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win_ce_semaphore.c
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win_ce_semaphore.c
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/* win_ce_semaphore.c
Copyright (c) 1998, Johnson M. Hart
(with corrections 2001 by Rainer Loritz)
Permission is granted for any and all use providing that this
copyright is properly acknowledged.
There are no assurances of suitability for any use whatsoever.
WINDOWS CE: There is a collection of Windows CE functions to simulate
semaphores using only a mutex and an event. As Windows CE events cannot
be named, these simulated semaphores cannot be named either.
Implementation notes:
1. All required internal data structures are allocated on the process's heap.
2. Where appropriate, a new error code is returned (see the header
file), or, if the error is a Win32 error, that code is unchanged.
3. Notice the new handle type "SYNCHHANDLE" that has handles, counters,
and other information. This structure will grow as new objects are added
to this set; some members are specific to only one or two of the objects.
4. Mutexes are used for critical sections. These could be replaced with
CRITICAL_SECTION objects but then this would give up the time out
capability.
5. The implementation shows several interesting aspects of synchronization, some
of which are specific to Win32 and some of which are general. These are pointed
out in the comments as appropriate.
6. The wait function emulates WaitForSingleObject only. An emulation of
WaitForMultipleObjects is much harder to implement outside the kernel,
and it is not clear how to handle a mixture of WCE semaphores and normal
events and mutexes. */
#include <windows.h>
#include "win_ce_semaphore.h"
static SYNCHHANDLE CleanUp (SYNCHHANDLE hSynch, DWORD Flags);
SYNCHHANDLE CreateSemaphoreCE (
LPSECURITY_ATTRIBUTES lpSemaphoreAttributes, /* pointer to security attributes */
LONG lInitialCount, /* initial count */
LONG lMaximumCount, /* maximum count */
LPCTSTR lpName )
/* Semaphore for use with Windows CE that does not support them directly.
Requires a counter, a mutex to protect the counter, and an
autoreset event.
Here are the rules that must always hold between the autoreset event
and the mutex (any violation of these rules by the CE semaphore functions
will, in all likelihood, result in a defect):
1. No thread can set, pulse, or reset the event,
nor can it access any part of the SYNCHHANDLE structure,
without first gaining ownership of the mutex.
BUT, a thread can wait on the event without owning the mutex
(this is clearly necessary or else the event could never be set).
2. The event is in a signaled state if and only if the current semaphore
count ("CurCount") is greater than zero.
3. The semaphore count is always >= 0 and <= the maximum count */
{
SYNCHHANDLE hSynch = NULL, result = NULL;
__try
{
if (lInitialCount > lMaximumCount || lMaximumCount < 0 || lInitialCount < 0)
{
/* Bad parameters */
SetLastError (SYNCH_ERROR);
__leave;
}
hSynch = HeapAlloc (GetProcessHeap(), HEAP_ZERO_MEMORY, SYNCH_HANDLE_SIZE);
if (hSynch == NULL) __leave;
hSynch->MaxCount = lMaximumCount;
hSynch->CurCount = lInitialCount;
hSynch->lpName = lpName;
hSynch->hMutex = CreateMutex (lpSemaphoreAttributes, FALSE, NULL);
WaitForSingleObject (hSynch->hMutex, INFINITE);
/* Create the event. It is initially signaled if and only if the
initial count is > 0 */
hSynch->hEvent = CreateEvent (lpSemaphoreAttributes, FALSE,
lInitialCount > 0, NULL);
ReleaseMutex (hSynch->hMutex);
hSynch->hSemph = NULL;
}
__finally
{
/* Return with the handle, or, if there was any error, return
a null after closing any open handles and freeing any allocated memory. */
result=CleanUp(hSynch, 6 /* An event and a mutex, but no semaphore. */);
}
