I'm coding in C++ a program that works as a socket between other programs in different languages (C#, Python till now). This socket reads data from the USB-port, do some stuff and stream it to the other programs.
My Idea: each program asks over a port-massage to be part of the stream. As response this program gets a pointer to a shared memory.
Is this possible? shared memory over different programing-languages? and can I just pass the pointer to the shared memory to the other program? and is there a way to do it cross-platform (Unix and Windows)?
Is this possible? shared memory over different programing-languages?
Yes. Memory is memory, so a shared-memory region created by language A can be accessed by a program written in language B.
and can I just passe the pointer to the shared memory to the other program?
It's not quite that simple; the other program won't be able to access the shared-memory region until it has mapped that same region into its own virtual-address space, using shmat() or MapViewOfFile() or similar.
Note that the shared-memory region will likely be mapped to a different range of addresses in each process that is attached to it, so don't rely on the pointer to a piece of shared data in process A being the same as a pointer to that same data in process B.
and is there a way to do it cross-platform ?
It looks like boost has an API for shared memory, if you want to go that route. Alternatively, you could write your own shared-memory API, with different code inside the .cpp file for Windows and POSIX OS's, with appropriate #ifdefs. That's how I did it in my library, and it works okay.
If you want to use shared memory, the answer above (by #JeremyFriesner) is a very good one.
However - as an alternaive to using shared memory, you can consider named pipes.
Some general information: Named pipe - Wikipedia.
Named pipes API is more high-level than shared memory.
Instead of having a raw block of memory you need to manage yourself (including using some kind of interporocess locking mechanism like named mutex to access shared data), you get a transport layer for messages.
Performance-wise they are quite efficient. But of course you will have to profile it for your specific case.
The API is available for all languages that you mentioned (and many more):
For C# (on Windows): How to: Use Named Pipes for Network Interprocess Communication.
For C++: on Windows: Named Pipes - Win32, and Linux: Named pipes - Linux.
For Python: Python named pipes.
Related
This is related to a previous post:
Allocating a large memory block in C++
I would like a single C++ server running that generates a giant matrix M. Then, on the same machine I would like to run other programs that can contact this server, and get the memory address for M. M is read only, and the server creates it once. I should be able to spawn a client ./test and this programs should be able to make read only access to M. The server should always be running, but I can run other programs like ./test at anytime.
I don't know much about C++ or OS, what is the best way to do this? Should I use POSIX threads? The matrix is a primitive type (double, float etc), and all programs know its dimensions. The client programs require the entire matrix, so I don't want latency from mem copy from the server to the client, I just want to share that pointer directly. What are my best options?
One mechanism of inter-process communication you could definitely use for sharing direct access to you matrix M is shared memory. It means that the OS lets multiple processes access a shared segment in the memory, as if it was in their address space, by mapping it for each one requesting. A solution that answers all your requirements, and is also cross-platform is boost::interprocess. It is a thin portable layer that wraps all of the necessary OS calls. See a working example right here in the docs.
Essentially, your server process just needs to create an object of type boost::interprocess::shared_memory_object, providing the constructor with a name for the shared segment. When calling its truncate() method, the OS will look for a large enough segement in the address space of this server process. From this moment, any other process can create an object of the same type and provide the same name. Now it too has access to the exact same memory. No copies involved.
If for some reason you are unable to use the portable Boost libraries, or for other reason want to restrict the supported platform to Linux, use the POSIX API around the mmap() function. Here's the Linux man page. Usage is basically not far from the Boost pipeline described above. you create the named segment with shm_open() and truncate with ftruncate(). From there onwards you receive the mapped pointer to this allocated space by calling mmap(). In simpler cases where you'll only be sharing between parent and child processes, you can use this code example from this very website.
Of course, no matter what approach you take, when using a shared resource, make sure to synchronize read/writes properly so to avoid any race condition -- as you would have done in the scenario of multiple threads of the same process.
Of course other programs cannot access the matrix as long as it is in the "normal" process memory.
Without questioning the design approach: Yes, you have to use shared memory. Lookup functions like shmget(), shmat() etc. Then you don't need to pass the pointer to another program (which would not work, actually), you simply use the same file in ftok() everywhere to get access to the shared memory.
