Does Windows offer any kind of mutex that can be placed in a memory mapped file and used across multiple processes?
Active6 years, 8 months ago
Ideally it must be completely self contained such that it can survive by itself in the file, even across a reboot.Also, no resources should be leaked if I simply remove the file manually while no processes are running.
If possible the solution should also offer the accompanying 'condition' concept which should also be an object that can sit in a shared memory mapped file.
In short, I need something similar to a PTHREADS mutex with the SHARED attribute. Sansui turntable manuals.
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In fact, the two are very similar in most respects. (Memory mapped files are committed to disk, too, so this could even be an advantage, yes?) With file locking or semaphores, data in a memory mapped file can easily be shared between multiple processes. Memory mapping a file on disk doesn’t make the disk I/O any faster, and disk sectors can be cached in memory as easily as pages can be stored in memory. However, most operating systems do not allow a program to trap its own page faults, so when a page has to be read in from.
As far as I understand, simply using a PTHREADS mutex is not possible because the SHARED attribute is unsupported in the Windows port of PTHREADS.
Kristian Spangsege
Kristian SpangsegeKristian Spangsege
3 Answers
To share a synchronization object, give it a name and use the same name in each process when you Create the object.
The following synchronization objects can be shared between process that way :
Critical sections cannot be shared, but are faster.
Testing or waiting on those objects is done with the wait family of functions, often WaitForMultipleObjects.
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Use the file as its own mutex: Use the Raymond ChenRaymond Chen
LockFileEx function and have everybody agree to lock byte 0 of the file when they want to claim the mutex.
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That's not possible. The mutex object itself lives in kernel space to protect it from user code messing with its state. The handle you acquired to it is only valid for the process that acquired it. Technically you could use DuplicateHandle() and put the returned handle in the mmf, but only if you have a handle to the other process that accesses the memory section. That's fairly brittle.
This is why you can specify a name for the mutex in the CreateMutex() function. The other process gets to it by using the same name in the OpenMutex call.
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Active5 years, 9 months ago
I am writing a C program that makes use of
mmap system call, running on Linux 3.12 64-bit.
If I have two processes mmaping the same region of a disk file with read/write access, and then modify the region content from both processes at the same time..
user972946
1 Answer
Yes, that's one of the purposes of memory mapping: as a form of 'instantaneous IPC'. Musafir song free mp3 download.
You must set the MAP_SHARED flag:
If you wish to use shared memory for this purpose, I would consider the shminit()/shmat() APIs instead:
Suggestion: check out Beej's Guide to *nix Interprocess Communication:
Windows Memory Mapped File Lock
And no, if you use the raw mmap() APIs, there's no 'before/after guarantee', and you must use some kind of locking (e.g. semaphores) if you wish to read/write data concurrently.
Also, from Understanding Memory Mapping:
Both the mmap and shmat services provide the capability for multiple processes to map the same region of an object such that they share addressability to that object. However, the mmap subroutine extends this capability beyond that provided by the shmat subroutine by allowing a relatively unlimited number of such mappings to be established. While this capability increases the number of mappings supported per file object or memory segment, it can prove inefficient for applications in which many processes map the same file data into their address space.
The mmap subroutine provides a unique object address for each process that maps to an object. The software accomplishes this by providing each process with a unique virtual address, known as an alias. The shmat subroutine allows processes to share the addresses of the mapped objects.
Mapped File Memory Windows 10
The above is true of any *nix variant, including both Linux and AIX.
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C++ Memory Mapped FileFoggyDay
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