This function determines whether the current thread is processing a message sent by another thread.

#define INCL_WINMESSAGEMGR /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HAB     hab;  /*  Anchor-block handle. */
BOOL    rc;   /*  Message-processing indicator. */

rc = WinInSendMsg(hab);