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> The OS/2 APIs DosWaitThread, DosEnterCritSec, and DosExitCritSec  
> 
> 1. DosWaitThread()
> 
>  It has two basic modes of operation. This function allows a thread to wait or check
>  for completion of another thread.
>  Places the current thread into a wait state until another thread in the 
>  current process has ended. It then returns the thread identifier of the ending 
>  thread. It can wait for a specific thread, by specifying the thread's ID, or it can wait
>  for any thread termiantion in the process, by spcifying 0 for the tread ID.  
> 
> 
> Syntax:
> 
> #define INCL_DOSPROCESS
> #include <os2.h>
>  
> PTID      ptid;    /*  Address of the thread identification. */
> ULONG     option;  /*  An indicator that specifies whether to return if no thread has ended. */
> APIRET    ulrc;    /*  Return Code. */
>  
> ulrc = DosWaitThread(ptid, option);
>  
> Prototype:
> 
> APIRET DosWaitThread(PTID ptid, Ulong option); 
> 
> 
> Parameters:
> 
> ptid (PTID) - in/out -   Address of the thread identification. 
> 
>      
> Input -  The address of the ThreadID of the thread of interest. If ptid 
> is 0, the current thread waits until the next thread in the 
> process has ended. If ptid is nonzero, the current thread waits 
> until the indicated thread has ended. 
> 
> Output - The ThreadID of the ended thread is returned in this field. 
>   
> option (ULONG) - input  
> An indicator that specifies whether to return if no thread has ended. 
> 
> The values of this field are shown in the following list: 
> 
> 0 - DCWW_WAIT 
> The current thread waits until a thread ends. If a thread has 
> already ended, the call returns immediately with the ptid. 
> 
> 1 -  DCWW_NOWAIT 
> The current thread does not wait if no threads have ended. 
>   
> ulrc (APIRET) - returns :   Return Code. 
> 
> DosWaitThread returns one of the following values: 
> 
> 0              NO_ERROR 
> 95             ERROR_INTERRUPT 
> 294            ERROR_THREAD_NOT_TERMINATED 
> 309            ERROR_INVALID_THREADID 
> 
> Remarks:
> 
> DosWaitThread is used to wait for termination of threads within a process. It 
> is usually used so that thread resources (for example, the stack) can be 
> recovered when a thread ends.
> 
> DosWaitThread waits on any thread within the current process, or on a specific thread within the process, based on the  ptid parameter's contents. option allows the caller the option of waiting until a thread ends, or getting immediate return and status.
> 
> If no thread has  ended and the DCWW_NOWAIT option is specified, the ptid field is preserved. 
> 
> If DosWaitThread is called with the input ptid set to the current thread (the 
> thread attempts to wait on its own termination), the  ERROR_INVALID_THREADID error
> code is returned.
> 
> ERROR_INVALID_THREADID is also returned if a caller attempts to wait on the termination
> of the thread  with a ptid of 1. 
> 
> Example:
> 
> This example creates a new thread within a process, sleeps for 1 second, 
> suspends the thread for 5 seconds, and then waits for the thread to 
> terminate. 
> 
> Compile this example with MULTITHREAD LIBRARIES. If you are using C/2, 
> use the /Gm+ switch. 
