 buf[0] = 0; 46 if (sendmsg(fd, &msg, 0) != 2) { 47 free (cmptr); 48 return -1; 49 } 50 51 free (cmptr); 52 return 0; 53 }
以上是发送句柄部分，重点位于 37-39 行，设置了控制消息的类型与句柄的值。
sendmsg 中的数据消息部分，用来兼容出错的场景（出错时可以提供一个-1~-255的错误码，及一段描述信息），关键信息位于控制部分。
下面来看消息的接收：
1 int recv_fd (int fd, uid_t *uidptr, ssize_t (*userfunc) (int, const void*, size_t)) 2 { 3 struct cmsghdr *cmptr = NULL; 4 int newfd, nr, status; 5 char *ptr; 6 char buf[MAXLINE]; 7 struct iovec iov[1]; 8 struct msghdr msg; 9 10 status = -1; 11 newfd = -1; 12 13 for (;;) { 14 iov[0].iov_base = buf; 15 iov[0].iov_len = sizeof (buf); 16 17 msg.msg_iov = iov; 18 msg.msg_iovlen = 1; 19 msg.msg_name = NULL; 20 msg.msg_namelen = 0; 21 22 if ((cmptr = malloc (CONTROLLEN)) == NULL) { 23 fprintf (stderr, "malloc error "); 24 return -1; 25 } 26 27 msg.msg_control = cmptr; 28 msg.msg_controllen = CONTROLLEN; 29 30 if ((nr = recvmsg (fd, &msg, 0)) < 0) { 31 fprintf (stderr, "recvmsg error "); 32 free (cmptr); 33 return -1; 34 } else if (nr == 0) { 35 fprintf (stderr, "connection closed by server "); 36 free (cmptr); 37 return -1; 38 } 39 40 for (ptr = buf; ptr < &buf[nr]; ) { 41 if (*ptr ++ == 0) { 42 if (ptr != &buf[nr-1]) { 43 fprintf (stderr, "message format error"); 44 free (cmptr); 45 return -1; 46 } 47 48 status = *ptr & 0xff; 49 if (status == 0) { 50 if (msg.msg_controllen != CONTROLLEN) { 51 fprintf (stderr, "status = 0 but no fd "); 52 free (cmptr); 53 return -1; 54 } 55 56 newfd = *(int *) CMSG_DATA(cmptr); 57 } else { 58 newfd = -status; 59 } 60 61 nr -= 2; 62 } 63 } 64 65 free(cmptr); 66 if (nr > 0 && (*userfunc)(STDERR_FILENO, buf, nr) != nr) 67 return -1; 68 69 if (status >= 0) 70 return newfd; 71 } 72 73 return -1; 74 }
接收部分的重点位于 56 行，这里取得了对方传递过来的文件句柄（注意不是简单的值传递！参考上篇文章）
其它一些代码则用来处理出错信息，当出现错误时，调用 userfunc 打印错误信息 (用户一般传递 write) 。
另外接口中 uidptr 参数并没有用，这个是为将来扩展预留的。
使用之前的 demo （spipe_server.c / spipe_client.c）编译、运行，输出结果如下：
./spipe_server ./spipe_client create pipe 3.4 3 7 create temp file /tmp/outliqA3i with fd 4 seek to head send fd 4 to peer recv fd 3, position 0 create temp file /tmp/inaLr30i with fd 4 source: 3 7 seek to head send fd 4 recv fd 5 from peer, position 0 10
可以看到通过新的方式传递的文件句柄值也发生了变化（从 4 变为 3），且也需要对文件偏移进行重置，否则还会掉到之前文章说的那个坑里。
问题出现在增加一些代码来传递发送进程凭证（如uid）时，此时发送方需要传递两个控制子消息（分别表示句柄与凭证），接收方也需要处理两个子消息。
新的发送代码如下：
1 #define MAXLINE 128 2 #if defined(SCM_CREDS) // on BSD 3 #define CREDSTRUCT cmsgcred 4 #define CR_UID cmcred_uid 5 #define CREDOPT LOCAL_PEERCRED 6 #define SCM_CREDTYPE SCM_CREDS 7 #elif defined(SCM_CREDENTIALS) // on linux 8 #define CREDSTRUCT ucred 9 #define CR_UID uid 10 #define CREDOPT SO_PASSCRED 11 #define SCM_CREDTYPE SCM_CREDENTIALS 12 #else 13 #error passing credentials is unsupported! 