Mutex blocks CreateFileMapping
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What did
CreateFileMappingfail with, callGetLastErrorand see what it says, maybe it helps to decypher the problem.> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
Code-o-mat wrote:
What did CreateFileMapping fail with, call GetLastError and see what it says, maybe it helps to decypher the problem.
When
CreateMutexis used in the code inCDemoApp::InitInstance,CreateFileMappingreturns a NULL value.GetLastError()returns0x00000006(invalid handle). WhenCreateMutexis NOT used in the code,CreateFileMappingreturns a valid handle to the mapped memory.Maxwell Chen
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Code-o-mat wrote:
What did CreateFileMapping fail with, call GetLastError and see what it says, maybe it helps to decypher the problem.
When
CreateMutexis used in the code inCDemoApp::InitInstance,CreateFileMappingreturns a NULL value.GetLastError()returns0x00000006(invalid handle). WhenCreateMutexis NOT used in the code,CreateFileMappingreturns a valid handle to the mapped memory.Maxwell Chen
This is strange, i find it very unlikely that
CreateMutexwould affectCreateFileMapping. I have worked with a project that used quite a few mutexes for synchronization and also used file mappings to access data (not using the paging file though) and there was no trouble with working altogether. Are you completely sure that the only difference between the two "versions" is the creation of the mutex?> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
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This is strange, i find it very unlikely that
CreateMutexwould affectCreateFileMapping. I have worked with a project that used quite a few mutexes for synchronization and also used file mappings to access data (not using the paging file though) and there was no trouble with working altogether. Are you completely sure that the only difference between the two "versions" is the creation of the mutex?> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
Code-o-mat wrote:
Are you completely sure that the only difference between the two "versions" is the creation of the mutex?
Yes! I just used
//to comment out the lines. :-DMaxwell Chen
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Code-o-mat wrote:
Are you completely sure that the only difference between the two "versions" is the creation of the mutex?
Yes! I just used
//to comment out the lines. :-DMaxwell Chen
Is it possible that you specified the same name for the mutex and the file mapping object?
> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
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Is it possible that you specified the same name for the mutex and the file mapping object?
> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
Code-o-mat wrote:
Is it possible that you specified the same name for the mutex and the file mapping object?
I had also considered this possibility, too. So I have tried using different names for the mutex and the mapped memory individually. Not working... :-D
Maxwell Chen
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Code-o-mat wrote:
Is it possible that you specified the same name for the mutex and the file mapping object?
I had also considered this possibility, too. So I have tried using different names for the mutex and the mapped memory individually. Not working... :-D
Maxwell Chen
Try creating an unnamed mutex and see if that changes anything. I know you will need a named one but this is just for testing.
> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
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Code-o-mat wrote:
Is it possible that you specified the same name for the mutex and the file mapping object?
I had also considered this possibility, too. So I have tried using different names for the mutex and the mapped memory individually. Not working... :-D
Maxwell Chen
Also, the documentation[^] of CreateFileMapping states this: "If lpName matches the name of an existing event, semaphore, mutex, waitable timer, or job object, the function fails, and the GetLastError function returns ERROR_INVALID_HANDLE. This occurs because these objects share the same namespace.", this should be a name-collision issue. Just to test this, try explicitly specifying "appleappleapple" for your mutex and "pearpearpear" for your mapping object, i mean, directly where you call the creation methods, not using a macro or any member variable.
> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
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Try creating an unnamed mutex and see if that changes anything. I know you will need a named one but this is just for testing.
> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
Code-o-mat wrote:
Try creating an unnamed mutex and see if that changes anything. I know you will need a named one but this is just for testing.
