下面的代码来自Git。它加入了一个多播组并接收数据包。
在这里,我们循环接收数据并将其存储在名为
在这里,我们循环接收数据并将其存储在名为
msgbuf
的缓冲区中:while (1)
{
char msgbuf[MSGBUFSIZE];
const int addrlen = sizeof(addr);
const int nbytes = recvfrom(fd, msgbuf, MSGBUFSIZE, 0, (struct sockaddr *) &addr, &addrlen);
我该如何选择缓冲区msgBuf
的大小?它只需要是最大数据包大小吗?还是我需要在处理第一个数据包时存储多个数据包?
完整代码:
int main(int argc, char *argv[])
{
if (argc != 3) {
printf("Command line args should be multicast group and port\n");
printf("(e.g. for SSDP, `listener 239.255.255.250 1900`)\n");
return 1;
}
char* group = argv[1]; // e.g. 239.255.255.250 for SSDP
int port = atoi(argv[2]); // 0 if error, which is an invalid port
if(port <= 0)
{
perror("Invalid port");
return 1;
}
// create what looks like an ordinary UDP socket
//
int fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd < 0)
{
perror("socket");
return 1;
}
// allow multiple sockets to use the same PORT number
//
u_int yes = 1;
if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (char*) &yes, sizeof(yes)) < 0)
{
perror("Reusing ADDR failed");
return 1;
}
// set up destination address
//
struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY); // differs from sender
addr.sin_port = htons(port);
// bind to receive address
//
if (bind(fd, (struct sockaddr*) &addr, sizeof(addr)) < 0)
{
perror("bind");
return 1;
}
// use setsockopt() to request that the kernel join a multicast group
//
struct ip_mreq mreq;
mreq.imr_multiaddr.s_addr = inet_addr(group);
mreq.imr_interface.s_addr = htonl(INADDR_ANY);
if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*) &mreq, sizeof(mreq)) < 0)
{
perror("setsockopt");
return 1;
}
// now just enter a read-print loop
//
while (1)
{
char msgbuf[MSGBUFSIZE];
const int addrlen = sizeof(addr);
const int nbytes = recvfrom(fd, msgbuf, MSGBUFSIZE, 0, (struct sockaddr *) &addr, &addrlen);
if (nbytes < 0)
{
perror("recvfrom");
return 1;
}
msgbuf[nbytes] = '\0';
puts(msgbuf);
}
return 0;
}
recvfrom()
读取了MSGBUFSIZE
字节,则该代码会导致UB。然后,msgbuf[nbytes] = '\0'
会导致缓冲区溢出。如果这是生产代码,那么实际上数据包可能会更小。 - Ingo Leonhardt