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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 | |
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 | |
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 | |
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 | |
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 | |
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 | } |