I just tried this, and it works. And I also tried again the mutex with name A and the mapped memory with name B. It also works now. I think my system was getting crazy. X| But thank you anyway. :)
Maxwell Chen
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I am adding a feature into my application programs which will be using shared (mapped) memory to communicate between two programs. I made a demo1 program to test and verify the class implementations. It works well. So I started to do one of the real application program. To prevent two instances in the system, I used
CreateMutexfor that.HANDLE hMutex = CreateMutex(NULL, FALSE, MUTEX\_NAME); if(hMutex) { if(ERROR\_ALREADY\_EXISTS == GetLastError()) { return FALSE; // Exit. } }Later during the initialization of the program, I found
CreateFileMappingfailed.m\_hFile = CreateFileMapping( INVALID\_HANDLE\_VALUE, // use paging file NULL, // default security PAGE\_READWRITE, // read/write access 0, // maximum object size (high-order DWORD) m\_dwBufSize, // maximum object size (low-order DWORD) m\_sMappingName); // name of mapping objectWhen I removed the
CreateMutexline,CreateFileMappingreturned a valid handle. Are they using the same system resource?Maxwell Chen
The APIs are not related in anyway. Just to be sure I tried to run the piece of code and it works perfectly. So the only thing relating both the APIs in the code above is the name. By giving the same name for both the mutex and the mapping, I'm able to reproduce the error with
GetLastErrorreturning the error code 6.«_Superman_»
I love work. It gives me something to do between weekends. -
What did
CreateFileMappingfail with, callGetLastErrorand see what it says, maybe it helps to decypher the problem.> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
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I've given this a 5, not 'cause of the original answer but because of the way code-o-mat followed the problem through to it's solution. Class act! Cheers, Ash
Thanks. :)
> The problem with computers is that they do what you tell them to do and not what you want them to do. < > "It doesn't work, fix it" does not qualify as a bug report. < > Amazing what new features none of the programmers working on the project ever heard of you can learn about when reading what the marketing guys wrote about it. <
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I am adding a feature into my application programs which will be using shared (mapped) memory to communicate between two programs. I made a demo1 program to test and verify the class implementations. It works well. So I started to do one of the real application program. To prevent two instances in the system, I used
CreateMutexfor that.HANDLE hMutex = CreateMutex(NULL, FALSE, MUTEX\_NAME); if(hMutex) { if(ERROR\_ALREADY\_EXISTS == GetLastError()) { return FALSE; // Exit. } }Later during the initialization of the program, I found
CreateFileMappingfailed.m\_hFile = CreateFileMapping( INVALID\_HANDLE\_VALUE, // use paging file NULL, // default security PAGE\_READWRITE, // read/write access 0, // maximum object size (high-order DWORD) m\_dwBufSize, // maximum object size (low-order DWORD) m\_sMappingName); // name of mapping objectWhen I removed the
CreateMutexline,CreateFileMappingreturned a valid handle. Are they using the same system resource?Maxwell Chen
Maxwell Chen wrote:
if(hMutex) {
shouldn't that be
if(!hMutex) {? :)Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, and improve readability.
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Maxwell Chen wrote:
if(hMutex) {
shouldn't that be
if(!hMutex) {? :)Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, and improve readability.
Luc Pattyn wrote:
shouldn't that be if(!hMutex) { ?
Nope, my original code snippet as the below is correct.
HANDLE hMutex = CreateMutex(NULL, FALSE, MUTEX\_NAME); if(hMutex) { // Returning a valid handle means that this mutex object has been successfully created. if(ERROR\_ALREADY\_EXISTS == GetLastError()) { // So we check the system error. // It says the mutex object has already been existing. // Meaning this is a second instance of the program. return FALSE; // So we have this one exit. } // The other case is no error. :-) }Maxwell Chen
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Luc Pattyn wrote:
shouldn't that be if(!hMutex) { ?
Nope, my original code snippet as the below is correct.
HANDLE hMutex = CreateMutex(NULL, FALSE, MUTEX\_NAME); if(hMutex) { // Returning a valid handle means that this mutex object has been successfully created. if(ERROR\_ALREADY\_EXISTS == GetLastError()) { // So we check the system error. // It says the mutex object has already been existing. // Meaning this is a second instance of the program. return FALSE; // So we have this one exit. } // The other case is no error. :-) }Maxwell Chen
My mistake. This is one of the exceptional cases where calling GetLastError makes sense even when the function succeeded. :)
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, and improve readability.
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My mistake. This is one of the exceptional cases where calling GetLastError makes sense even when the function succeeded. :)
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, and improve readability.
:)
Maxwell Chen
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Luc Pattyn wrote:
shouldn't that be if(!hMutex) { ?
Nope, my original code snippet as the below is correct.
HANDLE hMutex = CreateMutex(NULL, FALSE, MUTEX\_NAME); if(hMutex) { // Returning a valid handle means that this mutex object has been successfully created. if(ERROR\_ALREADY\_EXISTS == GetLastError()) { // So we check the system error. // It says the mutex object has already been existing. // Meaning this is a second instance of the program. return FALSE; // So we have this one exit. } // The other case is no error. :-) }Maxwell Chen
What happens after you return FALSE ? Otherwise, I think you just leaked a system object handle. Your program should technically close the mutex handle if you are not going to use it.