return result;
}
BOOL ReleaseSemaphoreCE (SYNCHHANDLE hSemCE, LONG cReleaseCount, LPLONG lpPreviousCount)
/* Windows CE equivalent to ReleaseSemaphore. */
{
BOOL Result = TRUE;
/* Gain access to the object to assure that the release count
would not cause the total count to exceed the maximum. */
__try
{
WaitForSingleObject (hSemCE->hMutex, INFINITE);
/* reply only if asked to */
if (lpPreviousCount!=NULL)
*lpPreviousCount = hSemCE->CurCount;
if (hSemCE->CurCount + cReleaseCount > hSemCE->MaxCount || cReleaseCount <= 0)
{
SetLastError (SYNCH_ERROR);
Result = FALSE;
__leave;
}
hSemCE->CurCount += cReleaseCount;
/* Set the autoreset event, releasing exactly one waiting thread, now or
in the future. */
SetEvent (hSemCE->hEvent);
}
__finally
{
ReleaseMutex (hSemCE->hMutex);
}
return Result;
}
DWORD WaitForSemaphoreCE (SYNCHHANDLE hSemCE, DWORD dwMilliseconds)
/* Windows CE semaphore equivalent of WaitForSingleObject. */
{
DWORD WaitResult;
WaitResult = WaitForSingleObject (hSemCE->hMutex, dwMilliseconds);
if (WaitResult != WAIT_OBJECT_0 && WaitResult != WAIT_ABANDONED_0) return WaitResult;
while (hSemCE->CurCount <= 0)
{
/* The count is 0, and the thread must wait on the event (which, by
the rules, is currently reset) for semaphore resources to become
available. First, of course, the mutex must be released so that another
thread will be capable of setting the event. */
ReleaseMutex (hSemCE->hMutex);
/* Wait for the event to be signaled, indicating a semaphore state change.
The event is autoreset and signaled with a SetEvent (not PulseEvent)
so exactly one waiting thread (whether or not there is currently
a waiting thread) is released as a result of the SetEvent. */
WaitResult = WaitForSingleObject (hSemCE->hEvent, dwMilliseconds);
if (WaitResult != WAIT_OBJECT_0) return WaitResult;
/* This is where the properties of setting of an autoreset event is critical
to assure that, even if the semaphore state changes between the
preceding Wait and the next, and even if NO threads are waiting
on the event at the time of the SetEvent, at least one thread
will be released.
Pulsing a manual reset event would appear to work, but it would have
a defect which could appear if the semaphore state changed between
the two waits. */
WaitResult = WaitForSingleObject (hSemCE->hMutex, dwMilliseconds);
if (WaitResult != WAIT_OBJECT_0 && WaitResult != WAIT_ABANDONED_0) return WaitResult;
}
/* The count is not zero and this thread owns the mutex. */
hSemCE->CurCount--;
/* The event is now unsignaled, BUT, the semaphore count may not be
zero, in which case the event should be signaled again
before releasing the mutex. */
if (hSemCE->CurCount > 0) SetEvent (hSemCE->hEvent);
ReleaseMutex (hSemCE->hMutex);
return WaitResult;
}
BOOL CloseSynchHandle (SYNCHHANDLE hSynch)
/* Close a synchronization handle.
Improvement: Test for a valid handle before dereferencing the handle. */
{
BOOL Result = TRUE;
if (hSynch->hEvent != NULL) Result = Result && CloseHandle (hSynch->hEvent);
if (hSynch->hMutex != NULL) Result = Result && CloseHandle (hSynch->hMutex);
if (hSynch->hSemph != NULL) Result = Result && CloseHandle (hSynch->hSemph);
HeapFree (GetProcessHeap (), 0, hSynch);
return (Result);
}
static SYNCHHANDLE CleanUp (SYNCHHANDLE hSynch, DWORD Flags)
{ /* Prepare to return from a create of a synchronization handle.
If there was any failure, free any allocated resources.
"Flags" indicates which Win32 objects are required in the
synchronization handle. */
BOOL ok = TRUE;
if (hSynch == NULL) return NULL;
if (Flags & 4 == 1 && hSynch->hEvent == NULL) ok = FALSE;
if (Flags & 2 == 1 && hSynch->hMutex == NULL) ok = FALSE;
if (Flags & 1 == 1 && hSynch->hEvent == NULL) ok = FALSE;
if (!ok)
{
CloseSynchHandle (hSynch);
return NULL;
}
/* Everything worked */
return hSynch;
}