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\n");
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\n");
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\n");
32 free (cmptr);
33 return -1;
34 } else if (nr == 0) {
35 fprintf (stderr, "connection closed by server\n");
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\n");
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\n");
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\n", 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\n");
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\n");
41 free (cmptr);
42 return -1;
43 } else if (nr == 0) {
44 fprintf (stderr, "connection closed by server\n");
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\n");
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\n", 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\n", 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\n");
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\n", 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\n", 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\n", 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\n");
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\n", RIGHTSLEN);
15 fprintf (stderr, "cmsghdr = %d, cmsglen = %d, after align = %d, control len = %d\n", sizeof(struct cmsghdr), CREDSLEN, CMSG_ALIGN(CREDSLEN), CONTROLLEN);
16
17 cmp = CMSG_NXTHDR(&msg, cmp);
18 fprintf (stderr, "cmp = %p\n", 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\n", 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\n", 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\n");
6 return 0;
7 }
8
9 fprintf (stderr, "%p: cmsg_len %u, cmsg_level %d, cmsg_type %d\n", __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\n");
13 return 0;
14 }
15
16 fprintf (stderr, "%p: cmsg_len %u, cmsg_level %d, cmsg_type %d\n", __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\n");
20 fprintf (stderr, "msg len %d, after align %d, msg control %d\n", __cmsg->cmsg_len, CMSG_ALIGN(__cmsg->cmsg_len), __mhdr->msg_controllen);
21 return 0;
22 }
23
24 fprintf (stderr, "in final step\n");
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！！
……
垃圾数据！！
……
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\n");
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~
来源：https://www.cnblogs.com/goodcitizen/p/12146060.html