> 
> #define INCL_DOSPROCESS     /* Process and thread values */
> #define INCL_DOSERRORS      /* DOS error values          */
> #include <os2.h>
> #include <stdio.h>
>  
> void _System CntThreadProc(ULONG LoopMax);  /* Count Thread */
>  
> int main(VOID) {
> 
> 
>     TID         tidCntThread = 0;     /* ID returned for newly created thread */
>     PFNTHREAD   pfnCntThread = &CntThreadProc; /* Address of thread program   */
>     ULONG       ulThreadParm = 100;        /* Parameter to thread routine     */
>     APIRET      rc           = NO_ERROR;   /* Return code                     */
>  
> rc = DosCreateThread(&tidCntThread,  /* Thread ID (returned by function)  */
>                          pfnCntThread,   /* Address of thread program         */
>                          ulThreadParm,   /* Parameter passed to ThreadProc    */
>                          CREATE_READY |  /* Thread is ready when created      */
>                          STACK_SPARSE,   /* Do not pre-commit stack pages     */
>                          8192L);         /* Stack size, rounded to page bdy   */
> 
>     if (rc != NO_ERROR) {
> 
>        printf("DosCreateThread error: return code = %u\n", rc);
>        return 1;
>     }
>  
> rc = DosSleep (1000);  /* Sleep for a second to allow thread to run a bit */
> rc = DosSuspendThread (tidCntThread);
> 
> if (rc != NO_ERROR) {
> 
> printf("DosSuspendThread error: return code = %u\n", rc);
> return 1;
>     }
>  
> rc = DosSleep (5000);  /* Sleep 5 seconds before resuming the thread     */
>  
> rc = DosResumeThread (tidCntThread);
> 
> if (rc != NO_ERROR) {
> 
> printf("DosResumeThread error: return code = %u\n", rc);
> return 1;
>     }
>  
> rc = DosWaitThread (&tidCntThread, DCWW_WAIT);
> 
> if (rc != NO_ERROR) {
> printf ("DosWaitThread error : return code = %u\n", rc);
>     }
>  
> printf ("Thread has completed!\n");
>  
> return NO_ERROR;
>  }
>  
> void _System CntThreadProc(ULONG LoopMax )  /* Count thread */
> {
>     ULONG i = 0;                 /* Loop index */
>  
>     for (i=0;i < LoopMax;i++ ) {
>        printf ("%d\n", i);
>     }
>  
>     return;
>  }
>  
> 
> 2. DosEnterCritSec()
> 
> The DosEnterCritSec() function disables all thread rescheduling for the current process.
> 
> Syntax:
>  
> #define INCL_DOSPROCESS
> #include <os2.h>
>  
> APIRET    ulrc;  /*  Return Code. */
>  
> ulrc = DosEnterCritSec();
> 
> Prototype:
> 
> APIRET DosEnterCritSec(void); 
>  
> 
> Parameters:
> 
> ulrc (APIRET) - returns:  Return Code. 
> 
> DosEnterCritSec returns one of the following values: 
> 
> 0              NO_ERROR 
> 309            ERROR_INVALID_THREADID 
> 484            ERROR_CRITSEC_OVERFLOW 
> 
> Remarks:
> 
> DosEnterCritSec causes all other threads in the process to block themselves 
> and give up their time slice.
> 
> After a DosEnterCritSec request is made, no functions should be called that depend on another thread to do processing  until DosExitCritSec has completed.
> 
> !! Great care should be taken when using compiler runtime functions and other OS/2 functions after a DosEnterCritSec  request has been made, since the underlying processing of the called function may require processing by another thread and thus cause a 
> deadlock. !! 
> 
> A thread can also execute code without having to give up time slices to other threads in
> its process if it requests a priority class that is higher than those of the other threads. A
> thread's priority is examined with DosGetInfoBlocks, and changed with DosSetPriority. 
> 
> A count is maintained of the number of times DosEnterCritSec is issued without a corresponding DosExitCritSec. The count is incremented by  DosEnterCritSec and decremented by DosExitCritSec.
> 
> Normal thread dispatching is not restored until the count is zero. The outstanding
> DosEnterCritSec count is maintained in a word. If an overflow occurs, the count is set to
> the maximum value, no operation is performed, and the request returns with ERROR_CRITSEC_OVERFLOW. 
> 
> If a signal occurs, thread 1 begins execution to process the signal even  though another thread in the process has a DosEnterCritSec active. Thread 1 of a process is its initial
> thread of execution, not a thread created with DosCreateThread.
> 
> Any processing done by thread 1 to satisfy the signal must not include accessing the
> critical resource intended to be protected by DosEnterCritSec. 
> 
> There is one exception to the exclusivity of critical sections. If a signal occurs, thread 1
> will be allowed to execute its exception handling code even if another thread in the
> same process is in a critical section. For this reason, execption handling code must be
> carefully written to avoid performing any operations that might interfere with a thread
> in a critical section.