I wrote a C++ program which read a file using file pointer. And I need to run multiple process at the same time. Since the size of file can be huge (100MB~), to reduce memory usage in multiple processes, I think I need use shared memory. (For example IPC library like boost::interprocess::shared_memory_object)
But does it really need? Because I think if multiple processes read same file, then virtual memory of each processes mapped to same physical memory of file thru page table.
I read a Linux doc and they said,
Shared Virtual Memory
Although virtual memory allows processes to have separate (virtual)
address spaces, there are times when you need processes to share
memory. For example there could be several processes in the system
running the bash command shell. Rather than have several copies of
bash, one in each processes virtual address space, it is better to
have only one copy in physical memory and all of the processes running
bash share it. Dynamic libraries are another common example of
executing code shared between several processes. Shared memory can
also be used as an Inter Process Communication (IPC) mechanism, with
two or more processes exchanging information via memory common to all
of them. Linux supports the Unix TM System V shared memory IPC.
Also, wiki said,
In computer software, shared memory is either
a method of inter-process communication (IPC), i.e. a way of exchanging data between programs running at the same time. One process
will create an area in RAM which other processes can access, or
a method of conserving memory space by directing accesses to what would ordinarily be copies of a piece of data to a single instance
instead, by using virtual memory mappings or with explicit support of
the program in question. This is most often used for shared libraries
and for XIP.
Therefore, what I really curious is that does shared virtual memory supported by OS level or not?
Thanks in advance.
Regarding your first question - if you want your data to be accessible by multiple processes without duplication you'll definitely need some kind of a shared storage.
In C++ I'd surely use boost's shared_memory_object. That's a valid option to share (large) data among processes and it has good documentation with examples (http://www.boost.org/doc/libs/1_55_0/doc/html/interprocess/sharedmemorybetweenprocesses.html).
Using mmap() is a more low-level approach usually used in C. To use it as an IPC you'll have to make the mapped region shared. From http://man7.org/linux/man-pages/man2/mmap.2.html:
MAP_SHARED
Share this mapping. Updates to the mapping are visible to
other processes that map this file, and are carried
through to the underlying file. The file may not actually
be updated until msync(2) or munmap() is called.
Also on that page there's an example of mapping a file to shared memory.
In either case there are at least two things to remember:
You need synchronization if there are multiple processes that modify the shared data.
You can't use pointers, only offsets from the beginning of the mapped region.
Here's an explanation from the boost docs:
If several processes map the same file/shared memory, the mapping address will be surely different in each process. Since each process might have used its address space in a different way (allocation of more or less dynamic memory, for example), there is no guarantee that the file/shared memory is going to be mapped in the same address.
If two processes map the same object in different addresses, this invalidates the use of pointers in that memory, since the pointer (which is an absolute address) would only make sense for the process that wrote it. The solution for this is to use offsets (distance) between objects instead of pointers: If two objects are placed in the same shared memory segment by one process, the address of each object will be different in another process but the distance between them (in bytes) will be the same.
Regarding the OS support - yes, shred memory is an OS specific feature.
In Linux mmap() is actually implemented in kernel and modules and can be used to transfer data between user and kernel-space.
Windows also has it's specifics:
Windows shared memory creation is a bit different from portable shared memory creation: the size of the segment must be specified when creating the object and can't be specified through truncate like with the shared memory object. Take in care that when the last process attached to a shared memory is destroyed the shared memory is destroyed so there is no persistency with native windows shared memory.
Your question doesn't make sense.
I think I need use shared memory. (For example IPC library like boost::interprocess::shared_memory_object).
If you use shared memroy, the memory is shared.
I think if multiple processes read same file, then virtual memory of each processes mapped to same physical memory of file thru page table.
Now you're talking about memory-mapped I/O. It isn't the same thing. However more probably it is what you need in this situation.
I'm looking for MemoryMappedViewAccessor ะก++ analog. It exists?
I want to send data from C++ app to .net app using shared memory. And I need random access. Is it possible?
Thank you!
There is no equivalent in C++. Mostly because none is needed, you can simply access the shared memory with a pointer. MemoryMappedViewAccessor is required to serialize garbage collected objects to the view.
