Question

I have problem with mutexes
I have this code and I dont any idea why it doesn’t work correctly…

#include <windows.h>
#include <process.h>
#include <stdio.h>
HANDLE mutex;
unsigned _stdcall t(void*){
printf(":D:D:D\n");
return NULL;
}
int main(){
mutex=CreateMutex(NULL,FALSE,NULL);
WaitForSingleObject(mutex,INFINITE);
_beginthreadex(NULL,NULL,&t,NULL,0,NULL);
WaitForSingleObject(mutex,INFINITE);
printf("HD\n");
}

the result is :

HD
:D:D:D

I expect not to see HD in console…..

but this code work correctly

HANDLE mutex;
unsigned _stdcall t(void*){
WaitForSingleObject(mutex,INFINITE);
printf(":D:D:D\n");
ReleaseMutex(mutex);
return NULL;
}
int main(){
mutex=CreateMutex(NULL,FALSE,NULL);
WaitForSingleObject(mutex,INFINITE);
_beginthreadex(NULL,NULL,&t,NULL,0,NULL);
printf("HD\n");
while(1){
}

} 

the result is:

HD

Thank you everyone….

Answer 1

As per MSDN:

The thread that owns a mutex can specify the same mutex in repeated wait function calls without blocking its execution.

Thus in your first sample, the second call to WaitForSingleObject() doesn’t block the main thread as it is the thread that owns the mutex.