14 #endif 15 16 #define RIGHTSLEN CMSG_LEN(sizeof(int)) 17 #define CREDSLEN CMSG_LEN(sizeof(struct CREDSTRUCT)) 18 #define CONTROLLEN (RIGHTSLEN+CREDSLEN) 19 20 21 int send_fd (int fd, int fd_to_send) 22 { 23 struct iovec iov[1]; 24 struct msghdr msg; 25 struct cmsghdr *cmptr = NULL; 26 char buf[2]; 27 struct CREDSTRUCT *credp; 28 struct cmsghdr *cmp; 29 30 iov[0].iov_base = buf; 31 iov[0].iov_len = 2; 32 33 msg.msg_iov = iov; 34 msg.msg_iovlen = 1; 35 msg.msg_name = NULL; 36 msg.msg_namelen = 0; 37 msg.msg_flags = 0; 38 39 if (fd_to_send < 0) { 40 msg.msg_control = NULL; 41 msg.msg_controllen = 0; 42 buf[1] = -fd_to_send; 43 if (buf[1] == 0) 44 buf[1] = 1; 45 } else { 46 if ((cmptr = malloc(CONTROLLEN)) == NULL) { 47 fprintf (stderr, "malloc memory failed "); 48 return -1; 49 } 50 51 msg.msg_control = cmptr; 52 msg.msg_controllen = CONTROLLEN; 53 54 cmp = cmptr; 55 cmp->cmsg_level = SOL_SOCKET; 56 cmp->cmsg_type = SCM_RIGHTS; 57 cmp->cmsg_len = RIGHTSLEN; 58 *(int *) CMSG_DATA(cmp) = fd_to_send; 59 60 cmp = CMSG_NXTHDR(&msg, cmp); 61 cmp->cmsg_level = SOL_SOCKET; 62 cmp->cmsg_type = SCM_CREDTYPE; 63 cmp->cmsg_len = CREDSLEN; 64 credp = (struct CREDSTRUCT *) CMSG_DATA(cmp); 65 66 # if defined(SCM_CREDENTIALS) 67 // only linux need to set members of this struct ! 68 credp->uid = getuid (); 69 credp->gid = getegid (); 70 credp->pid = getpid (); 71 # endif 72 buf[1] = 0; 73 } 74 75 buf[0] = 0; 76 if (sendmsg(fd, &msg, 0) != 2) { 77 free (cmptr); 78 return -1; 79 } 80 81 free (cmptr); 82 return 0; 83 }
最开始的一些宏定义，是用来区分 linux 与 bsd 上一些细节，重点在 55-64 行，这两段代码分别设置了句柄与凭证。
然后控制消息的大小 CONTROLLEN 由两部分消息的长度（RIGHTSLEN 与 CREDSLEN）累加得到，分配的内存也是这么大。
再来看接收部分：
1 int recv_fd (int fd, uid_t *uidptr, ssize_t (*userfunc) (int, const void*, size_t)) 2 { 3 struct cmsghdr *cmptr = NULL; 4 5 int newfd, nr, status; 6 char *ptr; 7 char buf[MAXLINE]; 8 struct iovec iov[1]; 9 struct msghdr msg; 10 11 status = -1; 12 newfd = -1; 13 14 const int on = -1; 15 struct cmsghdr *cmp; 16 struct CREDSTRUCT *credp; 17 if (setsockopt (fd, SOL_SOCKET, CREDOPT, &on, sizeof(int)) < 0) { 18 fprintf (stderr, "setsockopt for %d failed ", CREDOPT); 19 return -1; 20 } 21 22 for (;;) { 23 iov[0].iov_base = buf; 24 iov[0].iov_len = sizeof (buf); 25 26 msg.msg_iov = iov; 27 msg.msg_iovlen = 1; 28 msg.msg_name = NULL; 29 msg.msg_namelen = 0; 30 31 if ((cmptr = malloc (CONTROLLEN)) == NULL) { 32 fprintf (stderr, "malloc error "); 33 return -1; 34 } 35 36 msg.msg_control = cmptr; 37 msg.msg_controllen = CONTROLLEN; 38 39 if ((nr = recvmsg (fd, &msg, 0)) < 0) { 40 fprintf (stderr, "recvmsg error "); 41 free (cmptr); 42 return -1; 43 } else if (nr == 0) { 44 fprintf (stderr, "connection closed by server "); 45 free (cmptr); 46 return -1; 47 } 48 49 for (ptr = buf; ptr < &buf[nr]; ) { 50 if (*ptr ++ == 0) { 51 if (ptr != &buf[nr-1]) { 52 fprintf (stderr, "message format error"); 53 free (cmptr); 54 return -1; 55 } 56 57 status = *ptr & 0xff; 58 if (status == 0) { 59 if (msg.msg_controllen != CONTROLLEN) { 