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Windows thread API in the C program
Threads are created in the Windows API using the CreateThread() function, and—just as in Pthreads—a set of attributes like security information, the size of the stack, and a flag for the thread is passed to this function. In the below program, we use the default values for these attributes. (The default values do not initially set the thread to a suspended state and instead make it eligible to be run by the CPU scheduler.) Once the summation thread is created, the parent must wait for it to complete before outputting the value of Sum, as the value is set by the summation thread. In Pthread program, we had the parent thread wait for the summation thread using the pthread join() statement Here, using the WaitForSingleObject() function, we perform the equivalent of this in the Windows API , which causes the creating thread to block until the summation thread has exited. In situations that require waiting for multiple threads to complete, the WaitForMultipleObjects() function is used. This function is passed four parameters −
- The number of objects to wait for
- A pointer to the array of objects
- A flag indicating whether all objects have been signaled.
- A timeout duration (or INFINITE)
For example, if THandles is an array of thread HANDLE objects of size N, the parent thread can wait for all its child threads to complete with this statement −
WaitForMultipleObjects(N, THandles, TRUE, INFINITE);
Multithreaded C program using the Windows API.
Example
#include<windows.h>
#include<stdio.h>
DWORD Sum;
/* data is shared by the thread(s) */
/* thread runs in this separate function */
DWORD WINAPI Summation(LPVOID Param){
DWORD Upper = *(DWORD*)Param;
for (DWORD i = 0; i <= Upper; i++)
Sum += i;
return 0;
}
int main(int argc, char *argv[]){
DWORD ThreadId;
HANDLE ThreadHandle;
int Param;
if (argc != 2){
fprintf(stderr,"An integer parameter is required
");
return -1;
}
Param = atoi(argv[1]);
if (Param < 0){
fprintf(stderr,"An integer >= 0 is required
");
return -1;
}
/* create the thread */
ThreadHandle = CreateThread( NULL, /* default security attributes */ 0, /* default stack size */
Summation, /* thread function */ &Param, /* parameter to thread function */ 0, /* default creation flags */ &ThreadId);
/* returns the thread identifier */
if (ThreadHandle != NULL){
/* now wait for the thread to finish */ WaitForSingleObject(ThreadHandle,INFINITE);
/* close the thread handle */
CloseHandle(ThreadHandle);
printf("sum = %d
",Sum);
}
}
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