1 /* win_ce_semaphore.c
2
3 Copyright (c) 1998, Johnson M. Hart
4 (with corrections 2001 by Rainer Loritz)
5 Permission is granted for any and all use providing that this
6 copyright is properly acknowledged.
7 There are no assurances of suitability for any use whatsoever.
8
9 WINDOWS CE: There is a collection of Windows CE functions to simulate
10 semaphores using only a mutex and an event. As Windows CE events cannot
11 be named, these simulated semaphores cannot be named either.
12
13 Implementation notes:
14 1. All required internal data structures are allocated on the process's heap.
15 2. Where appropriate, a new error code is returned (see the header
16 file), or, if the error is a Win32 error, that code is unchanged.
17 3. Notice the new handle type "SYNCHHANDLE" that has handles, counters,
18 and other information. This structure will grow as new objects are added
19 to this set; some members are specific to only one or two of the objects.
20 4. Mutexes are used for critical sections. These could be replaced with
21 CRITICAL_SECTION objects but then this would give up the time out
22 capability.
23 5. The implementation shows several interesting aspects of synchronization, some
24 of which are specific to Win32 and some of which are general. These are pointed
25 out in the comments as appropriate.
26 6. The wait function emulates WaitForSingleObject only. An emulation of
27 WaitForMultipleObjects is much harder to implement outside the kernel,
28 and it is not clear how to handle a mixture of WCE semaphores and normal
29 events and mutexes. */
30
31 #define WIN32_LEAN_AND_MEAN
32 #include <windows.h>
33
34 #include "win_ce_semaphore.h"
35
36 static SYNCHHANDLE CleanUp (SYNCHHANDLE hSynch, DWORD Flags);
37
CreateSemaphoreCE(LPSECURITY_ATTRIBUTES lpSemaphoreAttributes,LONG lInitialCount,LONG lMaximumCount,LPCTSTR lpName)38 SYNCHHANDLE CreateSemaphoreCE (
39
40 LPSECURITY_ATTRIBUTES lpSemaphoreAttributes, /* pointer to security attributes */
41 LONG lInitialCount, /* initial count */
42 LONG lMaximumCount, /* maximum count */
43 LPCTSTR lpName )
44
45 /* Semaphore for use with Windows CE that does not support them directly.
46 Requires a counter, a mutex to protect the counter, and an
47 autoreset event.
48
49 Here are the rules that must always hold between the autoreset event
50 and the mutex (any violation of these rules by the CE semaphore functions
51 will, in all likelihood, result in a defect):
52 1. No thread can set, pulse, or reset the event,
53 nor can it access any part of the SYNCHHANDLE structure,
54 without first gaining ownership of the mutex.
55 BUT, a thread can wait on the event without owning the mutex
56 (this is clearly necessary or else the event could never be set).
57 2. The event is in a signaled state if and only if the current semaphore
58 count ("CurCount") is greater than zero.
59 3. The semaphore count is always >= 0 and <= the maximum count */
60
61 {
62 SYNCHHANDLE hSynch = NULL, result = NULL;
63
64 __try
65 {
66 if (lInitialCount > lMaximumCount || lMaximumCount < 0 || lInitialCount < 0)
67 {
68 /* Bad parameters */
69 SetLastError (SYNCH_ERROR);
70 __leave;
71 }
72
73 hSynch = HeapAlloc (GetProcessHeap(), HEAP_ZERO_MEMORY, SYNCH_HANDLE_SIZE);
74 if (hSynch == NULL) __leave;
75
76 hSynch->MaxCount = lMaximumCount;
77 hSynch->CurCount = lInitialCount;
78 hSynch->lpName = lpName;
79
80 hSynch->hMutex = CreateMutex (lpSemaphoreAttributes, FALSE, NULL);
81
82 WaitForSingleObject (hSynch->hMutex, INFINITE);
83 /* Create the event. It is initially signaled if and only if the
84 initial count is > 0 */
85 hSynch->hEvent = CreateEvent (lpSemaphoreAttributes, FALSE,
86 lInitialCount > 0, NULL);
87 ReleaseMutex (hSynch->hMutex);
88 hSynch->hSemph = NULL;
89 }
90 __finally
91 {
92 /* Return with the handle, or, if there was any error, return
93 a null after closing any open handles and freeing any allocated memory. */
94 result=CleanUp(hSynch, 6 /* An event and a mutex, but no semaphore. */);
95 }
96
97 return result;
98 }
99
ReleaseSemaphoreCE(SYNCHHANDLE hSemCE,LONG cReleaseCount,LPLONG lpPreviousCount)100 BOOL ReleaseSemaphoreCE (SYNCHHANDLE hSemCE, LONG cReleaseCount, LPLONG lpPreviousCount)
101 /* Windows CE equivalent to ReleaseSemaphore. */
