How to use WaitForSingleObject - c++

Question

In order to try out how to program with the Win32 API, I wrote a program that creates a process.
Then I want to check if my process waits for the newly created process, close the handle and then check WaitForSingleObject again (the second process is sleeping for 700 ms)
First process:
#include <iostream>
#include <windows.h>
#include <string>
using namespace std;
void main()
{
bool ret;
bool retwait;
STARTUPINFO startupinfo;
GetStartupInfo (&startupinfo);
PROCESS_INFORMATION pro2info;
wchar_t wcsCommandLine[] = L"D:\\betriebssystemePRA1PRO2.exe";
ret = CreateProcess(NULL, wcsCommandLine, NULL, NULL, false, CREATE_NEW_CONSOLE, NULL,
NULL, &startupinfo, &pro2info);
cout<<"hProcess: "<<pro2info.hProcess<<endl;
cout<<"dwProcessId: "<<pro2info.dwProcessId <<endl;
if (retwait= WaitForSingleObject (pro2info.hProcess, INFINITE)==true)
cout<<"waitprocess:true"<<endl; //The process is finished
else
cout<<"waitprocess:false"<<endl;
CloseHandle (pro2info.hProcess);//prozesshandle schließen, "verliert connection"
if (retwait= WaitForSingleObject (pro2info.hProcess, INFINITE)==true) //When the process has finished
cout<<"waitprocess:true"<<endl;
else
cout<<"waitprocess:false"<<endl;
//cout<<GetLastError()<<endl; //Output the last error.
ExitProcess(0);
}
Second process:
#include <iostream>
#include <windows.h>
#include <string>
using namespace std;
void main()
{
int b;
b = GetCurrentProcessId();
cout << b << endl;
cout << "Druecken Sie Enter zum Beenden" << endl;
cin.get();
//Wait until the user confirms
Sleep (700);
ExitProcess(0);
cout<<"test";
}
The first process prints false, false ; but it should print true, false.
Instead of the if-else statement, I used this:
//switch(WaitForSingleObject (pro2info.hProcess, INFINITE)){
// case WAIT_OBJECT_0: cout << "ja";
// break;
// case WAIT_FAILED:cout << "nein";
// break;
// case WAIT_TIMEOUT:
// break;
//}
// cout<<"waitprocess:true"<<endl;//prozess ist fertig
//else
// cout<<"waitprocess:false"<<endl;
And this seems to work. What did I do wrong with my if-else statement?

Answer 1

You really need to pay attention to the meaning for the return value of the API functions. You cannot ignore a FALSE return from CreateProcess(). WaitForSingleObject() can return several values, it returns 0 if the wait completed successfully. Which makes you print "false".

Answer 2

According to MSDN, WaitForSingleObject will return WAIT_OBJECT_0 if the wait wasn't aborted. If you check the documentation, the value of WAIT_OBJECT_0 happens to be 0x00000000L, which happens to be the value commonly converted to false, not true. Hence your comparison fails.
Promoting the return value of WaitForSingleObject to a bool is IMHO not a good idea given that you get several potentially illuminating non-zero return codes that indicate why the wait expired.
If you still want to keep the above code as using a boolean check, change the tests to !WaitForSingleObject(...) instead.

Answer 3

I think you sort of answered your question yourself. The point is that WaitForSingleObject doesn't return true or false, but WAIT_OBJECT_0 et al.
So instead of
if (retwait= WaitForSingleObject (pro2info.hProcess, INFINITE)==true)
you need
if (retwait= WaitForSingleObject (pro2info.hProcess, INFINITE)==WAIT_OBJECT_0)