以下是我迄今为止所做的(剥离错误检查):
struct sockaddr_in addr, ss, dest;
int port, s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
fcntl(s, F_SETFL, fcntl(s, F_GETFL, 0) | O_NONBLOCK);
memset((char*) &addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
inet_aton("127.0.0.1", &addr.sin_addr);
bind(s, (struct sockaddr*) &addr, sizeof(addr));
unsigned int len = sizeof(ss);
getsockname(s, (struct sockaddr*) &ss, &len);
port = ss.sin_port;
memset((char*) &dest, 0, sizeof(dest));
dest.sin_family = AF_INET;
dest.sin_port = htons(port);
inet_aton("127.0.0.1", &dest.sin_addr);
sendto(s, "test", 5, 0, (struct sockaddr*) &dest, sizeof(dest));
char buf[5];
recv(s, buf, 5, 0);
最后一句话失败,消息为Resource temporarily unavailable
(因为O_NONBLOCK
标志)。
在这个片段中,我让操作系统绑定一个随机端口,然后用getsockname
获得它。如果我使用固定端口,并删除对getsockname
的调用,那么它就可以工作了。
附言:我在一台linux机器上。
port = ss.sin_port
应给出网络订购的端口号。当您用dest.sin_port = htons(port)
分配端口时,您正在将htons()
应用于已按网络字节顺序排列的短端口。改用dest.sin_port = port
,一切都会好起来的。
或者,如果您想从getsockname()
结果中获得主机订购的端口号,您应该使用ntohs()
:
getsockname(s, (struct sockaddr*) &ss, &len);
port = ntohs(ss.sin_port);
/*...*/
dest.sin_port = htons(port);
当您捕获操作系统分配的端口时,忘记了使用ntohs
#include <netinet/in.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
int main()
{
struct sockaddr_in addr = {}, ss, dest = {};
int port, s = socket(AF_INET, SOCK_DGRAM | SOCK_NONBLOCK, 0);
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
bind(s, (struct sockaddr*) &addr, sizeof(addr));
unsigned int len = sizeof(ss);
getsockname(s, (struct sockaddr*) &ss, &len);
port = ntohs(ss.sin_port);
dest.sin_family = AF_INET;
dest.sin_port = htons(port);
dest.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
sendto(s, "test", 5, 0, (struct sockaddr*) &dest, sizeof(dest));
char buf[5];
int got = recv(s, buf, 5, 0);
printf("got: %d, errno: %sn", got, strerror(errno));
return 0;
}