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Comments on Write to same file from multiple threads

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Write to same file from multiple threads

+8
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I want to write a text file from multiple threads. The file structure is line-oriented. This means writing of lines should be atomic. I am using Qt 5.15.2.

Is it enough to protect a shared QTextStream/QFile pair using a QMutex?

EDIT:

For now using dmckee's‭ solution I have solved it (well, a bit more complex but it boils down exactly to your idea). However moving the file work into a single writer thread does not scale.

I have a lot of threads writing very big amounts of data. This means the writer thread receives a big load and (more important) queued events need to carry a lot of data (which means a lot of RAM consumption). This gets even more nasty in case the writer thread is too slow. It will pile up events consuming virtually all RAM in the system. This is why I would prefer a mutex like solution very much. Alas, is that possible?

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General comments (7 comments)
General comments
deleted user wrote over 3 years ago

Its off-topic in this site ~ i think. I think this question belongs to code golf

r~~‭ wrote over 3 years ago

@Istiak, you are either trolling or profoundly confused about what is on topic for software development. Either way, please ease up.

deleted user wrote over 3 years ago

@r~~‭ doesn't Software Codidact is for only solving problems? Code Golf is for completing tasks, isn't it?

Alexei‭ wrote over 3 years ago

@Istiak Code golf is about solving problems with the least amount of code (bytes). The software development community is (among others) about getting solutions to programming problems.

dmckee‭ wrote over 3 years ago

Strangely I got notifications for the comments you didn’t manage to actually post. Probably a bug in the engine.

dmckee‭ wrote over 3 years ago

How you respond to the load problem depends on where the Qt signals/slots solution is being too slow. If the signals queue is the issue the you can keep using QFile/QTextSteam, but if those classes are the bottleneck you may have to go to a low-level IO solution.

Silicomancer‭ wrote over 3 years ago

@dmckee I am not sure about QFile/QTextStream performance. Current code uses them, so I will give them a try. For the moment I would like to get rid of the central writer thread. Question is still open: Is it allowed to call QFile and QTextStream methods from multiple threads (with or without a mutex) on all platforms?

Skipping 2 deleted comments.