您可以使用IP_PKTINFO来获取数据包发送到的多播组的地址 - 即使套接字订阅了一堆多播组。有了这个，您将按顺序获取数据包并能够按组地址进行过滤。请参见下面的示例：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/stat.h>
#include <ctype.h>
#include <errno.h>
#define PORT 1234
#define PPANIC(msg) perror(msg); exit(1);
#define STATS_PATCH 0
int main(int argc, char **argv)
{
fd_set master;
fd_set read_fds;
struct sockaddr_in serveraddr;
int sock;
int opt = 1;
size_t i;
int rc;
char *mcast_groups[] = {
"226.0.0.1",
"226.0.0.2",
NULL
};
#if STATS_PATCH
struct stat stat_buf;
#endif
struct ip_mreq imreq;
FD_ZERO(&master);
FD_ZERO(&read_fds);
rc = sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if(rc == -1)
{
PPANIC("socket() failed");
}
rc = setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
if(rc == -1)
{
PPANIC("setsockopt(reuse) failed");
}
memset(&serveraddr, 0, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
serveraddr.sin_port = htons(PORT);
serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
rc = bind(sock, (struct sockaddr *)&serveraddr, sizeof(serveraddr));
if(rc == -1)
{
PPANIC("bind() failed");
}
rc = setsockopt(sock, IPPROTO_IP, IP_PKTINFO, &opt, sizeof(opt));
if(rc == -1)
{
PPANIC("setsockopt(IP_PKTINFO) failed");
}
for (i = 0; mcast_groups[i] != NULL; i++)
{
imreq.imr_multiaddr.s_addr = inet_addr(mcast_groups[i]);
imreq.imr_interface.s_addr = INADDR_ANY;
rc = setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (const void *)&imreq, sizeof(struct ip_mreq));
if (rc != 0)
{
PPANIC("joing mcast group failed");
}
}
FD_SET(sock, &master);
while(1)
{
read_fds = master;
rc = select(sock + 1, &read_fds, NULL, NULL, NULL);
if (rc == 0)
{
continue;
}
if(rc == -1)
{
PPANIC("select() failed");
}
if(FD_ISSET(sock, &read_fds))
{
char buf[1024];
int inb;
char ctrl_msg_buf[1024];
struct iovec iov[1];
iov[0].iov_base = buf;
iov[0].iov_len = 1024;
struct msghdr msg_hdr = {
.msg_iov = iov,
.msg_iovlen = 1,
.msg_name = NULL,
.msg_namelen = 0,
.msg_control = ctrl_msg_buf,
.msg_controllen = sizeof(ctrl_msg_buf),
};
struct cmsghdr *ctrl_msg_hdr;
inb = recvmsg(sock, &msg_hdr, 0);
if (inb < 0)
{
PPANIC("recvmsg() failed");
}
for (ctrl_msg_hdr = CMSG_FIRSTHDR(&msg_hdr); ctrl_msg_hdr != NULL; ctrl_msg_hdr = CMSG_NXTHDR(&msg_hdr, ctrl_msg_hdr))
{
if (ctrl_msg_hdr->cmsg_level == IPPROTO_IP && ctrl_msg_hdr->cmsg_type == IP_PKTINFO)
{
struct in_pktinfo *pckt_info = (struct in_pktinfo *)CMSG_DATA(ctrl_msg_hdr);
printf("got data for mcast group: %s\n", inet_ntoa(pckt_info->ipi_addr));
break;
}
}
printf("|");
for (i = 0; i < inb; i++)
printf("%c", isprint(buf[i])?buf[i]:'?');
printf("|\n");
#if STATS_PATCH
rc = fstat(sock, &stat_buf);
if (rc == -1)
{
perror("fstat() failed");
} else {
printf("st_atime: %d\n", stat_buf.st_atime);
printf("st_mtime: %d\n", stat_buf.st_mtime);
printf("st_ctime: %d\n", stat_buf.st_ctime);
}
#endif
}
}
return 0;
}
下面的代码不会解决 OPs 问题，但可以指导处理类似需求的人
（编辑）一个人不应该在深夜做这样的事情......即使使用该解决方案，您也只会获得 select 处理 fd 的顺序 - 这不会给您任何有关帧到达时间的指示。
如此处所述，目前无法检索套接字的顺序或它们更改的时间戳，因为未为套接字 inode 设置所需的回调。但是如果你能够修补你的内核，你可以通过在 select 系统调用中设置时间来解决这个问题。
以下补丁可能会给您一个想法：
diff --git a/fs/select.c b/fs/select.c
index 467bb1c..3f2927e 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -435,6 +435,9 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
unsigned long in, out, ex, all_bits, bit = 1, mask, j;
unsigned long res_in = 0, res_out = 0, res_ex = 0;
+ struct timeval tv;
+
+ do_gettimeofday(&tv);
in = *inp++; out = *outp++; ex = *exp++;
all_bits = in | out | ex;
@@ -452,6 +455,16 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
f = fdget(i);
if (f.file) {
const struct file_operations *f_op;
+ struct kstat stat;
+
+ int ret;
+ u8 is_sock = 0;
+
+ ret = vfs_getattr(&f.file->f_path, &stat);
+ if(ret == 0 && S_ISSOCK(stat.mode)) {
+ is_sock = 1;
+ }
+
f_op = f.file->f_op;
mask = DEFAULT_POLLMASK;
if (f_op->poll) {
@@ -464,16 +477,22 @@ int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
res_in |= bit;
retval++;
wait->_qproc = NULL;
+ if(is_sock && f.file->f_inode)
+ f.file->f_inode->i_ctime.tv_sec = tv.tv_sec;
}
if ((mask & POLLOUT_SET) && (out & bit)) {
res_out |= bit;
retval++;
wait->_qproc = NULL;
+ if(is_sock && f.file->f_inode)
+ f.file->f_inode->i_ctime.tv_sec = tv.tv_sec;
}
if ((mask & POLLEX_SET) && (ex & bit)) {
res_ex |= bit;
retval++;
wait->_qproc = NULL;
+ if(is_sock && f.file->f_inode)
+ f.file->f_inode->i_ctime.tv_sec = tv.tv_sec;
}
/* got something, stop busy polling */
if (retval) {
笔记：
这是......只为你:) - 不要指望它出现在主线中
在测试每个相关 fd之前调用 do_gettimeofday() 。为了获得更高的粒度，这应该在每次迭代中完成（并且仅在需要时）。由于 stat-interface 仅提供一秒的粒度，您可以（！UGLY！）使用剩余时间属性将秒的小数部分映射到这些字段。
这是使用内核 3.16.0 完成的，并且没有经过很好的测试。不要在太空船或医疗设备中使用它。如果您想尝试一下，请获取文件系统映像（例如https://people.debian.org/~aurel32/qemu/amd64/debian_wheezy_amd64_standard.qcow2）并使用 qemu 对其进行测试：
sudo qemu-system-x86_64 -kernel arch/x86/boot/bzImage -hda debian_wheezy_amd64_standard.qcow2 -append "root=/dev/sda1"