问题描述
我已经建立了 Winsock2 连接,但我需要涵盖互联网中断的情况。这是我的代码;
#include <winsock2.h>
#include <windows.h>
#include <ctime>
int main()
{
WSADATA w;
if(WSAStartup(MAKEWORD(2,2),&w)) return 0;
sockaddr_in sad;
sad.sin_family=AF_INET;
sad.sin_addr.s_addr=inet_addr("200.20.186.76");
sad.sin_port=htons(123);
sockaddr saddr;
int saddr_l=sizeof(saddr);
int s=socket(PF_INET,SOCK_DGRAM,IPPROTO_UDP);
if(s==INVALID_SOCKET) return 0;
char msg[48]={8};
if(sendto(s,msg,sizeof(msg),(sockaddr*)&sad,sizeof(sad))==SOCKET_ERROR) return 0;
if(recvfrom(s,48,&saddr,&saddr_l)==SOCKET_ERROR) return 0;
if(closesocket(s)==SOCKET_ERROR) return 0;
if(WSACleanup()) return 0;
return 0;
}
- 我可以像使用
select
时那样设置超时吗 - 我还能如何防止等待并使其立即返回?文档指出:
在发出诸如 sendto 之类的阻塞 Winsock 调用时,Winsock 可能需要等待网络事件才能完成调用。在这种情况下,Winsock 会执行警报等待,该等待可能会被安排在同一线程上的异步过程调用 (APC) 中断。
怎么做?
解决方法
如果你想发出一个 recvfrom() 并让它立即返回,那么你自己决定等待多长时间(我假设 Windows 因为你包含了 winsock2.h),你可以发出一个异步的 OVERLAPPED 请求,然后通过等待 OVERLAPPED 结构的 hEvent 成员发出信号,随时等待完成。
这是根据您的原始代码更新的示例。
- 您可以使用 WaitForSingleObject 设置超时,只要您需要等待(下面我等待 10 秒 6 次)
- 通过传递一个 OVERLAPPED 指针,您表示您将自己等待完成。请注意,在发出 hEvent 信号之前,OVERLAPPED 结构不能超出范围。 (或释放,如果 OVERLAPPED 是动态分配的)。
- 在保证 IO 完成之前让 OVERLAPPED 超出范围是一个常见的 Winsock 错误(我已经在 Winsock 上工作了 10 多年 - 我已经看到了这个错误的许多变体)
- 如下所述,如果您不知道 hEvent 已发出信号,则在调用 closesocket 后,您必须 等待 hEvent 发出信号后再继续 - closesocket 并不能保证所有异步 IO 请求都已完成回来之前。
#define _WINSOCK_DEPRECATED_NO_WARNINGS
#include <winsock2.h>
#include <windows.h>
#include <ctime>
int main()
{
WSADATA w;
if (WSAStartup(MAKEWORD(2,2),&w)) return 0;
sockaddr_in sad;
sad.sin_family = AF_INET;
sad.sin_addr.s_addr = inet_addr("200.20.186.76");
sad.sin_port = htons(123);
sockaddr saddr;
int saddr_l = sizeof(saddr);
int s = socket(PF_INET,SOCK_DGRAM,IPPROTO_UDP);
if (s == INVALID_SOCKET) return 0;
char msg[48] = { 8 };
if (sendto(s,msg,sizeof(msg),(sockaddr*)&sad,sizeof(sad)) == SOCKET_ERROR) return 0;
OVERLAPPED ov{};
ov.hEvent = CreateEvent(nullptr,TRUE,FALSE,nullptr);
if (ov.hEvent == nullptr) return 0;
WSABUF wsabuffer{};
wsabuffer.buf = msg;
wsabuffer.len = 48;
DWORD flags = 0;
if (WSARecvFrom(s,&wsabuffer,1,nullptr,&flags,&saddr,&saddr_l,&ov,nullptr) == SOCKET_ERROR)
{
DWORD gle = WSAGetLastError();
if (gle != WSA_IO_PENDING) return 0;
}
for (DWORD recv_count = 0; recv_count < 6; ++recv_count)
{
DWORD wait = WaitForSingleObject(ov.hEvent,10000);
if (wait == WAIT_FAILED) return 0;
if (wait == WAIT_OBJECT_0) break; // WSARecvFrom completed
if (wait == WAIT_TIMEOUT) continue; // WSARecvFrom is still pended waiting for data
}
// assuming WSARecvFrom completed - i.e. ov.hEvent was signaled
DWORD transferred;
if (WSAGetOverlappedResult(s,&transferred,&flags))
{
// WSARecvFrom completed successfully - 'transferred' shows the # of bytes that were received
}
else
{
DWORD gle = WSAGetLastError();
gle;
// WSARecvFrom failed with the error code in 'gle'
}
if (closesocket(s) == SOCKET_ERROR) return 0;
// with real code,we must guarantee that hEvent is set after calling closesocket
// e.g. if we get here in an error path
// closesocket() won't guarantee all async IO has completed before returning
WaitForSingleObject(ov.hEvent,INFINITE);
if (WSACleanup()) return 0;
return 0;
}