102 {
103 BOOL Result = TRUE;
104
105 /* Gain access to the object to assure that the release count
106 would not cause the total count to exceed the maximum. */
107
108 __try
109 {
110 WaitForSingleObject (hSemCE->hMutex, INFINITE);
111 /* reply only if asked to */
112 if (lpPreviousCount!=NULL)
113 *lpPreviousCount = hSemCE->CurCount;
114 if (hSemCE->CurCount + cReleaseCount > hSemCE->MaxCount || cReleaseCount <= 0)
115 {
116 SetLastError (SYNCH_ERROR);
117 Result = FALSE;
118 __leave;
119 }
120 hSemCE->CurCount += cReleaseCount;
121
122 /* Set the autoreset event, releasing exactly one waiting thread, now or
123 in the future. */
124
125 SetEvent (hSemCE->hEvent);
126 }
127 __finally
128 {
129 ReleaseMutex (hSemCE->hMutex);
130 }
131
132 return Result;
133 }
134
WaitForSemaphoreCE(SYNCHHANDLE hSemCE,DWORD dwMilliseconds)135 DWORD WaitForSemaphoreCE (SYNCHHANDLE hSemCE, DWORD dwMilliseconds)
136 /* Windows CE semaphore equivalent of WaitForSingleObject. */
137 {
138 DWORD WaitResult;
139
140 WaitResult = WaitForSingleObject (hSemCE->hMutex, dwMilliseconds);
141 if (WaitResult != WAIT_OBJECT_0 && WaitResult != WAIT_ABANDONED_0) return WaitResult;
142 while (hSemCE->CurCount <= 0)
143 {
144
145 /* The count is 0, and the thread must wait on the event (which, by
146 the rules, is currently reset) for semaphore resources to become
147 available. First, of course, the mutex must be released so that another
148 thread will be capable of setting the event. */
149
150 ReleaseMutex (hSemCE->hMutex);
151
152 /* Wait for the event to be signaled, indicating a semaphore state change.
153 The event is autoreset and signaled with a SetEvent (not PulseEvent)
154 so exactly one waiting thread (whether or not there is currently
155 a waiting thread) is released as a result of the SetEvent. */
156
157 WaitResult = WaitForSingleObject (hSemCE->hEvent, dwMilliseconds);
158 if (WaitResult != WAIT_OBJECT_0) return WaitResult;
159
160 /* This is where the properties of setting of an autoreset event is critical
161 to assure that, even if the semaphore state changes between the
162 preceding Wait and the next, and even if NO threads are waiting
163 on the event at the time of the SetEvent, at least one thread
164 will be released.
165 Pulsing a manual reset event would appear to work, but it would have
166 a defect which could appear if the semaphore state changed between
167 the two waits. */
168
169 WaitResult = WaitForSingleObject (hSemCE->hMutex, dwMilliseconds);
170 if (WaitResult != WAIT_OBJECT_0 && WaitResult != WAIT_ABANDONED_0) return WaitResult;
171
172 }
173 /* The count is not zero and this thread owns the mutex. */
174
175 hSemCE->CurCount--;
176 /* The event is now unsignaled, BUT, the semaphore count may not be
177 zero, in which case the event should be signaled again
178 before releasing the mutex. */
179
180 if (hSemCE->CurCount > 0) SetEvent (hSemCE->hEvent);
181 ReleaseMutex (hSemCE->hMutex);
182 return WaitResult;
183 }
184
CloseSynchHandle(SYNCHHANDLE hSynch)185 BOOL CloseSynchHandle (SYNCHHANDLE hSynch)
186 /* Close a synchronization handle.
187 Improvement: Test for a valid handle before dereferencing the handle. */
188 {
189 BOOL Result = TRUE;
190 if (hSynch->hEvent != NULL) Result = Result && CloseHandle (hSynch->hEvent);
191 if (hSynch->hMutex != NULL) Result = Result && CloseHandle (hSynch->hMutex);
192 if (hSynch->hSemph != NULL) Result = Result && CloseHandle (hSynch->hSemph);
193 HeapFree (GetProcessHeap (), 0, hSynch);
194 return (Result);
195 }
196
CleanUp(SYNCHHANDLE hSynch,DWORD Flags)197 static SYNCHHANDLE CleanUp (SYNCHHANDLE hSynch, DWORD Flags)
198 { /* Prepare to return from a create of a synchronization handle.
199 If there was any failure, free any allocated resources.
200 "Flags" indicates which Win32 objects are required in the
201 synchronization handle. */
202
203 BOOL ok = TRUE;
204
205 if (hSynch == NULL) return NULL;
206 if ((Flags & 4) == 1 && (hSynch->hEvent == NULL)) ok = FALSE;
207 if ((Flags & 2) == 1 && (hSynch->hMutex == NULL)) ok = FALSE;
208 if ((Flags & 1) == 1 && (hSynch->hEvent == NULL)) ok = FALSE;
209 if (!ok)
210 {
211 CloseSynchHandle (hSynch);
212 return NULL;
213 }
214 /* Everything worked */
215 return hSynch;
216 }
217