60 fprintf (stderr, "status = 0 but no fd "); 61 free (cmptr); 62 return -1; 63 } 64 65 for (cmp = CMSG_FIRSTHDR(&msg); cmp != NULL; cmp = CMSG_NXTHDR(&msg, cmp)) { 66 if (cmp->cmsg_level != SOL_SOCKET) { 67 fprintf (stderr, "ignore unknown socket level %d ", cmp->cmsg_level); 68 continue; 69 } 70 71 switch (cmp->cmsg_type) { 72 case SCM_RIGHTS: 73 newfd = *(int *) CMSG_DATA(cmp); 74 break; 75 case SCM_CREDTYPE: 76 credp = (struct CREDSTRUCT *) CMSG_DATA(cmp); 77 *uidptr = credp->CR_UID; 78 break; 79 default: 80 fprintf (stderr, "ignore unknown msg type %d ", cmp->cmsg_type); 81 break; 82 } 83 } 84 } else { 85 newfd = -status; 86 } 87 88 nr -= 2; 89 } 90 } 91 92 free(cmptr); 93 if (nr > 0 && (*userfunc)(STDERR_FILENO, buf, nr) != nr) 94 return -1; 95 96 if (status >= 0) 97 return newfd; 98 } 99 100 return -1; 101 }
重点分为两个部分：
14-20 行，设置 unix domain socket 可以接收凭证信息；
65-83 行，分别读取控制消息中的句柄与凭证信息，这里我们取了发送进程的 uid 信息作为凭证返回给上层调用者；
与发送消息类似，这里使用系统提供的 CMSG_FIRSTHDR、CMSG_NXTHDR 在控制消息中遍历各个子部分。
重新编译、运行 demo，却发现出错了：
./spipe_server ./spipe_client create pipe 3.4 3 7 create temp file /tmp/outgQY1Y4 with fd 4 seek to head send fd 4 to peer recv fd 3, uid 500, position 0 create temp file /tmp/invVgKW4 with fd 4 source: 3 7 seek to head connection closed by server recv fd from peer failed, error -1
从输出日志看，第一次从 server 发往 client 的句柄及凭证是可以的（line 7），再之后 client 处理完消息回传时，就出错了。
首先定位出错代码位置，在 client 回传这里 (send_fd)，加入一些日志：
1 if ((cmptr = malloc(CONTROLLEN)) == NULL) { 2 fprintf (stderr, "malloc memory failed "); 3 return -1; 4 } 5 6 msg.msg_control = cmptr; 7 msg.msg_controllen = CONTROLLEN; 8 9 cmp = cmptr; 10 cmp->cmsg_level = SOL_SOCKET; 11 cmp->cmsg_type = SCM_RIGHTS; 12 cmp->cmsg_len = RIGHTSLEN; 13 *(int *) CMSG_DATA(cmp) = fd_to_send; 14 fprintf (stderr, "add fd with len %d ", RIGHTSLEN); 15 16 cmp = CMSG_NXTHDR(&msg, cmp); 17 cmp->cmsg_level = SOL_SOCKET; 18 cmp->cmsg_type = SCM_CREDTYPE; 19 cmp->cmsg_len = CREDSLEN; 20 credp = (struct CREDSTRUCT *) CMSG_DATA(cmp); 21 fprintf (stderr, "add credential with len %d ", CREDSLEN); 22 23 # if defined(SCM_CREDENTIALS) 24 // only linux need to set members of this struct ! 25 credp->uid = getuid (); 26 credp->gid = getegid (); 27 credp->pid = getpid (); 28 fprintf (stderr, "set uid %d, gid %d, pid %d ", credp->uid, credp->gid, credp->pid); 29 # endif 30 buf[1] = 0;
标黄的是新加入的输出日志，再次编译运行：
./spipe_server ./spipe_client create pipe 3.4 3 7 create temp file /tmp/outivt2Og with fd 4 seek to head add fd with len 16 add credential with len 24 set uid 500, gid 500, pid 12071 send fd 4 to peer recv fd 3, uid 500, position 0 create temp file /tmp/inHqRwMg with fd 4 source: 3 7 seek to head add fd with len 16 connection closed by server recv fd from peer failed, error -1
可以看到，第一次传递时，这三条日志全都正确输出了，而回传时，只输出了第一条日志。
所以明显是在第一条日志与第二条日志之间的代码出了问题。左看右看，看不出这块有什么问题，难道系统提供的 CMSG_NXTHDR 会出错?
