DosSuspendThread

#define INCL_DOSPROCESS

USHORT  rc = DosSuspendThread(ThreadID);

TID              ThreadID;      /* Thread ID */

USHORT           rc;            /* return code */

Example 

The following example shows how to suspend and resume execution of a 
thread within a process.  The main thread creates Thread2 and allows it to 
begin executing.  Thread2 iterates through a loop that prints a line and 
then sleeps, relinquishing its time slice to the main thread. After one 
iteration by Thread2, the main thread suspends Thread2 and then resumes 
it.  Subsequently, Thread2 completes the remaining three iterations. 

#define INCL_DOSPROCESS

#include <os2.h>

#define   SEGSIZE       4000   /* Number of bytes requested in segment */
#define   ALLOCFLAGS    0      /* Segment allocation flags - no sharing */
#define   SLEEPSHORT    5L     /* Sleep interval - 5 milliseconds */
#define   SLEEPLONG     75L    /* Sleep interval - 75 milliseconds */
#define   RETURN_CODE   0      /* Return code for DosExit() */

VOID APIENTRY Thread2()
{
  USHORT     i;

  /* Loop with four iterations */
  for(i=1; i<5; i++)
  {
    printf("In Thread2, i is now %d\n", i);

    /* Sleep to relinquish time slice to main thread */
    DosSleep(SLEEPSHORT);          /* Sleep interval */
  }
  DosExit(EXIT_THREAD,             /* Action code - end a thread */
          RETURN_CODE);            /* Return code */
}

main()
{
  TID        ThreadID;             /* Thread identification */
  SEL        ThreadStackSel;       /* Segment selector for thread stack */
  PBYTE      StackEnd;             /* Ptr. to end of thread stack */
  USHORT     rc;

  /** Allocate segment for thread stack; make pointer to end of stack. **/
  /**  We must allocate a segment in order to preserve segment
  /**  protection for the thread.  **/

  rc = DosAllocSeg(SEGSIZE,             /* Number of bytes requested */
                   &ThreadStackSel,     /* Segment selector
                                                (returned) */
                   ALLOCFLAGS);         /* Allocation flags - no sharing */
  StackEnd = MAKEP(ThreadStackSel, SEGSIZE-1);

  /** Start Thread2 **/
  if(!(rc=DosCreateThread((PFNTHREAD) Thread2,    /* Thread address */
                       &ThreadID,                 /* Thread ID
                                                          (returned) */
                       StackEnd)))   /* End of thread stack */
    printf("Thread2 created.\n");

  /* Sleep to relinquish time slice to Thread2 */
  if(!(DosSleep(SLEEPSHORT)))                  /* Sleep interval */
    printf("Slept a little to let Thread2 execute.\n");

  /***** Suspend Thread2, do some work, then resume Thread2 *****/
  if(!(rc=DosSuspendThread(ThreadID)))         /* Thread ID */
    printf("Thread2 SUSPENDED.\n");
  printf("Perform work that will not be interrupted by Thread2.\n");
  if(!(rc=DosResumeThread(ThreadID)))          /* Thread ID */
    printf("Thread2 RESUMED.\n");
  printf("Now we may be interrupted by Thread2.\n");

  /* Sleep to allow Thread2 to complete */
  DosSleep(SLEEPLONG);                         /* Sleep interval */
}