QsharedMemory with no Qt application

Question

I have an application A, and I want to share some information with an application B. Application A write information each ~150ms. Application B read information at any times.

I searched and found QSharedMemory, which looks great, but the application B will not be developed by my company, so I can't choose the programming langage.

Is QSharedMemory a good idea ? How can I do that ?

Answer 1

QSharedMemory is a thin wrapper around named and unnamed platform shared memory. When named, there's simply a file that the other application can memory-map and use from any programming language, as long as said language supports binary buffers.

I do wonder if it wouldn't be easier, though, if you used a pipe for IPC. QLocalSocket encapsulates that on Qt's end, and the other side simply uses a native pipe.

Shared memory makes sense only in certain scenarios, like, say, pushing images that may not change all that much between applications - where the cost of pushing the entire image all the time would be prohibitive in the light of small-on-average bandwidth of changes. The image doesn't need to mean a visual image, it may be an industrial process image, etc.

In many cases, shared memory is a premature pseudo-optimization that makes things much harder than necessary, and can, in case of a multitude of communicating processes, become a pessimization - you do pay the cost in virtual memory for each shared memory segment.

Answer 2

Sounds like you need to implement a simple server, using local sockets it should be pretty fast in terms of bandwidth and easy to develop. The server will act to store data from A and deliver it to B upon request.

Obviously, it won't work "with no application" in between. Whether you go for shared memory or a local socket, you will need some server code to run at all time to service A and B. If A is running all the time, it can well be a part of it, but it can also be standalone.

It would be preferable to use a local socket, because the API for that is more portable across different programming languages, in that case A and B can be implemented in arbitrary languages and frameworks and communicate at the socket protocol level. With QSharedMemory it won't be as portable in your scenario.