这边再加两条日志：
1 if ((cmptr = malloc(CONTROLLEN)) == NULL) { 2 fprintf (stderr, "malloc memory failed "); 3 return -1; 4 } 5 6 msg.msg_control = cmptr; 7 msg.msg_controllen = CONTROLLEN; 8 9 cmp = cmptr; 10 cmp->cmsg_level = SOL_SOCKET; 11 cmp->cmsg_type = SCM_RIGHTS; 12 cmp->cmsg_len = RIGHTSLEN; 13 *(int *) CMSG_DATA(cmp) = fd_to_send; 14 fprintf (stderr, "add fd with len %d ", RIGHTSLEN); 15 fprintf (stderr, "cmsghdr = %d, cmsglen = %d, after align = %d, control len = %d ", sizeof(struct cmsghdr), CREDSLEN, CMSG_ALIGN(CREDSLEN), CONTROLLEN); 16 17 cmp = CMSG_NXTHDR(&msg, cmp); 18 fprintf (stderr, "cmp = %p ", cmp); 19 cmp->cmsg_level = SOL_SOCKET; 20 cmp->cmsg_type = SCM_CREDTYPE; 21 cmp->cmsg_len = CREDSLEN; 22 credp = (struct CREDSTRUCT *) CMSG_DATA(cmp); 23 fprintf (stderr, "add credential with len %d ", CREDSLEN); 24 25 # if defined(SCM_CREDENTIALS) 26 // only linux need to set members of this struct ! 27 credp->uid = getuid (); 28 credp->gid = getegid (); 29 credp->pid = getpid (); 30 fprintf (stderr, "set uid %d, gid %d, pid %d ", credp->uid, credp->gid, credp->pid); 31 # endif 32 buf[1] = 0;
第二条日志是主要怀疑的地方，看指针是否为空；第一条日志则是怀疑块大小计算有误，导致分配的内存不够大，指针递增时出现了范围错误，所以这里打印各种长度做验证。
再次运行后，又多了一些输出：
./spipe_server ./spipe_client create pipe 3.4 3 7 create temp file /tmp/out7UgSYZ with fd 4 seek to head add fd with len 16 cmsghdr = 12, cmsglen = 24, after align = 24, control len = 40 cmp = 0x9ded018 add credential with len 24 set uid 500, gid 500, pid 12100 send fd 4 to peer recv fd 3, uid 500, position 0 create temp file /tmp/inC3nyWZ with fd 4 source: 3 7 seek to head add fd with len 16 cmsghdr = 12, cmsglen = 24, after align = 24, control len = 40 cmp = (nil) connection closed by server recv fd from peer failed, error -1
神奇的地方出现了，同样的代码，相同的尺寸，第一次指针正常；第二次就为空了！
崩溃点找到了，但是还是一头雾水，看起来数据块都对齐了，计算也没毛病，难道是这个系统提供的宏 (CMSG_NXTHDR) 出问题了吗？
翻看头文件，找到这一段的定义 (我所在的系统，位于 /usr/include/bits/socket.h (L311))：
1 __EXTERN_INLINE struct cmsghdr * 2 __NTH (__cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg)) 3 { 4 if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr)) 5 /* The kernel header does this so there may be a reason. */ 6 return 0; 7 8 __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg 9 + CMSG_ALIGN (__cmsg->cmsg_len)); 10 if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control 11 + __mhdr->msg_controllen) 12 || ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len) 13 > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen))) 14 /* No more entries. */ 15 return 0; 16 return __cmsg; 17 }
这段 INLINE 函数主要包含三个判断，
1）子消息长度小于消息头长度，返回 null；
2）下一个子消息的消息头超出消息尾部，返回null；
3）下一个子消息的消息体超出消息尾部，返回null；
直接修改系统代码不方便，将这个函数拷贝到本地并重全名为 my_cmsg_nxthdr，在各个判断下面添加日志输出：
1 struct cmsghdr *my_cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg) 2 { 3 if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr)) { 4 /* The kernel header does this so there may be a reason. */ 5 fprintf (stderr, "in step1 "); 6 return 0; 7 } 8 9 fprintf (stderr, "%p: cmsg_len %u, cmsg_level %d, cmsg_type %d ", __cmsg, __cmsg->cmsg_len, __cmsg->cmsg_level, __cmsg->cmsg_type); 10 __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len)); 11 if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen)) { 