select with other objects than sockets on windows - windows

Question

I'm facing an issue doing a select() call waiting on a socket + pipe.
I know there are already some topics on that but I have read lots of things and their opposite and I can't figure out what is the best solution for my problem.
The best for me would be to use WaitForMultipleObjects() listening on these two objects but when I try to call it only on the WSAEvent object, it fails and last error catch is code 6 (Invalid Handle).
WSAEVENT sockEvent = WSACreateEvent();
sockEvent = WSAEventSelect(fd, sockEvent, FD_WRITE);
HANDLE *pHandles = &sockEvent;
DWORD dwEvent = WaitForMultipleObjects(1, pHandles, FALSE, amqp_time_ms_until(deadline));
switch (dwEvent)
{
// ghEvents[0] was signaled
case WAIT_OBJECT_0 + 0:
// TODO: Perform tasks required by this event
return AMQP_STATUS_OK;
// ghEvents[1] was signaled
case WAIT_OBJECT_0 + 1:
// TODO: Perform tasks required by this event
return AMQP_STATUS_POLL_EXTERNAL_WAKE;
case WAIT_TIMEOUT:
return AMQP_STATUS_TIMEOUT;
// Return value is invalid.
default:
return AMQP_STATUS_SOCKET_ERROR;
}
So WaitForMultipleObjects doesn't seems to Work with WinSocks events, however I have already seen some examples on the net working with it.
And the of WSACreateEvent documentation (https://msdn.microsoft.com/en-us/library/windows/desktop/ms741561%28v=vs.85%29.aspx) says this :
Windows Sockets 2 event objects are system objects in Windows
environments. Therefore, if a Windows application wants to use an
auto-reset event rather than a manual-reset event, the application can
call the CreateEvent function directly.
This doesn't mean that WSAEvent are based on regular windows events ? If it's the case why it doesn't work with WaitForMultipleObjects ? The doc says it can handle regular events.
Thanks for helping.

Answer 1

This is your problem:
sockEvent = WSAEventSelect(fd, sockEvent, FD_WRITE);
You're overwriting the event handle! (As documented, the return value for WSAEventSelect is either 0 or SOCKET_ERROR. It is not a new event handle.)
Try something like
if (WSAEventSelect(fd, sockEvent, FD_WRITE) != 0) return SOCKET_ERROR;

Answer 2

Looking at the declaration of WSAEVENT revealed that WSAEVENT is simply an alias for HANDLE. This explains the note of the WSACreateEvent documentation you added to your post. So WSACreateEvent simply creates a manual reset event by calling CreateEvent(..., TRUE, FALSE, ...);.
Therefore an event returned by WSACreateEvent has to work along with WaitForMultipleObjects(..).
According to the code you've posted I cannot see any reason why WaitForMultipleObjects(..) should return "invalid handle" when supplied with an event returned by WSACreateEvent...
It may be though that pipes do not work with WaitForMultipleObjects(..). I remember having problems with that a long time ago but I cannot remember the details right now. But maybe it is another place to start digging...
Here is the code of my little test application which creates two threads (one event thread signalling a normal event and a simple TCP/IP server sending data). In the main loop a connection to the server is established and signalled events are processed.
#include <winsock2.h>
#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#pragma comment(lib, "Ws2_32.lib");
#define SERVER_PORT 5000
HANDLE hSomeEvent;
HANDLE hSocketEvent;
DWORD WINAPI eventThread(LPVOID pData)
{
while (1)
{
SleepEx(2250, FALSE);
SetEvent(hSomeEvent);
}
return (0);
}
DWORD WINAPI serverThread(LPVOID pData)
{
SOCKET listener;
struct sockaddr_in sockaddr;
int size;
SOCKET client;
listener = socket(AF_INET, SOCK_STREAM, 0);
if (listener == INVALID_SOCKET)
{
printf("Could not create socket : %d" , WSAGetLastError());
}
sockaddr.sin_family = AF_INET;
sockaddr.sin_addr.s_addr = INADDR_ANY;
sockaddr.sin_port = htons(SERVER_PORT);
if (bind(listener, (struct sockaddr *)&sockaddr , sizeof(sockaddr)) == SOCKET_ERROR)
{
printf("Bind failed with error code : %d" , WSAGetLastError());
}
listen(listener, 1);
while (listener)
{
size = sizeof(struct sockaddr_in);
client = accept(listener, (struct sockaddr *)&sockaddr, &size);
printf("client connected\n");
while (client != INVALID_SOCKET)
{
SleepEx(5000, FALSE);
if (send(client, "hello\0", 6, 0) != 6)
{
closesocket(client);
shutdown(client, 2);
client = INVALID_SOCKET;
}
}
SetEvent(hSomeEvent);
}
return (0);
}
int main()
{
WSADATA wsaData;
HANDLE events[2];
DWORD result;
SOCKET s;
struct hostent *hp;
struct sockaddr_in sockaddr;
int len;
char buff[1024 * 16];
HANDLE *evtPtr;
WSAStartup(MAKEWORD(2, 2), &wsaData);
hSocketEvent = WSACreateEvent();
//hSocketEvent = CreateEvent(NULL, FALSE, FALSE, "socket_event");
hSomeEvent = CreateEvent(NULL, FALSE, FALSE, "some_event");
CreateThread(NULL, 0, eventThread, NULL, 0, &result);
CreateThread(NULL, 0, serverThread, NULL, 0, &result);
s = socket(AF_INET, SOCK_STREAM, 0);
if (s == INVALID_SOCKET)
{
printf("Could not create socket : %d" , WSAGetLastError());
}
hp = gethostbyname("127.0.0.1");
sockaddr.sin_addr.s_addr = *((unsigned long*)hp->h_addr);
sockaddr.sin_family = AF_INET;
sockaddr.sin_port = htons(SERVER_PORT);
if (connect(s, (struct sockaddr*)&sockaddr, sizeof(sockaddr)))
{
closesocket(s);
printf("Could not connect socket : %d" , WSAGetLastError());
}
WSAEventSelect(s, hSocketEvent, FD_READ);
do
{
//events[0] = hSocketEvent;
//events[1] = hSomeEvent;
//result = WaitForMultipleObjects(2, events, FALSE, 1000);
evtPtr = &hSocketEvent;
result = WaitForMultipleObjects(1, evtPtr, FALSE, 1000);
switch (result)
{
case WAIT_OBJECT_0 + 0:
printf("hSocketEvent is signalled!\n");
len = recv(s, buff, sizeof(buff), 0);
printf(" %d bytes received\n", len);
WSAResetEvent(hSocketEvent);
break;
case WAIT_OBJECT_0 + 1:
printf("hSomeEvent is signalled!\n");
break;
case WAIT_TIMEOUT:
printf("timeout\n");
break;
default:
printf("error = %d\n", GetLastError());
break;
}
}
while (1);
printf("\n\nend.");
getch();
return (0);
}
Note that if you use WSACreateEvent you have to manually reset the event after readinng the data (otherwise WaitForMultipleObjects(..) will go nuts).