Which makes shared memory pretty inefficient from managed code. Do make sure that a pipe or a socket doesn't solve your problem first. They usually do, dealing with concurrency is also much easier with them. Shared memory requires a raft of named mutexes to arbitrate access.
What is the fastest possible Interprocess Communication (IPC) method on Windows 7? We would like to share only a memory blocks (two-way).
Is it ReadProcessMemory or something else?
We would like to use plain C but, for example, what does Boost library use for IPC?
ReadProcessMemory shouldn't even be listed as an IPC method; yes, it can be used as such, but it exists mainly for debugging purposes (if you check its reference, it's under the category "Debugging functions"), and it's surely slower than "real" shared memory because it copies the memory of a process into the specified buffer, while real shared memory doesn't have this overhead.
The full list of IPC methods supported by Windows is available on the MSDN; still, if you just have two applications that want to share a memory block, you should create a named memory-mapped file (backed by the paging file) with CreateFileMapping/MapViewOfFile, that should be the most straightforward and fastest method. The details of file mapping are described on its page on MSDN.
The relevant Boost IPC classes can act as a thin wrapper around shared memory, AFAIK it only encapsulates the calls to the relevant system-specific APIs, but in the end you get the usual pointer to the shared memory block, so operation should be as fast as using the native APIs.
Because of this I advise you to use Boost.Interprocess, since it's portable, C++-friendly (it provides RAII semantics) and does not give you any performance penalty after the shared memory block has been created (it can provide additional functionalities on shared memory, but they are all opt-in - if you just want shared memory you get just it).
I've got a project that consists of two processes and I need to pass some data between them in a fast and efficent manner.
I'm aware that I could use sockets to do this using TCP, even though both processes will always exist on the same computer, however this does not seem to be a very efficient solution.
I see lots of information about using "pipes" on Linux. However I primarily want this for Windows and Linux (preferably via a cross platform library), ideally in a type safe,
non-blocking manner.
Another important thing is I need to support multiple instances of the whole application (i.e. both processes), each with their own independent copy of the communication objects.
Also is there a cross platform way to spawn a new process?
Take a look at Boost.Interprocess
Boost.Interprocess simplifies the use of common interprocess communication and synchronization mechanisms and offers a wide range of them:
Shared memory.
Memory-mapped files.
Semaphores, mutexes, condition variables and upgradable mutex types to place them in shared memory and memory mapped files.
Named versions of those synchronization objects, similar to UNIX/Windows sem_open/CreateSemaphore API.
File locking.
Relative pointers.
Message queues.
Boost.Interprocess also offers higher-level interprocess mechanisms to allocate dynamically portions of a shared memory or a memory mapped file (in general, to allocate portions of a fixed size memory segment). Using these mechanisms, Boost.Interprocess offers useful tools to construct C++ objects, including STL-like containers, in shared memory and memory mapped files:
Dynamic creation of anonymous and named objects in a shared memory or memory mapped file.
STL-like containers compatible with shared memory/memory-mapped files.
STL-like allocators ready for shared memory/memory-mapped files implementing several memory allocation patterns (like pooling).
Boost.Interprocess has been tested in the following compilers/platforms:
Visual 7.1 Windows XP
Visual 8.0 Windows XP
GCC 4.1.1 MinGW
GCC 3.4.4 Cygwin
Intel 9.1 Windows XP
GCC 4.1.2 Linux
GCC 3.4.3 Solaris 11
GCC 4.0 MacOs 10.4.1
For IPC, Windows supports named pipes just like Linux does, except that the pipe names follow a different format, owing to the difference in path formats between the two operating systems. This is something that you could overcome with simple preprocessor defines. Both operating systems also support non-blocking IO on pipes and IO multiplexing with select().
Plain old TCP should work fairly efficiently; as I understand it, modern OS's will detect when both ends of a TCP connection are located on the same machine, and will internally route that data through a fast, lightweight (pipe-like) mechanism rather than through the ordinary TCP stack.
So if you already have code that works over TCP, I say stick with that and avoid spending a lot of extra development time for not much payoff.
It may be overkill, but you could use the Apache Portable Runtime; here are the thread and process functions.