12 fprintf (stderr, "in step2 "); 13 return 0; 14 } 15 16 fprintf (stderr, "%p: cmsg_len %u, cmsg_level %d, cmsg_type %d ", __cmsg, __cmsg->cmsg_len, __cmsg->cmsg_level, __cmsg->cmsg_type); 17 if (((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len) > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen))) { 18 /* No more entries. */ 19 fprintf (stderr, "in step3 "); 20 fprintf (stderr, "msg len %d, after align %d, msg control %d ", __cmsg->cmsg_len, CMSG_ALIGN(__cmsg->cmsg_len), __mhdr->msg_controllen); 21 return 0; 22 } 23 24 fprintf (stderr, "in final step "); 25 return __cmsg; 26 }
为了便于根据不同的判断条件输出日志，这里对判断条件进行了拆分。
再次运行 demo，输出如下：
./spipe_server ./spipe_client create pipe 3.4 3 7 create temp file /tmp/outh7NhIs with fd 4 seek to head add fd with len 16 cmsghdr = 12, cmsglen = 24, after align = 24, control len = 40 0x9336008: cmsg_len 16, cmsg_level 1, cmsg_type 1 0x9336018: cmsg_len 0, cmsg_level 0, cmsg_type 0 in final step cmp = 0x9336018 add credential with len 24 set uid 500, gid 500, pid 12171 send fd 4 to peer recv fd 3, uid 500, position 0 create temp file /tmp/inoJMmKs with fd 4 source: 3 7 seek to head add fd with len 16 cmsghdr = 12, cmsglen = 24, after align = 24, control len = 40 0x904d008: cmsg_len 16, cmsg_level 1, cmsg_type 1 0x904d018: cmsg_len 500, cmsg_level 500, cmsg_type 16 in step3 msg len 500, after align 500, msg control 40 cmp = (nil) connection closed by server recv fd from peer failed, error -1
原来是第三个判断出现了问题(Line 24)！
消息总长度是 16 + 24 = 40，而这里的第二个子消息单个的长度达到 500，明显越界了。
但是第二个子消息的长度明明是 24 呀，哪里跑出来的 500 呢？
而且它的其它字段也明显不对，例如消息 level 也是 500，消息类型是 16 ！
初步可以确定是这块内存被弄乱了，而从前面打印的消息指针(0x904d008 与 0x904d018)看，分配的大小是没问题的，因此内存越界问题先排除掉；
其次是我们设置好的内容……等等……我们好像还没有设置第二个子消息的内容！！
……
malloc 之后没有清空的垃圾数据！
……
这也是第一次调用没问题而第二次掉坑里的原因，随着系统内存的分配回收而存在一定的随机性！
找到原因之后，修改就简单了，可以将 malloc 替换为 calloc，或者简单的加一句 memset 来清空内存：
1 if ((cmptr = malloc(CONTROLLEN)) == NULL) { 2 fprintf (stderr, "malloc memory failed "); 3 return -1; 4 } 5 6 // important on linux, garbage data may mess cmsg_len fields, 7 // and cause CMSG_NXTHDR return null on protection. 8 memset (cmptr, 0, CONTROLLEN); 9 msg.msg_control = cmptr; 10 msg.msg_controllen = CONTROLLEN;
再次运行 demo，一切正常：
./spipe_server ./spipe_client create pipe 3.4 3 7 create temp file /tmp/outqsTYkp with fd 4 seek to head add fd with len 16 cmsghdr = 12, cmsglen = 24, after align = 24, control len = 40 0x814c008: cmsg_len 16, cmsg_level 1, cmsg_type 1 0x814c018: cmsg_len 0, cmsg_level 0, cmsg_type 0 in final step cmp = 0x814c018 add credential with len 24 set uid 500, gid 500, pid 12207 send fd 4 to peer recv fd 3, uid 500, position 0 create temp file /tmp/in3ntkip with fd 4 source: 3 7 seek to head add fd with len 16 cmsghdr = 12, cmsglen = 24, after align = 24, control len = 40 0x8389008: cmsg_len 16, cmsg_level 1, cmsg_type 1 0x8389018: cmsg_len 0, cmsg_level 0, cmsg_type 0 in final step cmp = 0x8389018 add credential with len 24 set uid 500, gid 500, pid 12208 send fd 4 recv fd 5, uid 500 from peer, position 0 10
通过这次 debug，找到了经典的 APUE 例子中的一个瑕疵 (随机性比较大，大师刚好没有遇到而已，可能你的机器也不复现)。
不过回过头来看这个场景，也不能全算在 coder 身上，我感觉系统提供的这个 CMSG_NXTHDR 宏也颇成问题：
如果我调用这个之前还没有设置下一个子消息，难道还不准我使用了么? 过多的检查反而弄巧成拙，总之一句话：差评！ 哈哈~