> 
> Note:  This function is very powerful and must be used with caution!
>  
> It should be used only in a most cooperative environment where the state of all threads in known.  While in the critical section, do not call other compiler runtime or OS/2 functions that could start another thread that it would depend on running before being able to
> return. 
> 
> DosQueryThreadContext can be used to obtain the context of other threads in the process
> once they have been blocked by DosEnterCritSec. 
> 
> Calls to DosEnterCritSec() can be nested. Thus, if three calls to DosEnterCritSec() are
> issued, three calls to DosExitSec() must be issued to reenable scheduling.
> 
> 
> ERROR_INVALID_THREADID is returned when an invalid attempt is made to enter a critical section of code in a signal handler or exception handler. 
> 
> ERROR_INVALID_THREADID is also returned when a dynamic link library (DLL) routine incorrectly issues DosEnterCritSec. 
> 
> Example:
> 
>  
> /* This example shows how a thread enters and exits a critical section of code. */
> 
> #define INCL_DOSPROCESS      /* Process values */
> #define INCL_DOSERRORS       /* Error values */
> #include <os2.h>
> #include <stdio.h>
>  
> int main(VOID)
> 
>     {
> 
> APIRET  rc = NO_ERROR;    /* Return code */
>  
> rc = DosEnterCritSec();
>  
> if (rc != NO_ERROR) {
> 
> printf("DosEnterCritSec error: return code = %u\n",rc);
> return 1;
>      }
> 
> /***********************************************/
> /* Add critical section code here.  While this */
> /* code is running, all other threads are      */
> /* stopped.  CALL NO LIBRARY OR OS/2 FUNCTIONS */
> /* HERE UNLESS YOU KNOW THEY DO NOT REQUIRE    */
> /* ACTION BY ANOTHER THREAD IN THE PROCESS.    */
> /***********************************************/
>  
> rc =  DosExitCritSec();
>  
> if (rc != NO_ERROR) {
> 
> printf("DosExitCritSec error: return code = %u\n",rc);
> return 1;
>      }
>  
> return NO_ERROR;
>      }
>  
> 
> 3. DosExitCritSec()
> 
> Syntax:
> 
> Restores normal thread dispatching for the current process. 
> 
> #define INCL_DOSPROCESS
> #include <os2.h>
>  
> APIRET    ulrc;  /*  Return Code. */
>  
> ulrc = DosExitCritSec();
>  
> Prototype:
> 
> APIRET DosExitCritSec(void); 
> 
> Parameters:
> 
> ulrc (APIRET) - returns : Return Code. 
> 
> DosExitCritSec returns one of the following values: 
> 
> 0              NO_ERROR 
> 309            ERROR_INVALID_THREADID 
> 485            ERROR_CRITSEC_UNDERFLOW 
> 
> Remarks:
> 
> DosExitCritSec is used following DosEnterCritSec to restore normal thread 
> switching to the threads of a process. 
> 
> A count is maintained of the number of times DosEnterCritSec is issued 
> without a corresponding DosExitCritSec. The count is incremented by 
> DosEnterCritSec, and decremented by DosExitCritSec. Normal thread 
> dispatching is not restored until the count is zero. 
> 
> If multiple DosEnterCritSec() calls have been issued, then the same number of 
> DosExitCritSec() calls are required before scheduling actually will be enabled.
> 
> The outstanding count is maintained in a word. If an underflow occurs (the 
> count is decremented below zero), the count is set to zero, no operation is 
> performed, and the request returns with ERROR_CRITSEC_UNDERFLOW. 
> 
> ERROR_INVALID_THREADID is returned when an invalid attempt is made to 
> exit a critical section of code in a signal handler or exception handler. 
> 
> ERROR_INVALID_THREADID is also returned when a dynamic link library (DLL) 
> routine incorrectly issues DosExitCritSec. 
> 
> 
> Example:
> 
> See the example concerning DosEnterCritSec() above.
>   
> 
> 
> 
> 
>  
>   
> 
> 
> 
> 
>  
>   
> 
> 
>

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